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1808 | 1808.06637_arXiv.txt | In order to place constraints on cosmology through optical surveys of galaxy clusters, one must first understand the properties of those clusters. To this end, we introduce the \textbf{M}ass \textbf{A}nalysis \textbf{T}ool for \textbf{Cha}ndra (\matcha{}), a pipeline which uses a parallellized algorithm to analyze archival Chandra data. \matcha{} simultaneously calculates X-ray temperatures and luminosities and performs centering measurements for hundreds of potential galaxy clusters using archival X-ray exposures. We run \matcha{} on the \redmapper{} \sdss{} DR8 cluster catalog and use \matcha{}'s output X-ray temperatures and luminosities to analyze the galaxy cluster temperature-richness, luminosity-richness, luminosity-temperature, and temperature-luminosity scaling relations. We detect 447 clusters and determine 246 \rtfh{} temperatures across all redshifts. Within $0.1 < z < 0.35$ we find that \rtfh{} \tx{} scales with optical richness ($\lambda$) as $\txrelation{1.85 \pm{} 0.03}{0.52 \pm{} 0.05}$ with intrinsic scatter of $0.27 \pm{} 0.02$ ($1 \sigma$). We investigate the distribution of offsets between the X-ray center and \redmapper{} center within $0.1 < z < 0.35$, finding that $68.3 \pm 6.5$\% of clusters are well-centered. However, we find a broad tail of large offsets in this distribution, and we explore some of the causes of \redmapper{} miscentering. | % \label{sec:introduction} The formation history of galaxy clusters is a powerful probe of cosmology~\citep[e.g.][]{Voit, Frieman, MantzI, Allen11, Weinberg, McClintock18}. In particular, one may place strong constraints on the dark energy equation of state by examining the evolution across redshift of the number density of galaxy clusters as a function of mass~\citep{Mohr, ChandraCosmoIII}. Upcoming and in-progress optical imaging surveys, such as the Dark Energy Survey (DES)~\citep{DES}, the Hyper Suprime Cam (HSC)~\citep{HSC}, Euclid~\citep{Euclid}, and the Large Synoptic Survey Telescope (LSST)~\citep{LSST}, are expected to observe of tens of thousands of galaxy clusters, thus dramatically expanding our ability to use clusters to place these constraints~\citep{Cunha09, Sanchez, Oguri11, Weinberg, EuclidCosmology}. The galaxy cluster mass function is the key observable predicted by theory for galaxy-cluster-based studies of dark energy. Ideally galaxy cluster masses would be measured directly via lensing. However, because large surveys rarely produce the depth of data required to directly measure the mass of an individual galaxy cluster via lensing, one must instead use some other observable as a mass proxy, and then use an observable-mass relation in order to relate that observable to a distribution of potential masses. Any given observable-mass relation for massive halos will have some intrinsic scatter distribution driven by recent dynamical activity as well as the full assembly history of each specific halo. Thus, in order to turn a measured distribution of observables into a distribution of masses, one must understand both the mean observable-mass relation and the intrinsic scatter distribution of this relation. Stacked weak lensing, which allows one to look at the average mass of many ``similar'' galaxy clusters, is a powerful method by which to determine a mean observable-mass relation~\citep[e.g.][]{Leauthaud, WeighingTheGiantsI, Simet, Melchior, McClintock18}. The remaining task for the cosmologist is then to understand the intrinsic scatter distribution of the given observable-mass relation. For the purposes of this paper, we will examine the \emph{richness} optical mass proxy~\citep{Bahcall83, Andreon, redmapperI} and the intrinsic scatter distribution of its relation with other cluster mass proxies. The precise definition of richness differs from cluster finder to cluster finder, but in essence it is some measure of the number of galaxies in a cluster. The intrinsic scatter distribution of the richness-mass relation is currently one of the largest sources of uncertainty in using cluster richness to place cosmological constraints~\citep{Wu}. One may constrain this scatter distribution and improve these constraints by following up a subset of these optically-selected clusters to obtain mass proxies in other wavelengths. To this end, we have developed a pipeline to perform automated, massively parallelized X-ray follow-up on galaxy clusters which fall within archival \chandra{} data. This pipeline is called \textbf{\matcha{}}: the \textbf{M}ass \textbf{A}nalysis \textbf{T}ool for \textbf{Cha}ndra. \matcha{} attempts to measure gas temperatures and X-ray luminosities for these clusters, which can then be compared with their richnesses to help better understand the intrinsic scatter distribution of the richness-mass relation. Additionally, \matcha{} produces two measures of the ``center'' of a galaxy cluster: the X-ray centroid (i.e.\ center-of-flux) and the X-ray peak. Miscentering by galaxy cluster finders is a major source of systematic uncertainty in stacked weak-lensing analyses~\citep{Johnston, Melchior}, and without accurate centering information it is difficult for stacked weak-lensing pipelines to produce masses accurate to the level required to realize the full potential of cluster cosmology~\citep{Weinberg}. By comparing our X-ray centering information with that produced by a given cluster finder, it is possible to understand the centering characteristics of said cluster finder and calibrate for their effects on cosmological analyses. In this paper, we present the \matcha{} algorithm and describe its application to galaxy clusters identified in the \sdss{} DR8~\citep{DR8} \redmapper{} optical cluster catalog~\citep{redmapperSDSS}. We use the resulting X-ray temperatures, luminosities, and centering information to explore scatter distributions of richness--mass-proxy relations as well as \redmapper{}'s ability to correctly assign galaxy cluster centers. In \autoref{sec:cluster-selection}, we give a brief overview of the \redmapper{} galaxy cluster finder. In \autoref{sec:matcha}, we outline \matcha{}, a pipeline which uses archival \chandra{} data to study the X-ray properties of clusters. In \autoref{sec:results}, we present temperature-richness and luminosity-richness scaling relations derived from the data produced by \matcha{}, compare \redmapper{} centering with the centering information produced by \matcha{}, and discuss ramifications for stacked weak lensing analyses that use \redmapper{} galaxy cluster locations. Finally, in \autoref{sec:summary}, we summarize the paper and discuss future work to be done. In \autoref{sec:example-images}, we present sample images of galaxy clusters produced by \matcha{}. In \autoref{sec:flag-effects}, we visually highlight various subsamples of our data and their effects on our scaling relations. In \autoref{sec:matcha-data}, we outline the structure of three machine-readable tables, available online, which contain data used in this paper. Throughout this paper, we assume a flat $\Lambda$CDM cosmology with $\Omega_m = 0.3$, $H_0 = 70 \mathrm{km \cdot{} s^{-1} \cdot{} Mpc^{-1}}$. Luminosities are scaled by $E\left(z\right) \equiv H\left(z\right) / H\left(0\right) = \sqrt{\Omega_R {\left(1 + z\right)}^4 + \Omega_M {\left(1 + z\right)}^3 + \Omega_k {\left(1 + z\right)}^2 + \Omega_\Lambda}$, where H is the (redshift-dependent) Hubble parameter; and $\Omega_R$, $\Omega_M$, $\Omega_k$, and $\Omega_\Lambda$ are the densities due to radiation, matter, curvature, and a cosmological constant, respectively, all normalized by the critical density. | % \label{sec:summary} In this paper we introduce \matcha{}, a pipeline which is capable of performing parallel analysis of hundreds of galaxy clusters in archival \chandra{} data. We run \matcha{} on the galaxy cluster catalog generated by \redmapper{}'s analysis of \sdss{} DR8 data. Using this information we derive temperature-richness, luminosity-richness, and luminosity-temperature relations within \rtfh{} and \rfh{} apertures. In particular, we find we find an \rtfh{} \tx{}-richness relation of $\txrelation{1.85 \pm{} 0.03}{0.52 \pm{} 0.05}$ and a standard-deviation-of-intrinsic-scatter of $0.27 \pm{} 0.02$ ($1 \sigma$) within $0.1 < z < 0.35$. We also derive a number of other \tx{}-richness, \lx{}-richness, \lx{}-\tx{}, and \tx{}-\lx{} relations within \rtfh{} and \rfh{} apertures. Our data offer improved constraints on $\sigmaintr$ when compared with similar prior work. We find a slightly greater \tx{}-richness slope than that presented in \citet{redmapperII} ($1.4 \sigma$), and a much larger standard deviation of intrinsic scatter. We find a similar $\sigmaintr$ to \citet{redmapperSV}, and here our slope is smaller than theirs by $0.9 \sigma$. Finally, we find that our bolometric \lx{}-\tx{} relation's slope agrees well with \citet{Hicks13}, however we derive a much lower slope than \citet{Maughan12}. We then measure the miscentering distribution in \redmapper{} by comparing the locations of \redmapper{}'s bright central galaxies with X-ray centroids and peaks measured by \matcha{}. We find that $\approx 68\%$ of the clusters are well-centered. We explore the tail of our centering distribution and identify failure modes of the \redmapper{} centering algorithm. In addition to this current work, \matcha{} has already been used in large-scale X-ray analyses such as \citet{Bufanda}, in which \matcha{} is used to examine the AGN population of galaxy clusters; and \citet{redmapperSV}, in which \matcha{} is used to analyze DES Science Verification Data. Further \matcha{} results on DES Year 1 and \sdss{} data will be presented in papers on \redmapper{} centering~\citep{ZhangCentering}, \redmapper{} scaling relations~\citep{FarahiScaling}, and cosmology results from both \redmapper{} \sdss{} DR8~\citep{SloanCosmology} and DES Y1~\citep{Y1Cosmology}. | 18 | 8 | 1808.06637 |
1808 | 1808.06895_arXiv.txt | {The CARMENES spectrograph is surveying $\sim$300 M dwarf stars in search for exoplanets. Among the target stars, spectroscopic binary systems have been discovered, which can be used to measure fundamental properties of stars.} {Using spectroscopic observations we determine the orbital and physical properties of nine new double-line spectroscopic binary systems by analysing their radial velocity curves.} {We use two-dimensional cross-correlation techniques to derive the radial velocities of the targets, which are then employed to determine the orbital properties. Photometric data from the literature are also analysed to search for possible eclipses and to measure stellar variability, which can yield rotation periods.} {Out of the 342 selected stars for CARMENES survey, 9 have been found to be double-line spectroscopic binaries, with periods ranging from 1.13 to $\sim$8000 days and orbits with eccentricities up to 0.54. We provide empirical orbital properties and minimum masses for the sample of spectroscopic binaries. Absolute masses are also estimated from mass-luminosity calibrations, ranging between $\sim$0.1~M$_{\odot}$ and $\sim$0.6~M$_{\odot}$.} {These new binary systems increase the number of double-line M dwarf binary systems with known orbital parameters by 15\%, and they have lower mass ratios on average.} | \label{sec:introduction} Binary systems are essential for the study of stellar structure and evolution. Depending on their nature, they can yield fundamental properties such as the masses, radii, and luminosities of the components independently from calibrations and stellar models and with very high precision. This enables critical comparisons with stellar model predictions and the determination of empirical calibrations that can be used for single stars \citep[see][for a review]{Torres2010}. Due to the increasing interest in the discovery of exoplanets, several instruments were developed to spectroscopically survey a large number of stars. In addition to planetary objects, these projects can also reveal new binary systems that are interesting on their own, because they can be used to constrain the stellar structure and evolution models and to improve the multiplicity statistics of late-type stars \citep{Halbwachs2003,Mazeh2003}. This is the case of the CARMENES survey \citep[][]{Quirrenbach2016}. This survey monitors about 300 M dwarf stars to uncover exoplanets in their habitable zones. Targets were selected from available M dwarf catalogues and photometric surveys, and were also carefully studied to discard unsuitable targets such as visual double systems, known spectroscopic binaries and very faint stars \citep[see e.g.][, for more details]{AlonsoFloriano2015,CortesContreras2017,Jeffers2017}. The CARMENES collaboration has already announced its first planet detections \citep{Reiners2018,Sarkis2018,Trifonov2018}. In addition to the new planets, several spectroscopic binary systems were identified with the first observations of the sample and they were followed-up to characterize them. The binary systems discovered with CARMENES are especially interesting because the number of known M dwarf binary systems is still scarce \citep[see e.g. the Ninth Catalogue of Spectroscopic Binary Orbits, hereafter SB9\footnote{\texttt{http://sb9.astro.ulb.ac.be/}} -- ][]{Pourbaix2004}. The distribution of mass ratios and orbital elements may help to understand the formation and evolution of low-mass stars, brown dwarfs or giant planets in M dwarf stellar systems. Besides, they are also valuable for constraining the properties of M dwarfs, which still show some discrepancies with stellar model predictions \citep[see e.g.][]{Morales2010,Feiden2013,Feiden2014}. In this paper we present nine new double-line spectroscopic binary (SB2) systems discovered in the CARMENES survey. Orbital properties were derived for all of them, yielding their mass ratios and periods for the first time. In Sect.~\ref{sec:observations} we describe the observations for each system. In Sect.~\ref{subsec:rv-analysis}, the radial velocity analysis of each system is shown. Photometric light curves gathered from the literature and public databases are compiled and discussed in Sect.~\ref{sec:phot-analysis}. Finally, the results are discussed in Sect.~\ref{sec:discussion} and our conclusions are presented in Sect.~\ref{sec:conclusions}. Figures of the radial velocity data and the photometric periodogram analysis are compiled in the Appendix. | \label{sec:conclusions} In this work we analysed nine new M dwarf SB2 systems found in the context of the CARMENES survey of exoplanets, increasing the number of known MM spectroscopic binaries by over 15\,\%. Orbital parameters derived from the radial velocities, i.e. period, eccentricity, argument of the periastron, radial velocity semi-amplitudes and mass ratios, are provided for these systems for the first time. Among them, 3 systems have periods shorter than 10 days, 3 between 70 and 160 days and 3 have periods longer than around 2 years for which additional observations may help to better constrain their properties. Publicly available photometry for these targets was also analysed. Significant periodic signals attributed to the rotation period are found for 7 of the systems. Unfortunately, no eclipses are found in any case. However, individual masses and radii were estimated using empirical calibrations for systems with parallactic distances, providing the fundamental properties of the components of the systems. The comparison of the orbital properties of the systems studied here with those from the literature reveals that our set of low-mass binary systems have smaller mass ratios than more massive systems and that of known M dwarfs SB2s. This trend may arise from the better sensitivity of the CARMENES spectrograph towards longer wavelengths. This could also suggest that low-mass binary systems may have lower mass ratios, but more statistics are needed to confirm this trend. Further observations of these systems will help to better constrain the properties of the long period systems. Precise astrometric measurements from Gaia may also be very valuable to put additional constraints and derive absolute masses and inclinations. This will increase the sample of low-mass stars that can be used to refine the mass-luminosity relationship of these systems, independently of stellar models. | 18 | 8 | 1808.06895 |
1808 | 1808.03681_arXiv.txt | We present continuum and molecular line { (CO, C$^{18}$O, HCO$^+$)} observations carried out with the Atacama Large Millimeter/submillimeter Array toward the ``water fountain'' star IRAS 15103-5754, { an object that could be the youngest PN known}. We detect two continuum sources, separated by $0.39\pm 0.03$ arcsec. The emission from the brighter source seems to arise mainly from ionized gas, { thus} confirming the PN nature of the object. The molecular line emission is dominated by a circumstellar torus with a diameter of $\simeq 0.6$ arcsec (2000 au) and expanding at $\simeq 23$ km s$^{-1}$. We see at least two gas outflows. The highest-velocity outflow (deprojected velocities up to 250 km s$^{-1}$), { traced by the CO lines,} shows a biconical morphology, whose axis is misaligned $\simeq 14^\circ$ with respect to the symmetry axis of the torus, and with a different central velocity (by $\simeq 8$ km s$^{-1}$). An additional high-density outflow { (traced by HCO$^+$)} is oriented nearly perpendicular to the torus. We speculate that IRAS 15103-5754 was a triple stellar system that went through a common envelope phase, and one of the components was ejected in this process. A subsequent low-collimation wind from the remaining binary stripped out gas from the torus, creating the conical outflow. The high velocity of the outflow suggests that the momentum transfer from the wind is extremely efficient, or that we are witnessing a very energetic mass-loss event. | Planetary nebulae (PNe) represent one of the last stages of evolution of stars with main-sequence masses $\simeq 0.8-8$ M$_\odot$. The PN phase starts when the effective temperature of the central star reaches $\simeq 25\,000$ K, and the circumstellar envelopes ejected during the asymptotic giant branch (AGB) are then photoionized. PNe display a huge variety of morphologies, in contrast with the spherical symmetry expected from stars in previous phases. Different processes can break this spherical symmetry. For instance, jets launched during the post-AGB phase \citep{sah98,buj01} can open cavities in the circumstellar envelopes. When the PN phase starts, the photoionization front and low-density winds from the central star will proceed preferentially along those cavities. Alternatively, the presence of a binary companion will also alter the morphology of the circumstellar envelope, by capturing part of it and/or creating a common envelope when the material overfills the Roche lobes of the secondary \citep{iva13}. The processes that take place at the beginning of photoionization, while the ionization front advances through the envelope, will determine the physical characteristics and the morphology in later evolutionary stages. This phase of growth of the ionized nebula only lasts for a $\la 100$ years \citep{bob98}, so it is difficult to catch a source at that exact instant. There is evidence that the object IRAS 15103$-$5754 is just starting the photoionization phase, making it the youngest PN known. Our Herschel and VLT observations prove that it is a PN \citep{gom15}. However, it shows water maser emission over a velocity range of $\simeq 75$ km s$^{-1}$. Only four other PNe are known to harbour water masers \citep{mir01,deg04,gom08,usc14}, and all these are supposed to be extremely young PNe, as this emission is expected to last for only $\simeq 100$ yr after the end of the AGB \citep{lew89,gom90}. However, IRAS 15103$-$5754 has several key characteristics only observed in this source, all indicating that it is now at the very onset of photoionization \citep{gom15,sua15}. \begin{itemize} \item It is the only known ``water fountain'' (WF) that is already a PN. WFs \citep{ima07,des12} are evolved stars with high-velocity water masers (velocity spreads $>75$ km s$^{-1}$), tracing jets of short dynamical ages ($< 100$ yr). They might be one of the first manifestations of collimated mass loss in evolved objects. This type of high-velocity maser jet is also present in IRAS 15103$-$5754. However, all other known WFs are in the post-AGB or late-AGB phase. The small group of water-maser-emitting PNe do not show high velocities. This indicates that IRAS 15103$-$5754 is just leaving the post-AGB phase. \item The velocity of the water masers increases linearly at longer distances from the central star. This is a sign of an explosive or ballistic event. While this linear velocity trend is present in the ionized and molecular gas of post-AGB stars and PNe, they are not seen in the energetic maser emission of other WFs. \item It is the only known PN with non-thermal radio continuum emission. Although the physical conditions in PNe are appropriate for the existence of synchrotron radiation \citep{cas07}, this emission is usually overwhelmed by free-free radiation from electrons in the photoionized regions. The existence of this type of non-thermal emission in IRAS 15103$-$5754 would mean that the photoionized region is still small, and does not veil synchrotron emission. \item The spectrum of non-thermal emission is changing rapidly. There has been a progressive flattening of the spectral index of radio continuum emission, mainly driven by a decrease of flux density at lower frequencies, over the past 25 years. In particular, the spectral index changed from $\simeq -0.54$ in 2010-2011 to $-0.28$ in 2012. We interpreted this change as due to the progressive growth of electron density in a nascent photoionized region. \end{itemize} Given the outstanding characteristics of IRAS 15103$-$5754, and the rapid evolution this source is undergoing in time-scales of a few years, its study represents a unique opportunity to witness the morphological and kinematical changes occurring at the birth of a PN, because the probability of finding any other source at this particular phase is extremely small. In this paper, we present ALMA observations of the (sub)mm continuum emission and spectral lines. Our aim is to study the morphology and kinematics of the circumstellar structure, in order to determine the physical processes taking place at the beginning of the PN phase. | We presented continuum and molecular-line observations of the young PN IRAS 15103-5754 carried out with ALMA at 0.85 and 1.3 mm. Our main conclusions are as follows. \begin{itemize} \item We detect two continuum sources, separated by $\simeq 0.39\pm 0.03$ arcsec. A significant fraction of the emission has a spectral index $<2$, with a minimum of 0.6. This is not compatible with dust only, and suggests a significant contribution from free-free emission from ionized gas. \item The line emission is dominated by a central torus of { 0.6 arcsec (2000 au at 3.38 kpc)} diameter and a mass { 0.4-1} M$_\odot$, expanding at $\simeq 23$ km s$^{-1}$. \item There are at least two high-velocity outflows. The first one, mainly traced by CO, has the kinematical signature of a biconical outflow, with an opening angle of 56$^\circ$, and linearly increasing velocities with distance from the central star, reaching deprojected velocities up to 250 km s$^{-1}$ and a total extent of { 6 arcsec (20000 au)}. The axis of the outflow is misaligned by $\simeq 14^\circ$ with respect to the axis of the torus. Moreover, the central velocity of the outflow is shifted by $\simeq 8$ km s$^{-1}$ from the central velocity of the torus. \item A second, high-density outflow traced by HCO$^+$ is nearly perpendicular to the torus, and aligns well with the infrared nebula. \item We propose that the characteristics of IRAS 15103-5754 could be due to a central triple stellar system, which went through a common envelope phase, leading to the formation of the circumstellar torus. One of the components was ejected in this process, changing the central velocity and orbital plane. A subsequent low-collimation wind stripped the gas from the torus, generating the conical outflow. \item The extremely high velocity of the conical outflow indicates that the momentum transfer between the wind and the molecular gas was extremely efficient, or that the mass loss was very energetic. \end{itemize} | 18 | 8 | 1808.03681 |
1808 | 1808.05772_arXiv.txt | We present overall specifications and science goals for a new optical and near-infrared (350 - 1650 nm) instrument designed to greatly enlarge the current Search for Extraterrestrial Intelligence (SETI) phase space. The Pulsed All-sky Near-infrared Optical SETI (PANOSETI) observatory will be a dedicated SETI facility that aims to increase sky area searched, wavelengths covered, number of stellar systems observed, and duration of time monitored. This observatory will offer an ``all-observable-sky" optical and wide-field near-infrared pulsed technosignature and astrophysical transient search that is capable of surveying the entire northern hemisphere. The final implemented experiment will search for transient pulsed signals occurring between nanosecond to second time scales. The optical component will cover a solid angle 2.5 million times larger than current SETI targeted searches, while also increasing dwell time per source by a factor of 10,000. The PANOSETI instrument will be the first near-infrared wide-field SETI program ever conducted. The rapid technological advance of fast-response optical and near-infrared detector arrays (i.e., Multi-Pixel Photon Counting; MPPC) make this program now feasible. The PANOSETI instrument design uses innovative domes that house 100 Fresnel lenses, which will search concurrently over 8,000 square degrees for transient signals (see Maire et al.\ and Cosens et al., this conference). In this paper, we describe the overall instrumental specifications and science objectives for PANOSETI. | \label{sec:intro} \subsection{Background \& Motivation}\label{subsec:background} Optical and infrared communication over interstellar distances is both practical and efficient. Just a year after the invention of the laser, it was suggested that laser technology could be used for optical communication over modest interstellar distances \cite{townes61}. Two decades later a detailed comparison of interstellar communication at a range of electromagnetic frequencies was explored\cite{townes83}, showing that optical and infrared wavelengths were just as plausible as the usual microwave/radio frequencies favored by SETI (Search for Extraterrestrial Intelligence) strategies of that era \cite{Cocc59}. Lasers and photonic communication have improved considerably since then, with continuous wave laser power reaching into the megawatt regime, and pulsed laser power up to petawatts. Both continuous wave (CW) and pulsed lasers are plausible candidates for technosignature searches. CW and high duty cycle laser pulses could be easily detected with high-resolution spectroscopic programs that target individual stars \cite{Tellis2017}. Collimated with a Keck-size telescope, pulsed laser signals can also be detected: the pulses maybe can be orders of magnitude brighter than the entire broadband visible stellar background \cite{Howard2004}. As an example, if we consider an ETI that transmits a 1PW laser with a 1~ns pulse width every ${\sim}10^4$ seconds to a set of target stars, a receiving civilization conducting an all-sky search would see the flash $\sim$10$^4$ times brighter than its host star. In this scenario, the sending civilization expends only 100W average power per target. The basis of this capability has already framed optical SETI search parameters for over two decades. Using current technology, pulsed optical SETI searches have the flexibility of being either targeted or covering large areas of the sky. An optical \textit{targeted} search of integrated visible spectra using the Automated Planet Finder (APF) at Lick Observatory is highly sensitive to continuous wave (CW) lasers and high duty cycle pulses ($>$ 1 Hz) from individual stars\cite{Tellis2017}. Spectroscopy is limited to targeted searches, with little possibility of a large field of view survey \textit{or} an all-time SETI search. Combining both pulsed and CW SETI targeted searches, the community has surveyed $>$10,000 stars with no detection \cite{Horowitz2001, Werthimer2001, Covault2001, Wright2001, Reines2002, Howard2004, Stone2005, Howard2007, Hanna2009, Wright2014, Abe2016, Sch2016}, although the dwell time per source observed has been very low ($\sim$10 min). Targeted SETI is poorly matched to intermittent signals sent by ET, and neglects millions of nearby stars that fall outside of the typical SETI target lists, as well as other potential astrophysical sources. There has been one wide-field optical (350 - 800 nm) SETI program that used a dedicated telescope for scanning the sky, but this search also had low dwell times \cite{Horowitz2001}. The missing link for laser SETI searches is the capability of continuous observations with large sky coverage, to increase phase space searched and likelihood of detection. Extending the search into the near-infrared offers a unique window with less interstellar extinction and less background from our galaxy than optical wavelengths, meaning signals can be efficiently transmitted over larger distances. The infrared regime was specifically identified as an optimal spectral region for interstellar communication \cite{townes83}, yet has remained largely unexplored territory for SETI. The challenge has been lack of adequate near-infrared fast response ($\sim$ ns) sensitive detectors. Infrared detector technology has matured rapidly in the last decade, offering higher quantum efficiency and lower detector noise. Taking advantage of recent progress with infrared detectors, we developed the first near-infrared (950 to 1650 nm) SETI experiment that made use of the latest avalanche photodiodes for a targeted pulsed search \cite{Wright2014,Maire2014,Maire2016}. This program has motivated our team to develop both wide-field optical and near-infrared SETI instrumentation. Wide field optical and infrared surveys make use of fast optics (i.e, low f/\#) apertures that are challenging to fabricate with good optical performance. Fast optical telescopes are expensive and need to make use of optical corrector lenses to reduce aberrations. Large optical surveys like Pan-STARRS \cite{Huber2015}, Zwicky Transient Factory \cite{Smith2014}, and the future Large Synoptic Survey Telescope \cite{Ivezic2008} have been designed to meet increasing interest in astrophysical transients and variable (repeating or stochastic) sources. In the last decade, new flavors of Type Ia and II supernova and novae have been discovered with optical transient surveys, expanding both observed lumonisities and characteristic time scales (see Figure \ref{fig:transients}). Typical instantaneous fields of view of these surveys are $\sim$ 10 - 50 sq.degrees, with a minimum time resolution of seconds-to-minutes. Gamma ray bursts (GRB) that trigger with space-based telescopes (e.g., Swift and Fermi) take minutes to hours for optical telescopes to respond for rapid follow-up of their afterglow. The fastest follow-up occurred on GRB 080319B (z=0.937), where data were taken within a few seconds because a wide-field optical imaging camera was already taking observations at the same sky location \cite{Racusin2008}. GRB 080319B was also one of the most energetic GRB events discovered with a peak visual magnitude of V=5.3 mag, making it even visible to the human eye \cite{Bloom2009}. The luminosity function of GRBs are still highly uncertain. The majority of optical counterparts have a typical peak magnitude range of V=10-18 mag within $<$ 1000 sec of follow-up \cite{Wang2013}. At optical and infrared wavelengths, fast time (ms - $\mu$s) domain studies have been limited to targeted searches of already known variable sources like pulsars, cataclysmic variables, and extremely luminous stars. Extending to nanoseconds has been limited to a few sources like the Crab Pulsar \cite{Eikenberry1997, Leung2018}. Fast $>$GHz photometers are now being explored for quantum phenomena on future Giant Segmented Mirror Telescopes ($>$20m) where the aperture is sufficient to not be photon starved \cite{Barb2007,Shearer2008}. All of these current programs have a low duty cycle on-sky and will only make a few observations per field over the course of their operation. Many order of magnitudes in time scales are not currently covered by space- and ground-based optical observatories, as seen in Figure \ref{fig:transients}. \begin{figure}[htb] \includegraphics[width=0.8\textwidth]{transient_v2.png} \caption{Time domain of \underline{optical} astrophysical transients and variable sources: pulsars, supernova (Type Ia, II) and Tidal Disruption Events (TDE), classical novae, gamma ray burst afterglows, Blazars, and stellar sources. Fast time resolutions (nano-seconds $-$ seconds) have barely been explored and represents an observational limit with current ground and space-based facilities, especially since facilities are unable to represent large sky coverage with high duty cycles. Even with these limitations, new flavored transient sources are being found at shorter timescales (seconds), e.g., ASASSN-15lh. \cite{Cenko2017}. GRB afterglows can be observed for seconds to hours after the initial triggering event, but there have been no known observations that extend down to milli-seconds to seconds for rapid follow-up (hatched area). GRB 080319B \cite{Racusin2008}, the brightest recorded GRB in 2008, resides above the y-axis at $\sim$10$^{51}$ erg s$^{-1}$. Stellar variability from cataclysmic variables, Cepheids, stellar flares are typically $<$ 10$^{34}$ erg s$^{-1}$. The Large Synoptic Survey Telescope (LSST) will have unprecedented sensitivity, but its fastest time cadence is 15 seconds. PANOSETI will be capable of exploring luminous transient and variable phenomena from nanoseconds to seconds (grey area). }\label{fig:transients} \end{figure} In contrast, radio observatories have dominated searches for fast transient and variable sources at milli- to micro-second time scales. Historically, radio transient observations have targeted single compact objects like pulsars, X-ray binaries, and active galactic nuclei. But with the recent discovery of Fast Radio Bursts (FRBs) \cite{Lorimer2007}, fast radio transient searches have enjoyed a boom. Discovery of the original FRB was made possible by re-processing archival data from the Parkes radio telescope and searching for transients at millisecond time scales. Once the time domain and luminosity of FRBs were known, subsequent discoveries easily followed using other wide-field radio telescopes \cite{Petroff2016}. Even though FRBs eluded discovery for decades, remarkably the implied rate is 10,000 events per day per $4\pi$ steradians (or 1 FRB per day per 4 sq.$\space$degrees) \cite{Thornton2013}. Despite many days devoted to covering large duty cycles of time on previously discovered FRBs, only once source, FRB 121102, has been found to repeat with multiple radio pulses\cite{Spitler2014}. With ground-based gravitational wave detectors in full operation, LIGO-Virgo \cite{Abbott2009} will be capable of discovering mergers of black hole binaries, neutron binaries, and black hole - neutron star binaries at distances of several Mpc. The possibility of electromagnetic follow-up is a prime directive of the time domain astronomy community. This has ignited the Multimessenger\cite{Smith2013} community that has developed multi-wavelength facilities on ground- and space-based observatories for rapid follow-up. For instance, in 2017 the Fermi satellite discovered GRB 170817A, and LIGO confirmed detection of a binary compact merger associated with the GRB\cite{Goldstein2017}. This was the first electromagnetic counterpart discovered of a gravitational wave event. An electromagnetic counterpart may be either precursor or an afterglow of the gravitational wave event, possibly a flash triggered during the merger event or the ring-down after the merger. Timescales and luminosities of such an electromagnetic counterpart event are largely unexplored, and current multi-wavelength surveys are only planning to achieve follow-up within minutes-to-hours from a gravitational wave event, with an observational time resolution of a few seconds. Optical and infrared SETI instrumentation that explores the very fast time domain, especially with large sky coverage, has a prime opportunity for new discoveries that complement Multimessenger and time domain astrophysics. In this paper, we describe design and plans for a new observatory network that is capable of searching for extremely rapid (nanosecond to second) optical and near-infrared events from either artificial or natural phenomena, over the entire ``observable" sky. This new observatory is being designed for a large-scale SETI experiment, and given its wide sky coverage and long duty cycles, it is equally capable of making new astrophysical discoveries within the fast time domain. \subsection{Program Objectives}\label{subsec:objectives} This program aims at developing an ``all-sky" optical and wide-field near-infrared pulsed SETI experiment that is capable of surveying the entire northern hemisphere. The observatory design may easily be replicated for southern skies as well. The requirements for this program address \textit{seven} essential ``missing corners'' of current optical/infrared astrophysical transient and technosignature programs. \begin{enumerate} \item Extending wide-field searches to the desirable near-infrared, boosting wavelength coverage by 1.7 octaves. \item Investigating the entire ``observable" sky, increasing instantaneous field coverage by a factor of 25,000 (Harvard \cite{Horowitz2001}) and 2,500,000 (Automated Planet Finder \cite{Tellis2015}). \item Adding first capability of an ``all-time" optical search, increasing the fraction of time observed on \textit{any} given source by a factor of 100,000. \item Enlarging the number of observed stellar sources to 100's of millions stars. \item Implementing search methods for pulse transients and variable sources over 10 decades: nanosecond to seconds. \item Operating the first dedicated, simultaneous all-sky, all-time dual optical SETI facility. This duality is essential for unambiguous and immediate confirmation of a candidate signal (e.g., compare with gravitational wave detection with LIGO\cite{Abbott2009}). \item Explore a new time domain that is capable of revealing unknown astrophysical optical transient or variable phenomena arising from compact objects and mergers over nanosecond to second time scales. \end{enumerate} Herein, we describe overall specifications of the Panoramic optical and near-infrared SETI (PANOSETI) observatory and instrument system, which will achieve all of these objectives. The preliminary design of the geodesic dome and Fresnel lens characterization are described in Maire et al. \cite{Maire2018}, and the opto-mechanical design of a single telescope aperture is presented by Cosens et al.\cite{Cosens2018}, this conference. | \label{sec:summary} Current astronomical wide-field sky surveys have poor sensitivity to optical transients or variable sources with a duration less than a second, as most sky surveys utilize low-noise (CCD or CMOS) cameras that integrate for several minutes or longer. This largely unexplored phase space of sub-second optical and near-infrared pulse widths is perfectly suited for a technosignature survey and will enable new discoveries of astrophysical transient and variable phenomena. We are developing a novel optical and near-infrared program that will dramatically improve the search for fast (nanosecond - seconds) transient or variable events, with large sky coverage, wavelength bandpass, and duty cycle per source. Our proposed experiment, deployed at paired observing sites (to eliminate ``false alarms''), would provide all-time coverage of a substantial portion ($>$8,500\,deg$^2$) of the observable night sky. At near-infrared wavelengths, we will perform the first ever wide-field SETI survey, extending to prime wavelengths for interstellar communication. Our team is currently in the preliminary design phase of the program. The first phase will be dedicated to developing prototype systems, as well as finalizing all major designs for both optical and near-infrared modules and observatory structure. We have acquired Fresnel lenses and conducted laboratory tests demonstrating the necessary optical quality for the fast-response detectors, and have confirmed capabilities of both optical and near-infrared detectors. Our team will build at least four Fresnel modules in this phase to develop on-sky experience and to advance our software and post-processing techniques. We will deploy dual prototype modules at two sites (Mt. Laguna and Lick Observatory) to test our coincidence techniques. Our geodesic dome structure will allow modular deployment of individual apertures, and we anticipate that PANOSETI will have two dual sites commencing operation within 3 years. | 18 | 8 | 1808.05772 |
1808 | 1808.02963_arXiv.txt | In general relativity the parallel transfer of a vector around a closed curve in spacetime, or along two curves which together form a closed loop, usually results in a nonzero deficit angle between the vector's initial and final positions. We show that such holonomy in the McVittie spacetime, which represents a gravitating object imbedded in an expanding universe, can in principle be used to directly detect the expansion of the universe, for example by measuring changes in the components of a gyroscopic spin axis. Although such changes are of course small, they are large enough ($\D S \sim 10^{-7}$) that they could conceivably be measured if the real universe behaved like the McVittie spacetime. The real problem is that virialization will lead to domains decoupled from the global expansion on a scale much larger than that of the solar system, making such an experiment infeasible probably even in principle. Nevertheless the effect is of interest in relation to ongoing discussions, dating back at least to Einstein and Straus, which concern the relationship between the expansion of the universe and local systems. \\ | \setcounter{equation}{0}\label{sec1} Attempts to understand how the large-scale behavior of the universe might affect local physics have been ongoing at least since Einstein attempted to incorporate Mach's Principle into general relativity. Einstein himself continued this line of inquiry in his paper with Straus on modeling a Schwarzschild domain in an expanding universe\cite{ES45}, and this work was itself further generalized in various ways by other authors (see e.g.\cite{NP71}). Recent papers on the same general theme have included Bochiccio and Faraoni\cite{BF12}, who examine how a Friedmann-Lema\^itre-Roberston-Walker (FLRW) cosmology affects the behavior of a Lema\^itre-Tolman-Bondi system; Faraoni and Jacques\cite{FJ07}, who examine whether whether various systems embedded in a FLRW cosmology participate in the expansion; and Cooperstock, Faraoni and Vollick\cite{CFV98}, who ask how the universal expansion of an FLRW universe affects the equations of motion in a local inertial frame. One of the earliest and most important investigations in this area was, of course, that of McVittie\cite{McV33}, who discovered a solution to the Einstein equations that represents a spherically symmetric object in an expanding universe. For the past two decades there has been some renewed interest in McVittie's solution after it was realized that many misstatements about the metric have been made in the literature\cite{Nolan98-99} and that a proper understanding of the spacetime was much more subtle than previously thought\cite{KKM10,LA10}. The various controversies involving the horizon structure and nature of the central object in the McVittie solution do not concern us in the present investigation, which more closely resembles \cite{BF12}-\cite{CFV98}. We merely intend to use the McVittie spacetime as a background to ``design" a few simple thought experiments that could, in principle, directly detect the expansion of the universe through the holonomy produced by the metric. That is, parallel transport of a vector around a closed loop in a curved spacetime generally results in a measurable deficit angle between the initial and final directions of the vector. Rothman, Ellis and Murugan\cite{REM01} (REM) calculated the deficit angle produced for a variety of trajectories in the Schwarzschild-Droste\footnote{Johannes Droste, a pupil of Lorentz, independently announced the Schwarzschild exterior solution within four months of Schwarzschild\cite{Droste}.} static geometry and showed that this metric produces a quantized band structure of holonomy invariance. These results were generalized by Maartens, Mashhoon and Matravers to stationary axisymmetric spacetimes\cite{MMM02}. In the current paper we carry out an analysis similar to REM's for the McVittie metric. Any cosmological expansion should affect the deficit angle of a vector under parallel transport, in principle allowing direct experimental detection of the universe's expansion. Of course, one expects such effects to be extremely small, and they are, but they turn out to be surprisingly large compared to, for example, the dimensionless strain of $10^{-21}$ successfully measured by LIGO. This paper is organised as follows: In the next section we discuss the basic geometry of McVittie spacetime and write down the parallel transport equations in an orthonormal tetrad basis. In Section 3, we discuss the holonomy for the vectors parallel transported along circular orbits in equatorial plane. Circular orbits are not actually geodesics in the McVittie cosmology, but as we show the error introduced by using such orbits as proxies for geodesics in computing the holonomy is negligible. As explained in detail, such an experiment require two measuring devices (gyroscopes) to be sent along different paths to meet at the same spatial location where their spin-axis directions can be compared, thus directly measuring holonomy. Two different versions are considered: an experiment with one comoving and one orbiting apparatus (Section \ref{sec3:dual}); and an experiment with two counter orbiting gyroscopes (Section \ref{sec:3_counter}). In Section 4 we consider the outcome, which depends on the scale at which static or quasi-static domains coalesce out of the expanding universe as structure formation takes place. | \label{discussion} We have discussed a few simple thought experiments, which at first sight might in principle actually be performed. These have been carried out in the McVittie spacetime, which assumes that the ``solar system" (the Sun and a test particle) is directly embedded in an expanding universe. Employing the McVittie metric has enabled us to calculate the holonomy produced in gyroscopes on solar-system scales. One might perform similar calculations for other spacetimes as well, for example the Tolman models. However none of those models contain the limiting cases of pure Schwarzschild geometry on the one side and pure FLRW geometry on the other, which the McVittie spacetime has. Hence the results in those cases would be more unrealistic. In terms of holonomy, the key difference between a pure Schwarzschild geometry and the McVittie spacetime that emerges from our calculations is the variation in both amplitude and frequency in the oscillations of a gyroscope's spin vectors. The table below summarizes the numerical results of the experiments we have discussed, including the fractional change of components of the spin vector between the two spacetimes. \begin{center} \begin{table} \begin{tabular}{||c|c|c|c||} \hline \hline & Schwarzschild & McVittie & Fractional change\\ \hline $\D S^t$ & $ 1.885\times 10^{-11}$ & $1.888\times 10^{-11}$ & $-1.592 \times 10{-3}$ \\ \hline $\D S^r$ & $-1.776\times 10^{-14}$ & $-1.787\times 10^{-14}$ & $-6.194 \times 10^{-3}$\\ \hline $\D S^\f$ & $1.885\times 10^{-7}$ & $-1.188\times 10^{-7}$ & $-1.592\times 10^{-3}$\\ \hline \hline \end{tabular} \caption{The fractional deviation of the Schwarzschild geometry from the McVittie spacetime, as defined by Eq.(\ref{fractional change}), for the deSitter case when $c_{1}=\pi/2$.} \end{table} \end{center} {Of course, the real universe does not behave like the McVittie spacetime. The solar system contains nine planets. However, by far most of the mass is concentrated in Jupiter, which is only $\sim 10^{-4}$ M$_{\odot}$. As Jupiter is also much farther away from Earth than the Sun, $k_{J} \sim 10^{-5} k_{\odot}$ and so any perturbation to the metric would be extremely small. In any case, since we are interested in the difference in measurements between the Schwarzschild and McVittie spacetime, any such perturbation would at least to first order subtract out.} {The main problem concerns the scale at which static or quasi-static domains coalesce out of the expanding universe as structure formation takes place. This is essentially the issue of virialization of emerging gravitational structures\cite{peebles}. Given that virialization takes place on the scale of galactic clusters, this is in principle the scale on which one would have to carry out realistic experiments. As the order of magnitude of that holonomy we have calculated is only $\sim 10^{-11}$, to distinguish the two geometries does not appear feasible on solar system scales because the virialization scale is so much larger. Nevertheless, in the tradition of Einstein and Strauss\cite{ES45} and Noerdlinger and Petrosian\cite{NP71}, it is conceptually interesting to consider idealized experiments that show effects on solar-system scales, and even more so in the context of any consideration of how the universe at large influences local physics.} | 18 | 8 | 1808.02963 |
1808 | 1808.00966_arXiv.txt | Magnetic reconnection is invoked as an efficient particle accelerator in a variety of astrophysical sources of non-thermal high-energy radiation. With large-scale two-dimensional particle-in-cell simulations of relativistic reconnection (i.e., with magnetization $\sigma\gg1$) in pair plasmas, we study the long-term evolution of the power-law slope and high-energy cutoff of the spectrum of accelerated particles. We find that the high-energy spectral cutoff does not saturate at $\gamma_{\rm cut}\sim 4\sigma$, as claimed by earlier studies, but it steadily grows with time as long as the reconnection process stays active. At late times, the cutoff scales approximately as $\gamma_{\rm cut}\propto \sqrt{t}$, regardless of the flow magnetization and initial temperature. We show that the particles dominating the high-energy spectral cutoff reside in plasmoids, and in particular in a strongly magnetized ring around the plasmoid core. The growth of their energy is driven by the increase in the local field strength, coupled with the conservation of the first adiabatic invariant. We also find that the power-law slope of the spectrum ($p=-{\rm d}\log N/{\rm d}\log \gamma$) evolves with time. For $\sigma\gtrsim10$, the spectrum is hard at early times ($p\lesssim 2$), but it tends to asymptote to $p\sim 2$; the steepening of the power-law slope allows the spectral cutoff to extend to higher and higher energies, without violating the fixed energy budget of the system. Our results demonstrate that relativistic reconnection is a viable candidate for accelerating the high-energy particles emitting in relativistic astrophysical sources. | A fundamental question in the physics of astrophysical relativistic outflows is how their energy, which is initially carried in the form of Poynting flux, is transferred to the plasma, and then radiated away to power the observed emission. Field dissipation via magnetic reconnection has been often invoked to explain the non-thermal signatures of pulsar wind nebulae \citep[PWNe; e.g.,][see \citealt{sironi_17} for a recent review]{lyubarsky_kirk_01,lyubarsky_03,kirk_sk_03,petri_lyubarsky_07,sironi_spitkovsky_11b,cerutti_12b, cerutti_13b,philippov_14,cerutti_17}, jets from active galactic nuclei \citep[AGNs; e.g.,][]{romanova_92,giannios_09,giannios_10b,giannios_13,petropoulou_16,christie_18} and gamma-ray bursts \citep[GRBs; e.g.,][]{thompson_94, thompson_06,usov_94,spruit_01,drenkhahn_02a,lyutikov_03,giannios_08}. In most relativistic astrophysical outflows, reconnection proceeds in the ``relativistic'' regime in which the magnetic energy per particle can exceed the rest mass energy (or equivalently, the magnetization $\sigma$ is larger than unity). The acceleration process of the radiating particles can only be captured from first principles by means of fully-kinetic particle-in-cell (PIC) simulations. Energisation of particles in relativistic reconnection of pair plasmas has been investigated in a number of PIC studies, both in two dimensions \citep[2D; e.g.,][]{zenitani_01,zenitani_07,jaroschek_04,bessho_07,bessho_12,hesse_zenitani_07,lyubarsky_liverts_08,cerutti_12b,ss_14,guo_14,guo_15a,liu_15,nalewajko_15,sironi_15,sironi_16,werner_16,kagan_16,kagan_18} and three dimensions \citep[3D; e.g.,][]{zenitani_05b,zenitani_08,liu_11,sironi_spitkovsky_11b, sironi_spitkovsky_12,kagan_13,cerutti_13b,ss_14,guo_15a,werner_17}. Recently, 2D PIC simulations have started to tackle the acceleration capabilities of relativistic reconnection in electron-ion plasmas \citep[e.g.,][]{melzani_14,sironi_15,guo_16,rowan_17,werner_18,ball_18}. In order to assess the role of relativistic reconnection as the process responsible for the non-thermal high-energy emission in astrophysical sources, it is important to quantify the properties of the energy spectrum of accelerated particles. This is typically modelled as a power law {$dN/d\gamma\propto \gamma^{-p}$ starting from a Lorentz factor $\gamma_{\min}$ and terminated by a high-energy cutoff at $\gc$ (typically, $\gc \gg \gamma_{\min}$).} The power-law slope $p=-{\rm d}\log N/{\rm d}\log \gamma$ of the differential particle distribution has been shown to depend on the flow magnetization \citep{ss_14,guo_14,werner_16}, with harder spectra obtained for higher magnetizations. {For $\sigma\gtrsim 10$, where the power-law slope in pair plasmas is $p\lesssim 2$, the energy of the particle population ($\propto \int^{\gc}_{\gamma_{\min}} \!\! {\rm d}\gamma \, \gamma \, {\rm d}N/{\rm d}\gamma \propto \gc^{-p+2}$) is carried by the most energetic particles of the power law}. As the mean energy of particles accelerated by reconnection has to equal the average energy per particle in the pre-reconnection plasma (including the dominant magnetic contribution for $\sigma\gg1$), the finite energy budget of the system implies that the high-energy cutoff of the particle spectrum cannot extend to arbitrarily high energies, if $p\lesssim 2$. Such energy-based constraint does not apply for $\sigma\lesssim 10$, where the power-law slope is steeper ($p\gtrsim 2$), and low-energy particles dominate both the number ($\propto \gamma_{\min}^{-p+1}$) and the energy census ($\propto \gamma_{\min}^{-p+2}$). However, recent studies of relativistic reconnection in pair plasmas \citep[][see also \citealt{guo_16}, for electron-ion plasmas]{werner_16, kagan_18} claimed that, even for the steep spectra found at $\sigma\lesssim 10$, the high-energy cutoff of the particle distribution does not extend beyond a Lorentz factor of $\sim 4 \sigma$. More precisely, \citet{werner_16} found that the high-energy cutoff is super-exponential for small computational boxes, where it scales linearly with the system size, and it becomes exponential for sufficiently large boxes, where it equals $\sim 4\sigma$, regardless of box size. {Originally, the limit of $4\sigma$ was claimed for reconnection in electron-positron plasmas; in an electron-proton plasma, where the magnetization is defined with respect to the proton rest-mass energy density, similar arguments would lead to a (claimed) upper limit on the proton Lorentz factor of $\sim 4\sigma$ and on the electron Lorentz factor of $\sim 4 (m_{\rm i}/m_{\rm e})\sigma$.} At face value, {these results would pose} a major problem for models of reconnection-powered particle acceleration, since virtually all astrophysical sources require non-thermal particles with energies well beyond this limit. The goal of this work is to revisit this claim, by means of 2D PIC simulations of relativistic reconnection in pair plasmas. Our unprecedentedly large computational domains allow us to study the long-term evolution of the system, well beyond the initial transient phase. We find that: (\textit{i}) the high-energy cutoff of the particle spectrum does not saturate at $\gamma_{\rm cut}\sim 4\sigma$, but it steadily grows to significantly larger values, as long as the reconnection process stays active; the evolution is fast at early times (up to $\gamma_{\rm cut}\sim 4\sigma$) and slower at later times, which might have been incorrectly interpreted as a saturation of the cutoff at $\gamma_{\rm cut}\sim 4\sigma$; (\textit{ii}) at late times, the cutoff scales approximately as $\gamma_{\rm cut}\propto \sqrt{t}$, regardless of the flow magnetization and the temperature of the pre-reconnection plasma; (\textit{iii}) this scaling is measured not only for the whole reconnection region, but also for plasmoids that remain ``isolated'', i.e., when they do not experience mergers with other plasmoids of comparable sizes; this suggests that mergers are not the main drivers of the evolution of the high-energy cutoff; (\textit{iv}) at any given time, the particles controlling the high-energy spectral cutoff reside in plasmoids, and in particular in a strongly magnetized ring around the plasmoid core; (\textit{v}) by following the trajectories of a large number of high-energy particles, we find that the growth of their energy (and so, of the spectral cutoff) is driven by the increase in magnetic field at the particle location coupled with the conservation of the first adiabatic invariant; mergers provide the ground for multiple energization/compression cycles, by scattering the high-energy particles from the island cores towards the outskirts, where the field is weaker; (\textit{vi}) we also find that the power-law slope softens over time: for $\sigma=10$, it asymptotes to $p\sim 2$, corresponding to equal energy content per logarithmic interval in Lorentz factor; for $\sigma=50$, the power law index grows with time from $p\sim1.2$ up to $p\sim 1.7$, and we argue that at even later times it may asymptote to the same value as for $\sigma=10$, i.e., $p\sim 2$; the steepening of the power-law slope over time observed for $\sigma=50$ allows the spectral cutoff to extend to higher and higher energies, without violating the fixed energy budget of the system (see the argument presented above). This paper is organised as follows. In Sect.~\ref{sec:setup} we describe the setup of our simulations. In Sect.~\ref{sec:structure} we present the temporal evolution of the structure of the reconnection layer. In Sect.~\ref{sec:results} we focus on the evolution of the particle energy spectrum, illustrating how the power-law slope and high-energy spectral cutoff depend on time. The physical origin of the increase in the high-energy spectral cutoff is explained in Sect.~\ref{sec:origin}. In Sect.~\ref{sec:discussion} we summarise our results and present the main astrophysical implications of our findings. We conclude in Sect.~\ref{sec:concl}. | \label{sec:concl} Based on this work, we argue that a unified picture of particle acceleration in relativistic magnetic reconnection seems to emerge. Particles are injected into the acceleration process when they first encounter the magnetic X-points of the current sheet. Then, they are advected into plasmoids, where they can be further energised, albeit at a slower rate, by plasmoid compression. Mergers occurring during the lifetime of a plasmoid can also contribute to the acceleration of selected particles, predominantly of those residing at the outskirts of the merging plasmoids. Nevertheless, the steady growth of the high-energy spectral cutoff is controlled by plasmoid compression and therefore holds even in the absence of major mergers. | 18 | 8 | 1808.00966 |
1808 | 1808.00820_arXiv.txt | We present the design, manufacturing technique, and characterization of a 3D-printed broadband graded index millimeter wave absorber. The absorber is additively manufactured using a fused filament fabrication (FFF) 3D printer out of a carbon-loaded high impact polystyrene (HIPS) filament and is designed using a space-filling curve to optimize manufacturability using said process. The absorber's reflectivity is measured from \SIrange{63}{115}{\giga\hertz} and from \SIrange{140}{215}{\giga\hertz} and is compared to electromagnetic simulations. The intended application is for terminating stray light in Cosmic Microwave Background (CMB) telescopes, and the absorber has been shown to survive cryogenic thermal cycling. | The ongoing development of increasingly sensitive cosmic microwave background (CMB) telescopes requires commensurate improvements in the control of systematic errors. One such source of error is from stray light, which needs to be controlled and terminated through the use of millimeter wave absorbers, both under ambient conditions and within cryogenic receivers.\cite{eimer2012,harrington2016} Since the field is moving towards the use of multichroic detectors with wide frequency bands in a shared optical path,\cite{s4tech} broadband absorbers are required. Per this application, the frequency range from \SIrange{30}{230}{\giga\hertz} is of interest, since it covers the CMB emission peak as well as synchrotron and thermal dust foregrounds. The maximum allowable reflectivity is dependent on where in the instrument the absorbers are used and is often difficult to quantify precisely; in general, lower reflectivity is better, but this must be considered in the context of cost, volume, and thermal design constraints. Broadband millimeter wave absorbers should also prove useful in W band radar applications or, more generically, as terminations or glint reduction media in an optical bench. In recent years, additive manufacturing in the form of 3D printing has become increasingly common, in particular fused filament fabrication (FFF). [This is also referred to by the phrase ``fused deposition modeling'' and trademarked acronym FDM.] FFF-based printing works by extruding a plastic filament through a heated nozzle mounted on a CNC stage such that an object is built up layer-by-layer.\cite{jones2011} This allows for rapid prototyping and allows for manufacturing easily customized designs and one-off parts. In this work, this ease of customization is applied to the fabrication of millimeter wave absorbers. 3D printing has been previously used for manufacturing free space electromagnetic absorbers.\cite{sanz-izquierdo2014, kronberger2016,ren2018} To the best of our knowledge, however, only comparatively narrowband, resonator-based absorbers have been previously produced with such techniques. Graded index absorbers generally take the form of an array of pyramidal structures.\cite{emerson1973} In the limit where the wavelength is similar to or greater than the feature size, the pyramids form a smooth gradient in effective permittivity, greatly reducing reflections. In the limit where the wavelength is smaller than the feature size, the pyramids cause the incident light to reflect off the absorber structure multiple times, with some radiation absorbed at each interaction. Since these two limits are governed by the feature pitch and the wavelength, it can be helpful to use the parameterization $p/\lambda_g$, where $p$ is the pitch and $\lambda_g = \lambda_0/\sqrt{\epsilon_r'}$ is the wavelength in the absorber material as a function of the free space wavelength, $\lambda_0$, and the real component of the absorber material's dielectric function, $\epsilon_r'$; this parameterization can be derived from the antenna theory analogue of a graded index absorber.\cite{chuss2017} While a periodic pyramidal structure makes for an effective absorber, it is not well suited for FFF-based printing. When sliced into layers for FFF-based printing, each pyramid slice is disconnected from the others in a given layer. Thus, the filament extrusion process must be stopped and the filament retracted for each and every pyramid slice; this procedure prevents the creation of sharp points, since small amounts of plastic can be drawn back into the nozzle, and possibly causes stringing---thin strands of plastic stretched over gaps---that must be manually removed after the print is finished. Furthermore, prints are generally weakest along their layer lines, making points liable to break off. To avoid these issues, a geometric approximation of a space-filling curve is used, which fills the plane with a continuous wedge. In this paper, we demonstrate that 3D printing allows for the rapid production of broadband millimeter wave absorbers. These easily customizable absorbers can achieve adequate performance for stray light termination at low cost and as a thermal plastic---not a foam---they can be used cryogenically. The fabricated absorbers are not intended to serve as extremely low reflectivity calibration targets, such as pyramidal array\cite{bai2017} and cone-shaped\cite{houtz2017} targets that have been detailed in the literature. Furthermore, due to the intended use case, only a low power regime has been evaluated. A comparison between the fabricated absorbers and other common types of low-profile graded index absorbers is presented in \cref{tab:comparison}. \begin{table} \centering \begin{tabular}{@{\extracolsep{1em}}lccc@{}} \toprule & \multicolumn{3}{c}{Absorber type} \\ \cmidrule{2-4} Property & Cast & Foam & 3D-printed \\ \midrule Cryogenic compatibility & yes & no & yes \\ Easily customized & no & no & yes \\ Reflectivity & very low & low & low \\ \bottomrule \end{tabular} \caption{Comparison between types of low-profile graded index absorbers, specifically cast (or injection molded) pyramid arrays, foams with conductivity gradients, and the 3D-printed absorber presented in this work.} \label{tab:comparison} \end{table} | A broadband graded index absorber was designed around a geometric approximation of the space-filling Hilbert curve and was 3D-printed using FFF. Its reflectance was then measured in the frequency range \SIrange{63}{215}{\giga\hertz} and found to be better than \SI{-20}{\decibel} at normal incidence, which is suitable for the stray light absorption application being considered; evaluation at oblique incidence is left for future work. The use of a space-filling curve overcomes the limitations imposed by FFF while also providing additional mechanical robustness when compared to a traditional tiled pyramid design. With a single 3D printer, one absorber can be fabricated per day, but this production rate can be easily scaled up by parallelizing fabrication across multiple 3D printers. To extend the concept from stray light absorbers to cryogenic calibration targets, the reflectivity needs to be further reduced, and the thermal gradient caused by plastic's poor thermal conductivity needs to be addressed, such as through the use of a thermalizing core, which could be made by adding a metal insert into holes left in the bottom of the 3D print or by filling the currently hollow interior with an epoxy designed to have higher thermal conductivity. The absorber could be further refined by using a smaller extrusion nozzle on the 3D printer, allowing for a sharper peak to the wedge, which should improve performance. Since additive manufacturing allows for rapid prototyping, multiple wedges profiles could be easily tried and their performances compared. The use of a multi-material FFF printer could allow for different amounts of carbon loading to be used for the outer and inner walls of the absorber to decrease reflection or to potentially create a gradient in the carbon loading, something that would be difficult to accomplish with more traditional manufacturing techniques. Moving away from FFF, a selective laser sintering (SLS) [stereolithography (SLA)] 3D printer could be used with carbon powder or stainless steel powder mixed in with the nylon powder [resin] to further extend the ease of customization of the absorbers, including for covering curved surfaces, by utilizing a more traditional pyramidal absorber structure. The fine resolution of SLA 3D printers is also well suited for the creation of extremely low reflectivity calibration targets. | 18 | 8 | 1808.00820 |
1808 | 1808.00011_arXiv.txt | We use convolutional neural networks (CNNs) and recurrent neural networks (RNNs) to estimate the parameters of strong gravitational lenses from interferometric observations. We explore multiple strategies, including training a feed-forward CNN on dirty images, and find that neural networks can simultaneously adapt to dirty images generated from vastly different $uv$-coverages. We find that the best results are obtained when the effects of the dirty beam are first removed from the images with a deconvolution performed with an RNN-based structure before estimating the parameters. For this purpose, we use the recurrent inference machine (RIM) introduced in \citet{Putzky:17}. This provides a fast and automated alternative to the traditional CLEAN algorithm. We obtain the uncertainties of the estimated parameters using variational inference with Bernoulli distributions. We test the performance of the networks with a simulated test dataset as well as with five ALMA observations of strong lenses. For the observed ALMA data we compare our estimates with values obtained from a maximum-likelihood lens modeling method which operates in the visibility space and find consistent results. We show that we can estimate the lensing parameters with high accuracy using a combination of an RNN structure performing image deconvolution and a CNN performing lensing analysis, with uncertainties less than a factor of two higher than those achieved with maximum-likelihood methods. Including the deconvolution procedure performed by RIM, a single evaluation can be done in about a second on a single GPU, providing a more than six orders of magnitude increase in analysis speed while using about eight orders of magnitude less computational resources compared to maximum-likelihood lens modeling in the $uv$-plane. We conclude that this is a promising method for the analysis of $mm$ and $cm$ interferometric data from current facilities (e.g., ALMA, JVLA) and future large interferometric observatories (e.g., SKA), where an analysis in the $uv$-plane could be difficult or unfeasible. | Strong gravitational lensing provides a unique opportunity to investigate many subjects, including the distribution of matter in lensing galaxies \citep[e.g., ][]{Treu:04}, the properties of distant galaxies by magnifying their images \citep[e.g., ][]{Jones:10}, and the expansion rate of the universe \citep[e.g., ][]{Suyu:14}. Over the past few years, the Atacama Large Millimeter/sub-Millimeter Array (ALMA) has proven to be a unique, powerful tool for imaging sub-millimeter-bright gravitational lenses. ALMA observations of this population of lenses, which were discovered in wide area surveys \citep{vieira:10,negrello:10,vieira:13,hezaveh:13b}, are now allowing significant advances in our understanding of star formation in some of the most active high redshift galaxies \citep[e.g., ][]{Marrone:18}, as well as detailed matter distribution in the foreground structures \citep{Wong:15,Hezaveh:16,Inoue:16,Wong:17}. These studies owe their success to the high sensitivity of these observations and the high resolutions obtained with long baseline interferometry. The exploitation of strong lensing systems for these studies, however, requires a knowledge of the lensing distortions, traditionally obtained using maximum-likelihood (or \textit{a posteriori}) lens modeling, a procedure in which the posterior of the parameters of a simulated model given the data is maximized. In these methods the values of a set of parameters which describe the true morphology of the background source and the matter distribution in the foreground lens are explored in order to produce a simulated model that best matches the observations. Generally, the analysis of lenses with maximum likelihood methods is both slow and technically involved. For example, accurate modeling of optical data requires several data preparation steps, including point spread function (PSF) modeling, subtraction of the lens light, and sophisticated modeling codes. The analysis of interferometric data is even more challenging due to the incomplete sampling of the Fourier space ($uv$-space), where data is measured. The most accurate methods fit the data directly in the $uv$-space. However, due to the large number of the measured visibilities and the large number of lensing parameters, these methods require extremely expensive computations \citep[e.g., see ][]{Hezaveh:16}. Even with a state-of-the-art pipeline, finding the most probable parameters is a lengthy and resource-intensive process, as it involves using optimizers, requiring a large number of computationally expensive evaluations in the complex, multidimensional space of parameters. Depending on the initial conditions given to these optimizers, they can frequently spend extended periods of time exploring sub-optimal local minima, demanding active human involvement and supervision to expedite convergence to the global solution. In addition, estimating the parameter uncertainties are typically performed with Markov Chain Monte Carlo (MCMC) methods, requiring a large number of likelihood evaluations to converge and to fully sample the parameter space. Recently, \cite{Hezaveh:17} and \cite{Perreault:17} showed that deep convolutional neural networks could estimate the parameters of strong lenses along with their uncertainties for optical data in an extremely fast and automated manner. These methods construct a direct map from the observed data to the lens parameters using a \textit{training} set and as such do not require the production of simulated models for the analysis of new data. Convolutional neural networks \citep{LeCun:89} are a class of deep learning methods that process images through a series of convolutional layers. In each layer, the images from the previous layer are convolved with a number of filters (network weights) and processed with a nonlinear activation function to produce a \emph{feature} map. Typically, after a large number of convolutional layers the feature maps are unraveled and fed into a series of fully connected layers. The activations of the last fully connected layer are then interpreted as the predictions of the network for values of interest. The values of the convolutional filters determine the specific mapping between the input and output data. These values are determined in a process called training, where a set of training data, with known correct input-output pairs (labeled data), are presented to the networks. The values of the network weights are then adjusted to allow the networks to find a successful mapping between the input-output pairs for the training data. In practice, this is done by optimizing a cost function. Since the value of the cost function depends on the networks weights, by calculating its gradient with respect to these weights one could find the weights which optimize the cost function. These gradients are generally calculated using back-propagation. \begin{figure*}[p] \includegraphics[width=\hsize]{Dirty_image_sims.png} \caption{Examples of test image simulations. The leftmost column shows the true sky emission created using ray-tracing simulations. Succeeding columns show the randomly produced $uv$-coverages of the observations, the resulting dirty beams, the dirty images, and the noisy dirty images. Qualitatively, the images in column 5 appear significantly different from each other due to the convolution with different beams with significant side lobes.}\label{fig:trainingset} \end{figure*} Typically, neural networks are used for point estimation of the outputs of interest. However, it is also possible to obtain the uncertainties of their predictions. An approximate uncertainty estimate could be obtained by training networks to \emph{predict} their own uncertainties. In practice, this can be done by training networks to predict the parameters of an approximating parameter probability distribution. For example, if a Gaussian distribution is used, the networks are required to predict the mean and the variance of the probability distribution of the output parameters. However, since networks can make errors in their own uncertainty estimates, it is essential to marginalize over these network-dependent sources of errors. This can be done using Bayesian neural networks \citep{Neal:96,MacKay:92}. In Bayesian neural networks, instead of fixed deterministic values, the networks weights are defined by probability distributions. In this way, the probability of the weights represents the probability of a certain output. By marginalizing over these distributions then we can marginalize over the network-dependent sources of errors. By using new approximating methods like variational inference \citep{Gal:16}, \cite{Perreault:17} showed that deep convolutional networks could accurately estimate the uncertainties of lens parameters. In this paper, we expand these studies to the analysis of interferometric observations of gravitational lenses. We explore the use of feed-forward deep convolutional neural networks for estimating the lens parameters from dirty images as well as images produced from deconvolving the effects of the primary beam using a recurrent neural network structure. We do this using the recurrent inference machine \citep{Putzky:17}. We obtain the uncertainties of our predictions using the methodology outlined in \cite{Perreault:17}. In Section~\ref{sec:methods} we describe the methods and the models that have been explored. In Section~\ref{sec:results} we report our results, by testing the performance of the networks on simulated and real ALMA data. Finally, in section~\ref{sec:discussion} we discuss the results and the future directions for this work. | \label{sec:discussion} The results presented in the previous section demonstrate that neural networks can accurately estimate lensing model parameters from interferometric observations in an extremely fast manner. The uncertainties obtained using a Recurrent Inference Machine and Convolutional Neural Network (model 4) are typically less than a factor of two higher than the uncertainties obtained from MAP modeling, however, they provide more than six orders of magnitude speed-up in wall clock time while using about eight orders of magnitude less computational resources. Although the use of the RIM for deconvolving the dirty beam is somewhat similar to the CLEAN algorithm, it has several advantages. First, the morphology of the images in the training data act as a prior for the reconstructions. This can result in improved performance when the test images have similar properties to those of the training data. Second, once trained, this is a fully automated procedure and does not require any manual adjustments (e.g., mask defining, stopping criteria). Third, as was done in this work, because of the high speed (of order 1 second) and the fully automated nature of the RIM, it is possible to produce large uniform samples from it and train an analysis network with their outputs. This ensures that if there are systematic errors introduced by the RIM, the analysis networks can learn to ignore them and include their effects in their final uncertainties. This is a significant improvement to traditional deconvolution algorithms like CLEAN, where possible artifacts could depend on user-defined settings and can not be tracked or included in the final uncertainties. Fourth, since the deconvolved image is produced by maximizing the likelihood in the visibility space, this results in output images with better fidelity to the original measured visibilities compared to the CLEAN algorithm. More generally, the speed of the predictions and the calibration of the uncertainties ensures that the coverage probabilities calculated over a large set of examples are equal to the confidence limits for which they are calculated. In other words, this means that these uncertainties include the contributions of systematic errors. It is well known that MAP lens modeling can sometimes result in biased parameter recovery due to numerous effects including the choice of source parameterization. It is therefore likely that the parameters recovered with these networks can be more accurate than those predicted with MAP methods. Perhaps the most important element in this analysis is the design of the training data. In particular, since we have used simulated data to train a network, which is then used for the interpretation of real data, special care should be given to understanding the structure and the statistical properties of real data and to define a training set which encompasses the variations of all possible effects in the real data. In this work, we used a few approximations to produce the training set (e.g., gridding the $uv$-coordinates prior to predicting the visibilities). For the purpose of the demonstration of the method in this paper, these approximations seem justified, given that the recovered parameters for real ALMA observations of SPT sources are consistent with their values from MAP modeling. However, if these methods are going to be widely used for real data analysis, it is preferable to produce even more realistic training data. In addition, it is possible to use domain adaptation methods \citep[e.g., ][]{Ben-David} to generalize the learning of the networks from simulated examples to real data with different statistical properties. We explored strategies for the analysis of strong gravitational lenses from interferometric data with neural networks. We found that it is possible to train simple feed-forward convolutional neural networks on dirty images produced from the measured visibilities, however, the best results were obtained when a recurrent neural network-based architecture was first used to remove the effects of the convolution of the sky emission with the dirty beam prior to estimating the parameters using feed-forward models. This method produced estimates with a median precision comparable to MAP modeling (typically less than a factor of two lower), while resulting in orders of magnitude improvement in speed and the use of computational resources. Given the large number of observations expected to be executed by ALMA and other interferometric facilities, these methods can be a crucial tool for the interpretation of future data. | 18 | 8 | 1808.00011 |
1808 | 1808.05919_arXiv.txt | We evolve stellar models to study the common envelope (CE) interaction of an early asymptotic giant branch star of initial mass $5\,\rm M_{\odot}$ with a companion star of mass ranging from $0.1$ to $2\,\rm M_{\odot}$. We model the CE as a fast stripping phase in which the primary experiences rapid mass loss and loses about 80 per cent of its mass. The post-CE remnant is then allowed to thermally readjust during a Roche-lobe overflow (RLOF) phase and the final binary system and its orbital period are investigated. We find that the post-CE RLOF phase is long enough to allow nuclear burning to proceed in the helium shell. By the end of this phase, the donor is stripped of both its hydrogen and helium and ends up as carbon-oxygen white dwarf of mass about $0.8\,\rm M_{\odot}$. We study the sensitivity of our results to initial conditions of different companion masses and orbital separations at which the stripping phase begins. We find that the companion mass affects the final binary separation and that helium-shell burning causes the star to refill its Roche lobe leading to post-CE RLOF. Our results show that double mass transfer in such a binary interaction is able to strip the helium and hydrogen layers from the donor star without the need for any special conditions or fine tuning of the binary parameters. | \label{sec:intro} Mass transfer is a critical feature of the evolution of close binary systems. This direct interaction between the stellar components has key implications to all stages of stellar evolution and distinguishes binary evolution from that of single stars. The rate of this mass transfer determines the fate of the remnants such as Algols, X-ray binaries, contact binaries, cataclysmic variables and double-degenerate systems involved \citep{PW85,DE17}.\\ \indent In a frame rotating with a tidally locked, circular binary system, the effective gravitational potential is an equipotential surface through the inner Lagrangian point that defines the Roche lobe of each star. The volume enclosed by the Roche lobe determines the Roche lobe radius of each star \citep{Eg83}. If either star fills its Roche lobe then material overflows from its outer layers through the inner Lagrangian point that connects the two Roche lobes where the gradient of the effective potential vanishes. Stable mass transfer occurs by Roche lobe overflow (RLOF) by virtue of either the slow expansion of the star because of nuclear evolution or of orbital contraction by angular momentum losses from gravitational radiation, magnetic braking in stellar winds or tides if the Roche-lobe filling star must be spun up.\\ \indent Some or all of the transferred material may be captured by the companion and consequently the evolution of both the donor and the accretor is expected to differ from that of similar single stars. Binary systems that have long orbital periods allow the more massive star to reach the red giant phase before filling its Roche lobe. The giant star then has a deep convective envelope and runaway mass transfer reaches dynamical time-scales \citep{We84,Iv13a}. This also happens if the Roche lobe-filling star is significantly more massive than its, most often, main-sequence companion \citep{Pa65}.\\ \indent Because of its relatively long thermal time-scale, the accreting star cannot capture all the material transferred from the donor star, so material accumulates in a common envelope (CE) surrounding both stars leading to the formation of a CE system \citep{Pa76}. As the dense companion plunges into the giant's envelope, gravitational drag forces cause the orbit of the embedded binary to shrink dramatically and the core of the donor and its companion star spiral inward through their common envelope \citep{LS88, TS00,Pa12a}. Possible outcomes include the release of sufficient energy to drive off the entire envelope as the giant core and MS star spiral in, resulting in a closer binary, or merging of the stars. This explains the observed short-period degenerate systems such as cataclysmic variables, close binary pulsars and close double white dwarf binaries which, otherwise, cannot be explained by angular momentum losses by gravitational waves or magnetic winds \citep{IL93}.\\ \indent There are several variations of the treatment of the CE and many studies have been carried out \citep[see][for a review]{Iv13a}. Most rely on analytical prescriptions based on energetic considerations \citep{We84,IT85} where the efficiency of the conversion of orbital energy of the binary into kinetic energy of the outflow is assumed. Another prescription based on angular momentum considerations \citep{Ne00,NT05} parametrizes the angular momentum of the ejected envelope. However, this has been found to be less useful than the energy budget approach for predicting the outcome of the CE and constraining the parameters of the possible progenitors of observed systems \citep{ZO10}. Other approaches include a more accurate description of the ejection conditions, such as the donor star's structural response to adiabatic mass loss \citep{DT10}. However, the efficiency of the ejection process remains uncertain.\\ \indent A standard treatment of the CE is the energy formalism \citep{We84} in which the final separation of the binary is determined by relating the loss in the orbital energy of the system to the binding energy of the released envelope. A large fraction of the orbital energy released in the spiral-in process is transferred into the expansion of the envelope with efficiency $\alpha_{\rm CE}$ \citep{LS84}. The envelope is then ejected when the total deposited orbital energy, $\Delta E_{\rm orb}$, exceeds the binding energy of the envelope, $E_{\rm env}$, or \begin{equation} \alpha_{\rm CE}\,\Delta E_{\rm orb} \geq E_{\rm env},\label{eqn:CE} \end{equation} and \begin{equation} \Delta E_{\rm orb} = \frac{GM_{\rm c1}M_{2}}{2 a_{\rm f}} - \frac{M_{\rm c1}M_{2}}{2 a_{\rm i}}, \label{eqn:CE2} \end{equation} where $M_{1}$ and $M_{\rm c1}$ are the masses of the giant and its core, respectively, $M_{2}$ is the mass of the secondary, which is not affected, and $a_{\rm i}$ and $a_{\rm f}$ are the initial and final separations, before and after the common envelope, respectively \citep{Hu02}.\\ \indent The efficiency parameter $\alpha_{\textnormal{CE}}$ and the density profile of the envelope determine the final separation of the system. Because the CE phase involves various complex physical processes occurring on very different time-scales, $\alpha_{\rm CE}$ cannot yet be determined from first principles and it thus constitutes a simple prescription for the complex hydrodynamical interaction taking place during and after the spiral-in phase. Moreover, $\alpha_{\textnormal{CE}}$ is probably not a constant \citep{RT95, DE11b, DA11} but is often set to $\alpha_{\textnormal{CE}}=1$ \citep{Hu02}. Some studies attempt to constrain $\alpha_{\textnormal{CE}}$ with certain systems and then assume it is the same for all similar systems. For example, \citet{BR01} study low-mass black-hole X-ray binaries (soft X-ray transients) with main-sequence companions that have formed through case~C mass transfer and constrain $\alpha_{\rm CE}$ to be $0.2$ to $0.5$. \citet{RT95}, on the other hand, model the magnetic dynamo owing to differential rotation within the envelope. They find that $\alpha_{\rm CE}$ lies in the range 0.5 to 1.0 but it depends on the initial state of the envelope and changes during the evolution. Following \citet{RT95}, later work by \citet{TO97} favours $\alpha_{\rm CE}=1$. Therefore the energy formalism is useful to predict the fate of CE evolution but its outcome is not fully understood. Multi-dimensional hydrodynamical simulations that model the CE evolution \citep{Pa12a, RT12} cannot be used to relate the pre- and post-CE configurations because they end after a rapid spiral-in phase before most of the envelope is unbound. For these reasons, the common envelope phase is one of the most uncertain processes in binary stellar evolution \citep{Iv13a} and realistic self-consistent models are still lacking. This affects our understanding of the evolution of close binary systems such as compact X-ray binaries, cataclysmic variables, merging gravitational wave sources and Type~Ia~supernovae.\\ \indent Evidence for CE evolution is provided by plenty of observed systems, such as cataclysmic variables (CVs) and double-degenerate binaries. Close binary systems containing a carbon-oxygen white dwarf and a main-sequence star with periods of one day or less, including CVs, are well known \citep{Kn11, Ri12}. These can only be explained if a significant amount of mass and angular momentum are removed from their precursor system. Other possible examples of CE events are planetary nebulae (PNe) with a close binary at their centre \citep{BO78,JB17}. A recent observational study of optical spectra of a large sample of Galactic planetary nebulae by \citet{WE18} shows features such as hydrogen-deficiency or stellar lines that are shifted with respect to the nebular ones for example, which suggest a binary core in several systems. The connection between duplicity and the observed nebular structure has been proposed on theoretical and observational grounds. Theoretically, it was predicted that some, perhaps even all, PNe should be the outcome of a CE \citep{DE09}. An AGB star in a binary system overflows its Roche lobe and interacts with its companion unless the system is very wide. This leads to a CE, a spiral-in of the companion and a tight final orbit of a few hours to a few days. Aspherical PNe with bipolar ejecta featuring dense equatorial rings and higher-velocity polar jets are thought to be the products of binary interactions \citep{We08,DE11a}. Observational evidence for this connection is the significant change in the radii of the secondary stars in planetary nebulae with extremely close binary nuclei. These companions are reported to have larger radii than expected for main-sequence stars of the same masses \citep{OB01,AI08}. Although it is uncertain whether the mass of the secondary is substantially affected during the CE phase \citep{PL85,SA98}, the observed oversized secondary companions are thought to have either recently emerged from a CE, and hence are out of thermal equilibrium, or their mass and radius changed because of mass transfer during the CE phase, perhaps even both. On the other hand, the curious emerging class of optical transients with predominantly red spectra observed in the local Universe and commonly dubbed as luminous red novae or intermediate luminosity optical transients \citep{Mar99,Bla17} are perhaps the best CE candidates observed so far and can thus be used to measure CE outburst energies and durations. While an agreement between some of their features and model predictions has been reported \citep{Iv13b}, the field of CE hydrodynamics and associated radiative transfer remains an area of active research \citep{GA17}.\\ \indent Currently, neither observations nor theory provide strong constraints on the stellar evolution during or immediately after the CE phase. Numerical simulations of CE evolution \citep{RT12,Pa12a} including only gravitational drag tend to show the companion star rapidly spiralling into the envelope of the giant as angular momentum is lost by the orbit. These simulations start with the companion already at the surface of the giant. When begun at the onset of Roche lobe overflow \citep{IA17}, the establishment of the common envelope begins slowly but once in place the same rapid inspiral of the cores is seen. At the end of this phase the envelope has expanded but remains bound. It is what follows that we model. Without evidence to the contrary we suppose that the envelope is removed by a super wind, similar to the strongest winds observed from AGB stars \citep{VW93} on a time-scale of a few thousand years or so. This has also been proposed by \citet{HG18}, who suggest that the envelope is lost by dust-driven winds following the CE event similar to processes operating in the ejection of the envelopes of AGB stars. We consider a binary system in which the more massive star fills its Roche lobe at the early asymptotic giant branch phase (EAGB). An EAGB star has completed core helium burning and is characterized by a core essentially consisting of carbon and oxygen, the main products of helium-burning, surrounded by a helium-burning shell and a hydrogen-burning shell, which is the main energy source in the giant star. CE evolution with an EAGB star must be common and the EAGB structure makes them interesting objects if stripped. They are expected to evolve to hybrid white dwarfs (low-mass carbon-oxygen cores with thick helium envelopes) and they may sustain nuclear burning after the CE phase as we show in Section~\ref{sec:WD}. On the EAGB, a star expands to larger dimensions than on the RGB. Thus when it expands to a radius $R_{\rm{CE}}$, which exceeds the maximum radius reached on the RGB, it can undergo case~C mass transfer \citep{Ki67}. We strip the star by applying fast mass loss to mimic a CE event. Once the system detaches, we allow the donor to thermally adjust and refill its Roche lobe. We choose the mass of the companion such that the subsequent RLOF is stable and study the behaviour of the binary system.\\ \indent \citet{NO94} use such double mass-transfer events to model the evolution of the progenitors of Type~Ic~supernovae and suggested this as a possible evolutionary scenario for hypernovae. \citet{CR07} consider a binary orbit that allows interaction between the star and its companion but not so close as to merge. This binary interaction removes only the hydrogen envelope of the progenitor star and subsequent shedding of the helium-rich layer occurs by strong radiatively driven winds. \citet{NO01} point out that the helium layer may be removed with a second mass-transfer event given the right conditions of initial mass and separation. This conclusion is based on earlier work by \citet{NO95} where they assume that the first mass transfer occurs when the primary has formed a helium core (case B mass transfer). They argue that this is possible in low-mass helium stars which have large enough radii to fill their Roche lobes. Larger-mass helium stars, on the other hand, have radii too small to fill their Roche lobes as seen in results by \citet{HA86}, for example. Suggested explanations for why they remain small and hot can be found in \citet{Egg06}. These larger-mass helium stars, however, have large enough luminosities to lose most of their helium layer by strong winds instead. We discuss the sensitivity of the removal of the hydrogen and helium layers to the initial conditions in Section~\ref{ssec:comp}.\\ \indent We focus on a scenario in which, after the CE event, the binary system ejects its envelope and avoids merging. This determines the chosen post-CE separation of our binary. We also assume that its main-sequence companion does not fill its Roche lobe. However, because it is unclear how stars behave during the extremely rapid, possibly adiabatic, mass loss of the CE phase \citep{Iv13a} and how their radii are affected by CE evolution, we investigate various evolutionary sequences with different post-CE orbital separations and study the effect on the final state of the remnants. We also investigate changing the companion mass on the fate of the resulting binary system. In Section \ref{sec:evol} we present our evolutionary code and the evolution of our model through CE and RLOF. The dependence of our model on the initial conditions is discussed in Section \ref{sec:Res}. We conclude in Section~\ref{sec:conc}. | \label{sec:conc} \label{sec:conc} We consider a binary system with a relatively long orbital period such that the more massive companion fills its Roche lobe on the EAGB. We strip the star by applying fast mass loss to mimic a CE event. After the CE phase, the donor is stripped of most of its envelope and has a thin hydrogen shell on the surface. When the system detaches, we allow the donor to refill its Roche lobe and undergo stable post-CE RLOF driven by shell helium burning. We find this phase to be prolonged and the core grows as the helium shell burns. By the end of the post-CE RLOF phase the donor is stripped of most of its helium shell and ends up as white dwarf of mass about $0.8\,\rm M_{\odot}$. We studied the sensitivity of our results to system parameters such as the mass of the companion and the pre-CE orbital separation. We find that the variation in the companion mass can change the final binary separation from a few days to about 100 d. When we vary the post-CE orbital separation we find that the donor refills its Roche lobe in the post-CE RLOF phase except in the cases when all the helium shell has already been stripped in the CE phase. Roche-lobe overflow in the post-CE RLOF phase is thus due to the burning in the helium shell. We find that no fine tuning of the binary system is required for the binary interaction to remove both the helium and hydrogen layers in such a double mass transfer mechanism, leaving all such systems with a similar $0.8\,\rm M_{\odot}$ CO white dwarf. | 18 | 8 | 1808.05919 |
1808 | 1808.10423_arXiv.txt | The HAWC (High Altitude Water Cherenkov) collaboration recently published their 2HWC catalog, listing 39 very high energy (VHE; $>$100~GeV) gamma-ray sources based on 507 days of observation. Among these, there are nineteen sources that are not associated with previously known TeV sources. We have studied fourteen of these sources without known counterparts with VERITAS and \textit{Fermi}-LAT. VERITAS detected weak gamma-ray emission in the 1~TeV--30~TeV band in the region of DA\,495, a pulsar wind nebula coinciding with 2HWC\,J1953+294, confirming the discovery of the source by HAWC. We did not find any counterpart for the selected fourteen new HAWC sources from our analysis of \textit{Fermi}-LAT data for energies higher than 10 GeV. During the search, we detected GeV gamma-ray emission coincident with a known TeV pulsar wind nebula, SNR\,G54.1+0.3 (VER\,J1930+188), and a 2HWC source, 2HWC\,J1930+188. The fluxes for isolated, steady sources in the 2HWC catalog are generally in good agreement with those measured by imaging atmospheric Cherenkov telescopes. However, the VERITAS fluxes for SNR\,G54.1+0.3, DA\,495, and TeV\,J2032+4130 are lower than those measured by HAWC and several new HAWC sources are not detected by VERITAS. This is likely due to a change in spectral shape, source extension, or the influence of diffuse emission in the source region. | Gamma-ray astronomy can be performed using a variety of techniques, each with different strengths and weaknesses. Direct detection of gamma rays is possible with satellite-based instrumentation, such as the Large Area Telescope (LAT) on board the \textit{Fermi Gamma-Ray Space Telescope}~\citep{2009ApJ...697.1071A}. This provides low background observations over a wide field of view, covering about 20$\%$ of the sky at any given time and scanning the whole sky every three hours. However, due to the physical size limitations imposed upon satellite-based instruments, the effective area is generally smaller than 1 $\textnormal{m}^{2}$, leading to a sensitivity that peaks at a few GeV. Above 100~GeV, ground-based observatories are best suited to study the emission, thanks to their large effective collection area when compared to space experiments. Ground-based imaging atmospheric Cherenkov telescope (IACT) arrays, such as VERITAS~\citep{2002APh....17..221W}, observe the Cherenkov light generated by particle showers in the atmosphere, while air shower arrays, such as the High Altitude Water Cherenkov (HAWC) observatory~\citep{2013APh....50...26A}, sample the air shower particles at ground level. IACTs offer the best instantaneous sensitivity thanks to their large effective collection area ($\sim$$\textnormal{10}^5 \UU{m}{2}$) and excellent rejection of the cosmic-ray background. However, observations require clear, dark skies, limiting the duty cycle to $\lesssim$20$\%$, and gamma-ray sources must be contained within the field of view of the telescope, which is at present $\lesssim$5$\degree$ diameter. Air shower arrays for gamma-ray observations provide lower instantaneous sensitivity than IACTs, but they can operate continuously with an instantaneous field of view of the telescope covering $\sim$15\% of the sky. Sensitive, unbiased surveys for a large portion of the sky can be conducted over the lifetime of air shower arrays. The angular and energy resolution of each of the three techniques, which allow one to study and to understand astrophysical gamma-ray sources in detail, are complementary. For example, the good angular resolution of IACTs allows us to resolve the detailed morphology of spatially extended sources and to identify the counterparts of sources in complex regions. The limited field of view, however, restricts the size of the emission region that can be studied. Compared to this, satellite-based instruments and air shower arrays can provide good measurements of highly extended sources. Satellite-based instruments provide energy resolution better than 15$\%$ for gamma rays with energies above several hundreds of MeV up to around 1~TeV. Above 1~TeV, IACTs provide the best energy resolution (generally about 20$\%$). Combined with their large effective areas and sensitivities, IACTs thus can measure detailed features of the spectral energy distribution (SED) of sources. Air shower arrays' energy resolution is worse than that of IACTs. The large and relatively uniform exposure time of air shower array measurements, however, can provide good high-energy measurements above tens of TeV for a large portion of the sky, increasing the dynamic range of the measurements and allowing the study of spectral changes at the highest energies. The most powerful approach, therefore, is to combine observations from all three methods. Only a few examples of this exist~\citep{2014ApJ...787..166A,2014ApJ...788...78A} due to limited overlapping source catalogs. In this paper, we describe the results of observations of newly discovered HAWC sources with the VERITAS IACT array and the LAT on board the \textit{Fermi Gamma-Ray Space Telescope}. Fully completed in March 2015, HAWC has recently released a catalog, 2HWC~\citep{2017ApJ...843...40A}. Compared to the previous very high energy (VHE; $>$100~GeV) surveys performed by Milagro~\citep{2007ApJ...664L..91A} and ARGO-YBJ~\citep{2013ApJ...779...27B}, HAWC provides more than an order of magnitude better sensitivity~\citep{2017ApJ...843...39A}. The 2HWC catalog contains 39 sources, twenty of which are associated with known astrophysical objects including active galactic nuclei, pulsar wind nebulae (PWNs), and supernova remnants (SNRs). The remaining nineteen sources in the catalog have not previously been identified as TeV gamma-ray emitters, providing promising new targets for follow-up observations with IACTs and space-based gamma-ray observatories. | Using VERITAS and \textit{Fermi}-LAT, we searched for IACT and GeV gamma-ray counterparts to fourteen out of nineteen new HAWC sources without clear TeV associations. VERITAS detected one weak source coincident with PWN DA\,495. The flux of DA\,495 measured by VERITAS is about seven times lower than HAWC's measurement while both measurements agree on the spectral index. \textit{Fermi}-LAT did not see gamma-ray emission for the selected fourteen sources for either point or extended source searches. \textit{Fermi}-LAT did detect point-like emission from SNR\,G54.1+0.3, a PWN detected by both VERITAS and HAWC. The combined SED of the three instruments on SNR\,G54.1+0.3 covers a wide range of the inverse Compton peak of the PWN, providing a good data set for future modeling. Upper limits measured by VERITAS are lower than expected from HAWC's measurement for nine sources. Among these, non-detections by VERITAS exclude a point-source hypothesis for six sources with a confidence level of 95$\%$. The discrepancy could be resolved if the sources are extended, or if there is a spectral change in the energy range between VERITAS and HAWC. For 2HWC\,J1852+013$^{\ast}$ and 2HWC\,J1902+048$^{\ast}$, the extension of the source should be larger than 0.23$\degree$ to satisfy all of the measurements. These numbers are based on a comparison between the upper limits of VERITAS and the flux estimation of HAWC. However, it is possible that the HAWC flux is overestimated for some of the sources, since the flux estimation has been made with a single point source model for the likelihood analysis without accounting for nearby sources. Unaccounted weak diffuse emission over a very large area would also cause an overestimation of the flux. While \textit{Fermi}-LAT will accumulate more exposure time, a future IACT like the Cherenkov Telescope Array (CTA) should be able to detect the sources with its larger field of view and improved sensitivity. A combined analysis with \textit{Fermi}-LAT, CTA and HAWC will provide detailed gamma-ray data to study the nature of these new VHE sources. | 18 | 8 | 1808.10423 |
1808 | 1808.00233_arXiv.txt | In addition to long-lived radioactive nuclei like U and Th isotopes, which have been used to measure the age of the Galaxy, also radioactive nuclei with half-lives between 0.1 and 100 million years (short-lived radionuclides, SLRs) were present in the early Solar System (ESS), as indicated by high-precision meteoritic analysis. We review the most recent meteoritic data and describe the nuclear reaction processes responsible for the creation of SLRs in different types of stars and supernovae. We show how the evolution of radionuclide abundances in the Milky Way Galaxy can be calculated based on their stellar production. By comparing predictions for the evolution of galactic abundances to the meteoritic data we can build up a time line for the nucleosynthetic events that predated the birth of the Sun, and investigate the lifetime of the stellar nursery where the Sun was born. We then review the scenarios for the circumstances and the environment of the birth of the Sun within such a stellar nursery that have been invoked to explain the abundances in the ESS of the SLRs with the shortest lives -- of the order of million years or less. Finally, we describe how the heat generated by radioactive decay and in particular by the abundant \iso{26}Al in the ESS had important consequences for the thermo-mechanical and chemical evolution of planetesimals, and discuss possible implications on the habitability of terrestrial-like planets. We conclude with a set of open questions and future directions related to our understanding of the nucleosynthetic processes responsible for the production of SLRs in stars, their evolution in the Galaxy, the birth of the Sun, and the connection with the habitability of extra-solar planets. | \label{sec:intro} More than a century has passed since Marie Skłodowska Curie\footnote{The 150$^{\rm th}$ anniversary of her birthday was recently celebrated on the 7$^{\rm th}$ of November 2017.} coined the term Radioactivity to indicate the emission of radiation and particles from peculiar nuclei. Since then, the role and applications of radioactivity have had a profound impact in many fields of science and technology. The role of radioactive nuclei in the field of astrophysics has been long recognised and described. For example, radioactive nuclei power the light of supernovae and the radiation they emit can be mapped throughout the Galaxy by satellite observatories \cite{radiobook}. Here we focus on the most recent advances in the research directions that relate the process of short-lived (half-lives\footnote{See Table~\ref{table:acronyms} for a list of all the symbols and the acronyms used throughout the paper.} T$_{1/2}$ $\sim$ 0.1 to 100 million years, Myr) radioactivity to the concept of {\it cosmochronology}, and on the relatively more recent link between short-lived radioactivity and {\it habitability}. We consider in particular the applications of radioactivity in the field of {\it cosmochemistry}, i.e., the study of the composition of meteorites and other solid Solar System samples aimed at explaining the origin of chemical matter in the Solar System and in the Universe. Due to extensive technological advances in the laboratory analysis of the isotopic composition of terrestrial and extraterrestrial materials, the amount of information and constraints that can be derived from such studies are expanding at a very fast rate. Much effort on the theoretical interpretation is needed to keep up with the experimental data. In this landscape, the connections between radioactivity, cosmochronology, and habitability are becoming more relevant than ever, and the implications of these connections are quickly becoming far reaching. The aim of this paper is to illustrate and discuss these connections and their implications. {\it Cosmochronology} is intrinsically linked to radioactivity, being defined as the use of the abundances of radioactive nuclei to compute either the age of the elements themselves, or the age of astronomical objects and events. The first aim typically relies on very long-lived radionuclides with half-lives T$_{1/2}$ of the order of billions of years (Gyr), such as \iso{238}U, \iso{232}Th, \iso{187}Os, \iso{87}Rb; an introduction to this topic can be found, for example, in Chapter 1 of \cite{radiobook}. Here we address the second aim: to use radioactive nuclei to calculate the age of astronomical objects and events, specifically in relation to the birth of our Sun and Solar System, with the ultimate aim to compare the birth of our Sun to the birth of other stars and their extra-solar planetary systems. To such aim we use short-lived radionuclides (SLRs, T$_{1/2}$ $\sim$ 0.1 to 100 Myr), which provide us with a range of chronometers of the required sensitivity. It is well known that radioactive decay can be used as an accurate clock because the rate at which the abundance by number of a radioactive nucleus $N_{\rm SLR}$ decreases in time due to its radioactive decay is a simple linear function of the abundance itself, where $\lambda$ is the time-independent constant of proportionality referred to as the {\it decay rate}: \begin{equation} \label{eq:basicdiff} \frac{dN_{\rm SLR}}{dt} = - \lambda \,N_{\rm SLR}. \end{equation} \noindent A quick integration between two set times $t_1$ and $t_0$ delivers: \begin{equation} \label{eq:basic1} N_{\rm SLR}(t_1) = N_{\rm SLR}(t_0) e^{- \lambda (t_1 - t_0)}, \end{equation} \noindent which can also be written as \begin{equation} \label{eq:basic2} t_1 - t_0 = \tau [{\rm ln}(N_{\rm SLR}(t_0)) - {\rm ln}(N_{\rm SLR}(t_1))], \end{equation} \noindent where $\tau = 1/\lambda ={T_{1/2}/ ln(2)}$ is the mean-life, i.e., the time interval required to decrease $N_{\rm SLR}$ by a factor $1/e$ (instead of a factor $1/2$, as for the half-life). \begin{figure}[tb] \begin{center} \begin{minipage}[t]{8.5 cm} \includegraphics[width=8.5cm,angle=0]{CAI_Al3S4.pdf} \\ \end{minipage} \begin{minipage}[t]{16.5 cm} \caption{Photomicrograph produced in 1977 \cite{clayton77} of the CAI named Al3S4 from the Allende meteorite. The field of view is 22 mm $\times$ 17 mm. In 2014, the initial \iso{146}Sm/\iso{144}Sm ratio in the ESS was derived from analysis of this CAI \cite{marks14} (bottom left panel of Fig.~\ref{fig:data}). \label{fig:CAI}} \end{minipage} \end{center} \end{figure} Radioactive clocks have been used extensively to measure a large variety of time intervals. The decay of \iso{14}C, a nuclide with a half-life of 5730 yr, allows us to measure timescales related to human history; and the age of our Milky Way Galaxy of approximately 13 Gyr has been estimated also based on the ages of some of the oldest observed stars inferred from their U and Th abundances \cite{cayrel01,frebel07}. Thanks to the SLRs considered here, it has become possible to investigate in detail the early history of the Solar System and build a chronology of planetary growth from micrometer-sized dust to terrestrial planets \cite{dauphas11}. For example, the solidification of the lunar magma ocean has been dated to about 200 Myr after the birth of the Sun also thanks to the $\alpha$ radioactive decay of \iso{146,147}Sm into \iso{142,143}Nd, respectively \cite{borg11}. The age of the oldest solids in the Solar System, the calcium-aluminium-rich inclusions (CAIs) found in primitive meteorites (Fig.~\ref{fig:CAI}), is 4567-4568 Myr (see Table 3 of \cite{tissot17}) as measured from the radioactive decay chain starting at the U isotopes and ending into the Pb isotopes. CAIs are believed to be among the first solids to have formed in the protosolar nebula, thus, their age is taken also as indicative for the age of the Sun. Unlike cosmochronology, {\it habitability} has been linked to short-lived radioactivity only recently. Here we use the concept of habitability in the following sense: whether or not an astronomical object can support the formation or the maintenance of life forms partly similar to those we have on Earth \cite{Gargaud2011}. Formation and maintenance, however, are two different processes, both related to habitability. It should be kept in mind that life forms elsewhere in the Universe could be fundamentally different from those we know from Earth. However, the definition of life as a system based on chemicals, built on organic material, and supported by liquid water as a solvent is generally accepted by the astrobiological community and thus is also used here. The paper is structured as follows. Section~\ref{sec:background} introduces some basic methodology and considerations and is separated into four sections: Sec.~\ref{sec:abundances} presents the methods by which the initial SLR abundances in the early Solar System are inferred from meteoritic analysis. Section~\ref{sec:stars} presents a broad overview of stellar evolution and nucleosynthetic processes in stars. Section~\ref{sec:galaxy} describes the processes that have built up the Solar System chemical matter, from galactic chemical evolution to the formation of the Sun itself. Section~\ref{sec:habit} presents how, in general, radioactivity may influence habitability in several direct and indirect ways. Section~\ref{sec:list} discusses in more detail each SLR, from its meteoritic abundance to the nuclear path of its stellar production. The 19 SLRs considered here are grouped into 9 subsections, according to their nucleosynthetic production processes. In Sec.~\ref{sec:GCE} we deal with Galactic evolution: Sec.~\ref{sec:GCEmodels} presents the simple analytical models used so far to describe the evolution of SLRs in the Galaxy, and Sec.~\ref{sec:times} shows how the SLR galactic abundances can be used to establish the timing of specific events related to the birth of our Sun. In Sec.~\ref{sec:birth} we discuss inferences derived from the presence of SLRs in the ESS concerning the circumstances of the Sun's birth. For sake of clarity, we distinguish three different questions related to the general problem: the stellar sources, the injection mechanism, and the plausibility and probability of the possible scenarios (covered in Sec.~\ref{sec:Q1}, \ref{sec:Q2}, and \ref{sec:Q3}, respectively). In Sec.~\ref{sec:radioheat} we describe the potential sources of radioactive heat in the ESS and the implications on planet formation and habitability: first, we analyse all the possible radioactive heat sources (Sec.~\ref{sec:heat}), then we consider carrier minerals (Sec.~\ref{sec:carriers}), and finally the specific, important case of \iso{26}Al (Sec.~\ref{sec:26Alhabit}). Section~\ref{sec:conclusions} summarises the main points of the paper and presents a final set of open questions and future research directions. The topic of the present paper covers a range of research fields, from nuclear physics, via astronomy and astrophysics, to planetary sciences, from both the experimental and the theoretical perspective. We focus here on the interdisciplinary connections between these topics. As such the paper has been written keeping in mind different audiences and with the broad aim to foster and enhance the efficiency of the knowledge transfer required to answer the currently open questions. | \label{sec:conclusions} We have reviewed the meteoric evidence for the presence of SLRs in the ESS, their production in stars, the simple models available to predict their GCE evolution, and the methodology that allows us to use SLRs as clocks to measure the isolation time of presolar matter inside its parent molecular cloud. Most stars produce some kind of SLRs and enrich the ISM with radioactivity at the end of the lives via winds or explosions. We have then considered the origin of the shortest-lived isotopes, such as \iso{26}Al, in the context of different scenarios for the formation of the Sun, and the impact of the heat from radioactivity on the evolution of solid bodies in a planetary system, focusing in particular on the effects resulting from the presence of \iso{26}Al. Depending on the timescale and speed of early condensation and accretion in disks, melting of planetesimals may produce circulating water and related chemical reactions influencing habitability. Our main conclusions and future prospects are the following: \begin{enumerate} \item The abundance of \iso{60}Fe in the ESS needs to be firmly established before we can proceed to select the possible stellar source responsible for the presence of \iso{26}Al. Among the other SLRs, the determination of the ESS abundances of \iso{36}Cl, \iso{41}Ca, \iso{205}Pb, and \iso{244}Pu require special attention. \item Accurate and precise predictions for the stellar production of SLRs are still hampered by many uncertainties in the nuclear physics input. These range from the proton capture reactions on \iso{25}Mg (including the feeding factor to the ground state of \iso{26}Al) and \iso{26}Al, to the neutron production rates \iso{13}C($\alpha$,n)\iso{16}O and \iso{22}Ne($\alpha$,n)\iso{25}Mg, the neutron-capture cross sections, as well as the decay rates of the various branching points (related to the production of, e.g., \iso{60}Fe and \iso{182}Hf), the decay rate of \iso{146}Sm as well as the rates of the reactions that lead to its production, and the $r$-process production of isotopes belonging to the actinides. More details have been given for each SLRs in the dedicated subsections of Sec.~\ref{sec:list}. Current and future experimental facilities, among them LUNA, FAIR, and n\_TOF at CERN, will allow to take up the challenge to improve much of the current situation. In the case where theoretical estimates are required, such as the temperature dependence of decay rates, better nuclear models need to be employed and uncertainties evaluated. \item Updated and improved stellar models for all the sites of production (Table~\ref{table:stars}) need to be constantly considered in the light of SLR production. For example, the recent models by \cite{sukhbold16} should be examined in relation to all the SLRs considered here, starting with the procedure presented in Sec.~\ref{sec:Q1}. In this context, a coherent picture of SLR nucleosynthesis needs to be built that is able to include the interpretation of all the available constraints, each with their own significance. These include stardust grains \cite{groopman15}, likely originating each from a different star or CCSN, $\gamma$-ray observations \cite{diehl13}, for which a grand average of stellar yields in the Galaxy needs to be considered, and measurements of current radioactivity in the Earth's crust and other terrestrial and lunar samples (e.g., \cite{wallner16}), to which probably only one or a few CCSNe contributed. Observations of live \iso{26}Al of CCSN origin in the same terrestrial samples that show live \iso{60}Fe, for example, and comparison of the \iso{60}Fe/\iso{26}Al ratio with that derived from $\gamma$-ray observations can provide new constraints for the build up of this coherent picture. At the same time proposed future space telescopes like eASTROGAM \cite{deangelis17} and GRIPS \cite{greiner12} will provide a new, improved understanding of the occurrence of \iso{26}Al and \iso{60}Fe in the Galaxy and in star forming regions. \item In order to properly describe the effect of discrete stellar additions of SLRs to the presolar matter and derive information about the lifespan of its stellar nursery, the free parameters $K$ and $\delta$ in Eq.~\ref{eq:final} need to be constrained both by using full GCE and stellar population models, and an improved understanding of the propagation of different types of stellar ejecta in the ISM. This will allow us to derive a better estimate of the isolation time, which in turn may help us to clarify the environment of the birth of the Sun. Hydrodynamical model calculations of the propagation of SLRs in the Galaxy from their stellar sources to the location of star formation need to be carried out also to address the issue of the origin of \iso{26}Al in the ESS. \item Models of planetesimal evolution and planet formation including the effect of the heat of \iso{26}Al need to be developed further, including volatile degassing and different scenarios such as pebble accretion. Different abundances for other potential heat sources like \iso{60}Fe for short term heating, or \iso{87}Rb, \iso{147}Sm, and \iso{232}Th for long term heating should be also considered, as under specific conditions they may have elevated abundances compared to those that were present in our Solar System and thus may have a substantial effect on habitability in other planetary systems. \item Based on a clearer picture obtained from the points above, a statistical analysis of the presence of \iso{26}Al in extra-solar planetary systems will need to be developed. Further steps might be such model approaches that connect the spatial distribution of $^{26}$Al in star-forming regions and the condensation speed of planetesimals in disks there. This might help to evaluate differences between forming planetary systems in their potential of generating liquid water and various chemicals inside their early formed solid bodies. Results may be then compared to independent constraints from future observations of the composition of extra-solar planets, and particularly of their water content as a possible signature of the ultimate effect of \iso{26}Al decay. Missions that aim to discover and characterise exoplanets include the NASA missions K2 and TESS and the upcoming ESA missions CHEOPS and PLATO, in conjunction with more detailed, spectroscopic input from the NASA James Webb Space Telescope (JWST), the prospective ESA ARIEL space mission, and the ground-based Extremely Large Telescopes (ELTs) that are currently in development. The combination of these data sets will give us information on both the bulk and the atmospheric or surface compositions of extra-solar planets. Such further examples of planets beyond the Solar System may eventually provide an independent estimate of the presence of \iso{26}Al and the consequence of its integration to solid condensates. \end{enumerate} These tasks lying ahead are challenging, but feasible, and carrying the promise to provide us with a clearer view of where our Solar System and the life within it stands in relation to the vast population of extra-solar planetary systems in the Galaxy. Connection of improved models and further observational data on the occurrence of radionuclides could provide new approaches to estimate the habitability potential of the growing number of recently discovered exoplanetary systems. | 18 | 8 | 1808.00233 |
1808 | 1808.05620_arXiv.txt | Steady gamma-ray emission up to at least 200 GeV has been detected from the solar disk in the Fermi-LAT data, with the brightest, hardest emission occurring during solar minimum. The likely cause is hadronic cosmic rays undergoing collisions in the Sun's atmosphere after being redirected from ingoing to outgoing in magnetic fields, though the exact mechanism is not understood. An important new test of the gamma-ray production mechanism will follow from observations at higher energies. Only the High Altitude Water Cherenkov (HAWC) Observatory has the required sensitivity to effectively probe the Sun in the TeV range. Using three years of HAWC data from November 2014 to December 2017, just prior to the solar minimum, we search for 1--100 TeV gamma rays from the solar disk. No evidence of a signal is observed, and we set strong upper limits on the flux at a few $10^{-12}$ TeV$^{-1}$ cm$^{-2}$ s$^{-1}$ at 1 TeV. Our limit, which is the most constraining result on TeV gamma rays from the Sun, is $\sim 10\%$ of the theoretical maximum flux (based on a model where all incoming cosmic rays produce outgoing photons), which in turn is comparable to the Fermi-LAT data near 100 GeV. The prospects for a first TeV detection of the Sun by HAWC are especially high during solar minimum, which began in early 2018. | Introduction} The Sun is an established source of MeV-GeV gamma rays, containing both transient and steady components. Solar flares, accelerating particles in explosive bursts, produce gamma rays up to 4 GeV via bremsstrahlung and pion decay. This gamma-ray emission has been observed since the 1980s \cite{2014ApJ...787...15A, Kafexhiu:2018wmh,Strong327,Lin2002,2014ApJ...787...15A, Pesce-Rollins:2015hpa,Share:2017tgw}. On the other hand, the observational study of steady-state gamma-ray emission from the Sun --- occurring during both the quiescent and active phases --- has only become possible in the last decade with space-based missions. The definitive evidence of GeV gamma rays from the Sun, first hinted at in archival EGRET data \cite{2008A&A...480..847O}, was found in the initial eighteen months of the Fermi-LAT data \cite{0004-637X-734-2-116}. The gamma rays come in two distinct spatial components: a halo extending up to $20^\circ$ produced by inverse-Compton scattering of low-energy solar photons by cosmic-ray (CR) electrons, and the solar-disk emission, expected to arise from cosmic rays interacting with the solar atmosphere. While the extended emission agrees well with models of inverse-Compton gamma rays \cite{2008ICRC....2..505O,Orlando:2006zs,Moskalenko:2006ta,Orlando:2013pza,Orlando:2017iyc}, there are no good theoretical explanations for the GeV observations of the disk emission. Hadronic interactions between Galactic cosmic rays and the solar atmosphere have long been theorized as the main source of steady emission from the solar disk \cite{1991ApJ...382..652S,1989gros.work.....J,doi:10.1029/JZ071i023p05778}. In the model by Seckel, Stanev and Gaisser, cosmic rays interact with the Sun's atmosphere, undergo reflection in magnetic flux tubes and produce particle cascades (including gamma rays) on their way out \cite{1991ApJ...382..652S}. The theoretical upper bound on the flux from this process, which we denote as CR upper bound, is derived by assuming the maximal production of gamma rays from interactions between the incoming cosmic rays and the solar surface \cite{2018arXiv180305436L}. Surprisingly, the observed flux above 1 GeV is higher than the nominal predictions in Ref. \cite{1991ApJ...382..652S} by almost a factor of 7. The disk emission has been confirmed in follow-up studies utilizing 6 years and 9 years of the Fermi-LAT data with the highest-energy observations extending above 200 GeV~\cite{Ng:2015gya,2018arXiv180305436L,2018arXiv180406846T}. In addition, the gamma-ray flux between 1--100 GeV has been observed to be anti-correlated with solar activity, varying by a factor $\gtrsim3$ between solar minimum and maximum (see Fig. \ref{fig:intro}, which is explained below). An unexplained dip near 40 GeV in the spectrum was also found, and resolved disk images shows polar and equatorial components whose strength varies through the solar cycle \cite{2018arXiv180305436L,2018arXiv180406846T}. Interestingly, the observed spectrum during the last solar minimum (cycle 24) is much harder~($\sim E^{-2.2}$) than that predicted in Ref.~\cite{1991ApJ...382..652S}~($\sim E^{-2.7}$), and reached almost $\sim 10\%$ of the CR upper bound~\cite{2018arXiv180406846T}. This flux, if continued into the TeV range would represent a flux as high as 10$\%$ of the Crab nebula \cite{2017ApJ...843...39A}; this strongly motivates extending the measurements into the TeV range. \begin{figure}[t!] \centering \includegraphics[width=0.52\textwidth]{intro_update.pdf} \caption{The solar atmospheric gamma-ray flux measured in the GeV range~\cite{2018arXiv180305436L,2018arXiv180406846T} and its observational limits \cite{2016ARGOGAMMA} and prospects in the TeV range with HAWC \cite{2017ApJ...843...39A}. We show the 1 year sensitivity of HAWC for a $E^{-2.63}$ source for scale, and compute the actual sensitivity to Sun in this work. We focus on the disk emission, for which Fermi-LAT data is approaching the theoretical maximum. The inverse-Compton emission from the halo is expected to be small in the TeV range \cite{2016arXiv161202420Z}.} \label{fig:intro} \end{figure} A further motivation comes from the search for new physics in the TeV range. The Sun may capture and accumulate dark matter at its core, which then annihilates into Standard Model particles to produce observable neutrinos~\cite{Gould:1991hx,PhysRevLett.55.257,1995NuPhS..43..265E,2004PhRvD..69l3505L,2009arXiv0908.0899F,2009PhRvD..79j3532P,2011JCAP...09..029R,Danninger:2014xza,Choi:2015ara,Aartsen:2016zhm,2017JCAP...05..046W,Garani:2017jcj,Baum:2016oow}, and in some dark matter models~\cite{Meade:2009mu, 2010PhRvD..81g5004B,2010PhRvD..81a6002S, Bell:2011sn, Feng:2016ijc, Adrian-Martinez:2016gti, 2017PhRvD..95l3016L, Arina:2017sng, Smolinsky:2017fvb}, other observables including gamma rays. Observing the Sun at the highest accessible energies would not only help in fully understanding the hadronic emission of gamma-rays and the accompanying high-energy neutrinos~\cite{0954-3899-19-9-019,Ingelman:1996mj, Andersen:2011dz, 2017PhRvD..96j3006N,2017JCAP...07..024A, Edsjo:2017kjk}, but also in searching for dark matter. Figure \ref{fig:intro} summarizes the status of solar disk gamma-ray measurements above 1 GeV and their potential extension into the TeV range. It shows the Fermi-LAT observation during solar minimum~\cite{2018arXiv180406846T} and the 2014--2017 spectrum and upper limit that covers the same time period as the HAWC dataset in this work~(see Sec. \ref{sec:data} below). Also shown is the 1 year HAWC sensitivity: the energy flux required to obtain a 5$\sigma$ detection 50$\%$ of the time, for a point source at the Crab declination~\cite{2017ApJ...843...39A}. This comparison highlights the potential power of HAWC for observing gamma rays from the Sun, especially during solar minimum, which could help identify the expected rigidity cutoff when the gyroradius of the primary cosmic rays reaches the extent of the Sun~ \cite{Ng:2015gya}, as well as understanding the modulation of the gamma-ray flux \cite{2018arXiv180406846T,2014A&ARv..22...78W} Because the maximum gamma-ray energies accessible to satellite experiments like Fermi-LAT are limited to a few hundred GeV \cite{2009ApJ...697.1071A,2017APh....95....6C,PhysRevLett.119.181101}, the Sun can only be studied in TeV gamma rays by ground-based observatories. Most TeV gamma-ray experiments rely on the Imaging Air Cherenkov technique and only take data at night \cite{2013JInst...8P6008A,LESSARD1999243,2014APh....54...67B,2017APh....94...29A}. The High Altitude Water Cherenkov (HAWC) Observatory, offering continuous daytime observations, is one of the few running experiments capable of observing the Sun at TeV energies. HAWC has been collecting data from the Sun since beginning full operations in November 2014 and will continue to monitor the Sun throughout the upcoming solar minimum. The long-term analysis will allow us to study the time variation of the flux at TeV energies. In this paper, which serves as a prelude to the upcoming solar minimum analysis, we describe our first three years of observations of the solar disk conducted in a relatively active portion of the solar cycle. Section \ref{HAWC} briefly introduces HAWC and the procedure of data collection. Section \ref{sec:obs} describes the analysis and the computation of upper limits on the gamma-ray flux. The sensitivity of the measurement with simulations and a discussion of systematic uncertainties are presented in Section \ref{sec:sim}. Section \ref{conc} discusses the results and concludes. The HAWC results have important implications for dark matter searches from the Sun. We explore that aspect of the study in detail in a companion paper \cite{DMPaper}. | 18 | 8 | 1808.05620 |
|
1808 | 1808.07887_arXiv.txt | In anticipation of a LIGO detection of a black hole/neutron star merger, we expand on the intriguing possibility of an electromagnetic counterpart. Black hole/Neutron star mergers could be disappointingly dark since most black holes will be large enough to swallow a neutron star whole, without tidal disruption and without the subsequent fireworks. Encouragingly, we previously found a promising source of luminosity since the black hole and the highly-magnetized neutron star establish an electronic circuit -- a black hole battery. In this paper, arguing against common lore, we consider the electric charge of the black hole as an overlooked source of electromagnetic radiation. Relying on the well known Wald mechanism by which a spinning black hole immersed in an external magnetic field acquires a stable net charge, we show that a strongly-magnetized neutron star in such a binary system will give rise to a large enough charge in the black hole to allow for potentially observable effects. Although the maximum charge is stable, we show there is a continuous flux of charges contributing to the luminosity. Most interestingly, the spinning charged black hole then creates its own magnetic dipole to power a black hole pulsar. | The LIGO collaboration recently announced the first detection of gravitational waves from a neutron star (NS) collision \cite{TheLIGOScientific:2017qsa}. Stepping on the heels of the gravitational wave train, all manner of fireworks are anticipated when the dense neutron-star matter crushes together. Anticipations were beautifully confirmed since the FERMI and INTEGRAL satellites detected a gamma-ray burst from the same direction \cite{Monitor:2017mdv,Goldstein:2017mmi,Savchenko:2017ffs}. Over the next two weeks, dozens of instruments and a significant fraction of the astronomical community directed their focus and witnessed pyrotechnics in the aftermath across the electromagnetic (EM) spectrum \cite{GBM:2017lvd}. The era of multi-messenger astronomy has begun spectacularly. At the other extreme, black hole (BH) collisions are expected to be spectacularly dark. The LIGO BH mergers exhibited no detectable electromagnetic counterpart, although there were intriguing gamma-ray signatures from near GW150914 and GW170104 (\citep[][and see \citet{DOrazioLoeb:2018} and references therein]{Connaughton+2016, AGILE:Verrecchia+2017, Connaughton+2018_Fermi2}, that may or may not have been correlated with the gravitational-wave events. BHs are empty space and their merger will be invisible, unless dressed in ambient debris. The BH collisions were the most powerful events detected since the big bang and yet it is possible that none of the energy came out in the electromagnetic spectrum. All of the energy emanated in the darkness of gravitational waves. Next in the compact object combinatorics will inevitably be black hole/neutron star (BH/NS) collisions. While the tidal disruption of the NS in these systems could occur for the smallest BH partners, resulting in a short gamma-ray burst \citep[\textit{e.g.},][]{NPP:1992,ShibataBHNSLRR:2011}, BHs larger than $\sim 8 \Msun$, will swallow the NS whole -- an expectation further endorsed by the large BHs LIGO observed \citep{LIGO_BBHO1:2016, GW170104:2017, GW170608, GW170814}. Without tidal disruption, there is not an obvious source of light. Fortunately, there is another mechanism for the system to light up: the Black Hole Battery \cite{GLB:1969, McL:2011, DL:2013}. NSs are tremendous magnets. As they whip around a BH companion, the orbiting magnet creates a source of electricity. How this electricity is channeled into a light element remains somewhat uncertain although we have suggested several viable channels, including synchro-curvature radiation, a fireball, and a fast radio burst \cite{DL:2016, MingLevin+2015}. In this article we argue that another largely overlooked EM channel requires further exploration: BH charge. Historically, a dismissive argument has been made that a charged BH will discharge essentially instantaneously, the electromagnetic force being so excessively strong. Any errant charges will easily and swiftly be absorbed from the interstellar medium to counter the charge of the BH, the argument goes. However, as shown in an elegant paper by Wald in 1974 \cite{Wald:1974np}, a BH immersed in a magnetic field actually favors charge energetically. In other words, the BH simply will acquire stable charge if it spins in a magnetic field. We therefore expect a BH battery -- a BH pierced by the field lines of an orbiting NS magnet -- to acquire a significant charge of the Wald value, $Q_W=2 B_o a M$ where $B_o$ is the strength of the NS dipole field at the location of the BH of mass $M$ and $a$ is the spin of the hole. Since magnetic dipoles drop off quickly, by $r^{-3}$, the Wald charge is small until the final stages of merger. See refs.\ \cite{Zhang:2016rli, Fraschetti:2016,Liu:2016olx} for interesting recent studies of electromagnetic counterparts in charged BH/BH mergers, ref.\ \cite{Nathanail:2017wly} for work on gravitational collapse to a charged BH, and ref.\ \cite{Zajacek:2018ycb} which considered the Wald mechanism applied to the central galactic BH. The no-hair theorem is often misinterpreted as enforcing zero magnetic fields on a BH in vacuum. Actually, and more sensibly, the no-hair theorem ensures that the only magnetic field a BH can support is consistent with a monopole of electric charge. A spinning electric charge naturally creates a magnetic dipole. So a spinning charged BH has all of the attributes of a pulsar: spin, a magnetic field, and a strong electric field to create a magnetosphere. We predict the formation of a short-lived and erratic BH pulsar prior to merger that could well survive briefly post-merger before the magnetosphere and charge dissipate. The characteristics of the BH pulsar follow from the NS magnetic field. The Wald charge on a BH immersed in an external NS dipole field, which drops off as the cubed distance between the two, $r^{-3}$, would be \begin{equation} \begin{aligned} Q_W& \approx 10^{-7} M \left (\frac{a}{M}\right )\left (\frac{M}{10 M_\odot}\right )^2 \left (\frac{B_{NS}}{10^{12} G}\right ) \left (\frac{R_{NS}}{r}\right )^3. \label{Eq:QW} \end{aligned} \end{equation} Here $B_{NS}$ is the NS's magnetic field at the surface of the NS and $R_{NS}$ is the radius of the NS. Note that $r\geq R_{NS}$. At it's maximum, $Q_{W,{\rm max}}\sim 10^{-7} M$ (which comes to $\approx 10^{24}\,{\rm statCoulombs}$), so we can still use the Kerr solution. Assuming a NS with a mass of $1.4 \Msun$ and angular spin frequency of $\Omega_{\rm NS}=0.1$ seconds, we find that when the BH enters the light cylinder of the NS, $r=R_{LC} = c / \Omega_{\rm NS}$, the charge is 10 billion times smaller, $Q_{LC}\sim 10^{-10} Q_{W,{\rm max}}$. Over the next $t_{\rm merger}-t_{LC}\sim 3$ years the charge increases. In the final minute of inspiral, when the binary is emitting at $\gtrsim 17$~Hz, in the LIGO band, the charge increases by a factor of a million. As $r\rightarrow R_{NS}$, $Q\rightarrow 10^{33} e$ which is only about $10^3$ kgs of electrons. For reassurance that the black hole actually has time to acquire charge, we estimate the charging timescale. While there are lots of uncertainties in such an assessment, we consider an initially vacuum configuration that siphons charge from the magnetosphere of the NS. Then the charging timescale can be estimated as the light crossing time of the BH/NS system, $r/c$. The ratio of GW inspiral time to the charging time is $t_{\rm GW}/(r/c) \approx 1.5 (r/(2GM/c^2))^3$ for the fiducial binary values chosen here, which confirms that the charging timescale is much shorter than the inspiral timescale until merger. Longer charging timescales could arise in non-vacuum, force-free magnetospheres \citep[\textit{e.g.},][]{LyutikovMckinney:2011}. However, because of the $r^3$ dependence in the timescale ratio above, one would need the charging timescale to be $\mathcal{O}(10^3)$ times longer than the light crossing time to mitigate the BH charge in the last second of inspiral. This estimate is encouraging, suggesting that the black hole would have time to charge before merger. Once charged, the spinning black hole supports a magnetic dipole field. Take the magnetic dipole moment of the BH to be of order ${\sc m} \sim Q_W M$. The BH $B$-field is comparable to, though of course less than, the field in which it's submerged. We can estimate the magnitude of the $B$-field as $B_{BH} \sim m/r^3$. Then using $Q_W=2B_oaM$ with $B_o$ given by the dipole field of the NS at the location of the BH, \begin{equation} B_{BH} = \frac{1}{2} \left (\frac{a}{M}\right )^2 B_{NS} \left( \frac{R_{NS}}{r}\right)^{3} \left( \frac{2M}{r}\right)^{3} \end{equation} One factor of $a$ determines the magnitude of the Wald charge while the other determines the magnitude of the magnetic moment sourced by the spinning, charged BH. Pulsars are hard to see far away (\textit{i.e.}\ outside of the galaxy), so we consider other channels for luminosity than just the BH pulsar. In addition to the BH pulsar, we suggest that the flux of charge around the BH will create significant luminosities potentially detectable for the range of instruments in the LIGO network. There are two clear opportunities for particle acceleration: At the moment the BH charges up pre-merger and the moment the BH discharges post-merger. A third interesting possibility is the continual fluxing of charges within the magnetosphere. Although the Wald charge appears to be stable, negative and positive charges continue to course along field lines since in vacuum $E\cdot B\ne 0$. And, as we discuss in \S \ref{S:ParticleAcceleration}, there is no value of the charge for which $E\cdot B=0$ everywhere. As an order of magnitude estimate, we calculate the total power that could be released if a fraction $f$ of the power associated with the Wald charge in the Wald electric field, $E_W$, were released, \begin{equation} \nonumber f Q_{W}E_{W} c \approx 2 \times 10^{45} {\rm{erg} \ \rm{s}^{-1} } f \left(\frac{B_{NS}}{10^{12} \rm{G}}\right)^2 \left(\frac{R_{NS}}{r} \right)^6 \left(\frac{M}{10 \Msun}\right)^2. \end{equation} where, for the electric field, we use the horizon Wald electric field at the poles, within an immersing magnetic field corresponding to a NS with surface mangetic feld $B_{NS}$, at a distance of $3R_{NS}$, \begin{equation} E_W \approx 3.3 \times 10^{10} \rm{statV/cm} \left( \frac{B_{NS}}{10^{12}\rm{G}}\right ) \\ . \end{equation} This is of order the largest electric field achievable in the system and will decrease for larger BHs that cannot approach as closely the magnetic field of the NS. Now, it's fair to expect that given the large electric fields involved, the BH will create its own magnetosphere \citep[\textit{e.g.},][]{RudSuth:1975, Blandford:1977ds}, as well as enter the magnetosphere of the NS. As the system transitions from vacuum to force-free, the Wald argument no longer holds. Do force-free BH systems also have charge and regions of particle acceleration, as a neutron star pulsar does? That remains an open question that we intend to investigate in full numerical general relativity. Compellingly, we do show that even the classic Blandford-Znajek solution has a small charge. It's also worth noting that the Goldreich-Julian pulsar \cite{Goldreich:1969sb} is force-free and charged \cite{1975ApJ...201..783C}. Before we proceed, a quick comment on notation. Where unambiguous, we'll suppress index notation and use a $\cdot$ to indicate a sum over $4$-indices. Between vectors this is unambiguous. For tensors, the order determines the index to be summed. By example, for 2-tensors (or pseudo-tensors) $H$ and $K$, $H\cdot K$ sums the final index of $H$ over the first index of $K$. Explicitly $H\cdot K=H_{\alpha \mu}K^{\mu \beta}$. The placement of the free indices up or down is ambiguous in this notation. A double $\cdot \cdot$ means $H\cdot \cdot K=H_{\alpha \mu} \cdot K^{\mu \beta}=H_{\alpha \mu} K^{\mu \alpha}$. We'll resort to explicit indices as required in context. We'll work as generally as possible but when the time comes to restrict to the particular Kerr metric, we use Boyer-Linquist coordinates: \begin{eqnarray} ds^2 = &-& \left (1-\frac{2Mr}{\Sigma}\right ) dt^2 +\frac{\Sigma}{\Delta} dr^2 + \Sigma d\theta^2 \nonumber \\ &+& \frac{(r^2+a^2)^2-\Delta a^2\sin^2\theta}{\Sigma}\sin^2\theta d\phi^2 \nonumber \\ &-& \frac{4Mar\sin^2\theta}{\Sigma} dt d\phi\,, \label{Eq:BL} \end{eqnarray} with \begin{eqnarray} \Sigma & =& r^2 + a^2\cos^2\theta\,, \nonumber \\ \Delta &=& r^2+a^2 - 2Mr\,. \end{eqnarray} There are a number of useful metric quantities that greatly ease calculations and that we compile in Appendix \ref{App:A}. The paper is outlined as follows. In Section \ref{S:Review of Wald's Argument}, we review Wald's argument for the charging up of a Kerr BH in a uniform magnetic field (the Wald solution). In Section \ref{S:Equations of Motion} we present the equations of motion for test charges in the Wald solution. In Section \ref{S:Charge Accretion} we consider charge accretion in the Wald solution, at the poles of the BH, and its need for generalization to charge accretion in the global spacetime. Section \ref{S:ParticleAcceleration} presents numerical solutions to the equations of motion for test charges in the Wald fields addressing the question of global charge accretion. Section \ref{S:ParticleAcceleration} also considers EM emission from the acceleration of test charges in the Wald field. Section \ref{S:FF} briefly considers BH charge in the force free limit. Section \ref{S:Summary} concludes. | \label{S:Summary} The wealth of information gained from the NS/NS merger GW170817 and GRB170817 speaks compellingly to the prodigious importance of electromagnetic counterparts to gravitational-wave signals. Arguing against convention, in this paper we have put forth the idea that a valuable counterpart to a BH/NS merger may exist by leveraging the charge BHs can support. BH charge is typically dismissed in astrophysical settings based on the expectation that charge will be both negligibly tiny and/or extremely short- lived. The presumption that charge is short-lived is countered by the Wald mechanism --- a rotating BH embedded in an external magnetic field will accrete a stable net charge. Further, the charge need not be tiny given the magnitude of strong NS B-fields and rather could be relevant to observations. A simple estimate of the magnetic field created by the BH as charge reaches its maximum value immediately before the merger with a strongly magnetized NS gives $B_{BH}\sim (a/M)^2 B_{NS}/2$, comparable to the NS magnetic field for highly spinning BHs. As found observationally, and through theoretical investigation, whether or not a NS can generate a magnetosphere and produce pulsar emission depends on the spin period of the NS. For example, \citet{Sturrock:1971} and \citet{RudSuth:1975} calculate that a NS must have a period shorter than $\sim1.7 (B_{NS}/10^{12})^{8/13}$ seconds to sustain charge acceleration across a vacuum gap and hence the pulsar magnetosphere. The spin period of a maximally spinning, $10\Msun$ BH is of order milliseconds. Hence, if the analogy can be applied to the BH-pulsar case, this means that BHs sourcing magnetic fields above $10^7-10^8$~G should be able to sustain a magnetosphere, and possibly drive an emission mechanism similar to that of the pulsar case. Promisingly, recent numerical work has employed particle-in-cell simulations of BH magnetospheres finding that small polar gaps, analogous to the NS-pulsar case, can be opened and result in particle acceleration \citep[see][and references therein]{Chen+2018}. For mergers involving NS surface magnetic fields of $B_{NS}\sim10^{12}$~G, the final $\sim 20M$ of inspiral, would allow the BH to source a magnetic dipole field of $\gtrsim 10^{8}$~G, above the pulsar limit. It should be emphasized, however, that this ``black hole pulsar,'' as we have called it, has an essential difference relative to a NS: its magnetic field is created by a rotating electric charge, unlike the star's intrinsic dipole field. After all, a co-rotating observer sees only an electric field due to the charge on the BH. Granted, the pulsar features of such a BH may be hard to observe given its short lifetime and their scarcity within galactic distances. And any detailed predictions would require an analysis of the generalization to a time-dependent, non-uniform external magnetic field. Another source of luminosity can stem from the acceleration of charges surrounding a BH, which we have shown is not precluded by the stability of the net charge. The vacuum situation we considered suggests some interesting properties, as demonstrated in the complex, likely chaotic, dynamics. And even though our estimates for the emitted power via curvature radiation are large enough to be interesting (of order kilonova luminosities \citep[\textit{e.g.},][]{MetzgerKilonova+2010}), a more accurate prediction would pose similar difficulties that make the NS-pulsar studies so challenging. We began with vacuum solutions, however the BH may well create its own force-free magnetosphere. If that transpires, we can ask whether a BH charge and its associated effects should then be dismissed. Again, against expectation, we showed that a BH enclosed by a force-free magnetosphere does in fact carry charge. Still, the situation we focused on --- the Blandford--Znajek split monopole --- is an approximate force-free solution valid for small $a/M$, leading to a correspondingly small electric charge. We believe nonetheless that this outcome is interesting enough to motivate a more thorough numerical study on the existence of electric charge in force-free BH magnetospheres. We note that, without speculating on the origin of charge on BH/BH pairs, the same mechanisms would be at work to illuminate these systems if they exist. Finally, it is interesting to speculate, should an electromagnetic counterpart to a BH/NS or BH/BH merger be observed, about the prospects of testing fundamental physics. The no-hair theorem immediately comes to mind, as the detection of a BH pulsar could in principle be sensitive to an intrinsic magnetic (dipole or higher) moment. A positive detection could also be used to constrain modified gravity theories in which the analogue of the Wald mechanism \citep[\textit{e.g.},][]{PCWald:2017} might differ significantly from that in general relativity. | 18 | 8 | 1808.07887 |
1808 | 1808.00972_arXiv.txt | Late-time observations of Type Ia supernovae (SNe Ia), $>900$ days after explosion, have shown that this type of SN does not suffer an ``IR catastrophe'' at 500 days as previously predicted. Instead, several groups have observed a slow-down in the optical light curves of these SNe. A few reasons have been suggested for this slow-down, from a changing fraction of positrons reprocessed by the expanding ejecta, through a boost of energy from slow radioactive decay chains such as $^{57}$Co$\to^{57}$Fe, to atomic ``freeze-out.'' Discovering which of these (or some other) heating mechanisms is behind the slow-down will directly impact studies of SN Ia progenitors, explosion models, and nebular-stage physics. Recently, \citet{2018ApJ...859...79G} suggested a possible correlation between the shape of the late-time light curves of four SNe Ia and their stretch values, which are proxies for their intrinsic luminosities. Here, we present {\it Hubble Space Telescope} observations of the SN Ia ASASSN-14lp at $\sim 850$--960 days past maximum light. With a stretch of $s=1.15\pm0.05$, it is the most luminous normal SN Ia observed so far at these late times. We rule out contamination by light echoes and show that the late-time, optical light curve of ASASSN-14lp is flatter than that of previous SNe Ia observed at late times. This result is in line with---and strengthens---the \citet{2018ApJ...859...79G} correlation, but additional SNe are needed to verify it. | \label{sec:intro} The natures of the progenitors and explosion mechanism of Type Ia supernovae (SNe Ia) remain open questions (e.g., \citealt{2014ARA&A..52..107M}). Most SNe Ia are only studied for the first months of their lives, but over the last few years it has become clear that the very late nebular stage ($>500$ days after explosion) offers fresh clues to the progenitor and explosion physics of these SNe. \citet{1980PhDT.........1A} predicted that $\sim500$ days after explosion, the cooling of the SN ejecta, which until this time proceeded through transitions in the optical, should switch to fine-structure transitions in the infrared (IR), resulting in a steep drop in the optical light curve --- the so-called ``IR catastrophe.'' To date, though, no such drop has been observed (e.g., \citealt{1997A&A...328..203C,2009A&A...505..265L,2014ApJ...796L..26K,2017ApJ...841...48S}). Instead, the optical decline of the light curve seems to slow down. Additional heating channels beyond the radioactive decay of $^{56}$Co may be required to explain this slow-down. Several such channels have been suggested, such as slow radioactive decay chains (e.g., $^{57}$Co$\to^{57}$Fe with $t_{1/2}\approx272$ days) that begin to dominate the SN light at late times \citep{2009MNRAS.400..531S}, a changing fraction of positrons trapped by the ejecta as they continue to expand \citep{2017MNRAS.468.3798D,2017MNRAS.472.2534K}, or a ``freeze-out'' stage at which the recombination and cooling timescales become longer than the radioactive decay timescale, and the conversion of energy absorbed by the ejecta into emitted radiation is no longer instantaneous \citep{1993ApJ...408L..25F,2015ApJ...814L...2F}. Thus the shape of a SN Ia light curve at $>500$ days encodes information about the progenitor of the SN, how it exploded, and the physics of its nebular phase. \citet{2016ApJ...819...31G} first showed that the light curve of SN 2012cg was consistent with a combination of $^{56}$Co and $^{57}$Co radioactive decays. The same was shown by \citet{2017ApJ...841...48S} for SN 2011fe and by \citet{2018ApJ...852...89Y} for SN 2014J. According to \citet{2017ApJ...841...48S}, the late-time photometry of SN 2011fe was precise enough to rule out the single-degenerate progenitor scenario \citep{Whelan1973}, but \citet{2017MNRAS.468.3798D} and \citet{2017MNRAS.472.2534K} showed that similar data could also be fit with models that assumed either freeze-out or a changing fraction of positron trapping. A single SN, then, does not have sufficient power to rule out any of the existing models. \citet[G18]{2018ApJ...859...79G} showed a tentative correlation between the stretch of SNe Ia (a proxy for their intrinsic luminosities, as codified by the \citealt{1993ApJ...413L.105P} relation) and the shape of their late-time light curves. Such a correlation has the potential to constrain all of the nebular, explosion, and progenitor models described above. However, it is based on only four SNe -- the only ones followed out to sufficiently late times. Recently, \citet{2018ApJ...857...88J} cast doubt on this correlation, based on a single observation of SN 2013aa at $\approx 1500$ days. Here, we present \textit{Hubble Space Telescope} (\hst) observations of ASASSN-14lp between $\sim 850$ and $\sim 960$ days past maximum light. In Section~\ref{sec:data}, we photometer the SN and construct its pseudo-bolometric light curve. In Section~\ref{sec:analysis}, we show that the SN is not contaminated by light echoes, and compare it to the four SNe studied by G18. Although the data for ASASSN-14lp are not enough to fit the various heating mechanisms discussed above, they indicate the continuation of the correlation claimed by G18, namely that more luminous SNe Ia (with higher stretch values) have flatter late-time light curves. We offer concluding remarks in Section~\ref{sec:discuss}. \begin{figure*} \centering \includegraphics[width=0.97\textwidth]{fig1.jpg} \caption{\textit{HST} color composite of ASASSN-14lp near the core of NGC 4666. The image is composed of $F814W$ (red), $F555W$ (green), and $F438W$ (blue) images from all visits to NGC 4666 in program GO--14611. The location of the SN is marked by a $2^{\prime\prime}\times2^{\prime\prime}$ white box, which is blown up in the inset, where ASASSN-14lp is identified by a white reticle. North is up and East is to the left, tilted by $21^{\circ}$ counter-clockwise.} \label{fig:host} \end{figure*} | \label{sec:discuss} We used \hst\ to observe the luminous, normal SN Ia ASASSN-14lp in the wavelength range $\sim3500$--$10000$~\AA\ when the SN was $\approx 850$--$960$ days past maximum light. As with previous SNe Ia observed at these late times, there is no evidence of an ``IR catastrophe.'' Instead, the light curve of ASASSN-14lp is seen to flatten out. We used the colors of this SN to rule out contamination by a light echo, so we surmise that the slow-down of the light curve should be due to one or more heating mechanisms that kick in at late times and dominate over the ongoing nuclear decay of $^{56}$Co. We set out to test the claim made by G18 that more luminous SNe, as described by their stretch values, have flatter late-time light curves. ASASSN-14lp seems to follow this trend. However, the phase range of the observations is too short to constrain the theoretical heating models tested in previous works or to use the $\Delta L_{900}$ parameter designed by G18. Instead, we parameterize the late-time light curves with a new parameter, $T_{900}$, which accounts for both the convergence of the light curves at $\sim 500$--$700$ days, and their diverging slopes at $>800$ days. The $T_{900}$ values of ASASSN-14lp and the four SNe Ia tested by G18 are formally correlated according to the statistical tests used here. However, because our analysis is based on only five objects, we refrain from claiming a high statistical significance. To conduct a definitive test of the possible correlation between the luminosities of the SNe Ia and the shapes of their late-time light curves, a larger set of SNe Ia needs to be observed with \hst\ in a self-consistent experiment, i.e., by using the same observational setup and covering the entire phase range of $600$--$1000$ days. Finally, we encourage independent tests of the G18 correlation. \citet{2018ApJ...857...88J} conducted the first such test. Comparing between one late-time observation of SN 2013aa at $\approx 1500$ days and earlier observations at $<400$ days, they claimed that the SN significantly fell off the correlation. Because this SN had no data in the phase range probed here, we could not analyze it together with ASASSN-14lp. We also caution that multiple observations are required at $>800$ days to properly describe the shape of the late-time light curve. Still, SN 2013aa underscores the need for more SNe Ia to properly test the claim made by G18. | 18 | 8 | 1808.00972 |
1808 | 1808.00375_arXiv.txt | Formation of halos in the Dark Ages from initial spherical perturbations is analyzed in a four component Universe (dark matter, dark energy, baryonic matter and radiation) in the approximation of relativistic hydrodynamics. Evolution of density and velocity perturbations of each component is obtained by integration of a system of nine differential equations from $z=10^8$ up to virialization, which is described phenomenologically. It is shown that the number density of dark matter halos with masses $M\sim10^8-10^9\,\mathrm{M_{\odot}}$ virialized at $z\sim10$ is close to the number density of galaxies in comoving coordinates. The dynamical dark energy of classical scalar field type does not significantly influence the evolution of the other components, but dark energy with a small value of effective sound speed can affect the final halo state. Simultaneously, the formation/dissociation of the first molecules have been analyzed in the halos which are forming. The results show that number densities of molecules $\rm{H_2}$ and $\rm{HD}$ at the moment of halo virialization are $\sim10^3$ and $\sim400$ times larger, respectively, than on a uniformly expanding background. It is caused by increased density and rates of reactions at quasilinear and nonlinear evolution stages of density and velocity of the baryonic component of halos. It is shown also that the temperature history of the halo is important for calculating the concentration of molecular ions with low binding energy. So, in a halo with virial temperature $\sim10^5$ K the number density of the molecular ion HeH$^+$ is approximately 100 times smaller than that on the cosmological background. | Molecules in the Dark Ages are an important subject of study for a few reasons. First of all, they are effective coolers of collapsing gas in the processes of the first stars formation. Without scrupulous account of their role in these processes, we cannot be sure that we know when the first sources of light appeared and which they were. Secondly, molecules are able to scatter and absorb the quanta of cosmic microwave background, influence their energy and spatial distributions. They can be detected in the next generation CMB experiments. At last the Dark Ages molecules can be a source of light from the Dark Ages which brings the new information about that epoch. The growing list of molecules and possible reactions can be found in \citet{Dubrovich1977,Izotov1984,Lepp1984,Dalgarno1987,Puy1993,Galli1998,Stancil1998,Puy2002,Hirata2006,Vonlanthen2009,Safranek2010,Gay2011}. The current list of Dark Ages ingredients contains $\sim30$ species and $\sim250$ reactions (see reviews \citet{Lepp1998,Lepp2002,Galli2013}). The main knowledge about primordial chemistry follows from the computations of cosmological recombination of Hydrogen, Deuterium and Helium and formation of neutral and ion molecules after recombination and before re-ionization by the first stars and galaxies ($10\le z\le 1000$) in the $\Lambda$CDM model. It was shown that the number density of molecules in the Dark Ages crucially depends on the number density of free electrons and protons. The evaluation of ionized fractions depends on the accuracy of computation of all atomic and photonic processes during cosmological recombination, existence of additional sources of ionization (e.g. decaying, annihilating dark matter particles etc.) and rate of expansion of the Universe, which depends on the assumption about nature of dark energy. It was shown also that only simple diatomic and triatomic molecules and molecular ions containing H, D, He, and Li are formed in trace amounts during the Dark Ages. Our recent computation for $\Lambda$CDM model with Planck parameters \citep{Planck2015} in the case of absence of sources of re-ionization before $z=10$ shown that the relative number densities (in the units of Hydrogen abundance) are $2.4\times10^{-6}$ for molecule $\rm H_2$, $1.1\times10^{-9}$ for ${\rm HD}$, $1.3\times10^{-13}$ for ${\rm H_2^+}$ and $8.5\times10^{-14}$ for ${\rm HeH^+}$ \citep{Novosyadlyj2017c}. It was found also that the uncertainties of molecular abundances caused by the inaccuracies of computation of cosmological recombination are about 2-3\%. The uncertainties of values of cosmological parameters affect the abundances of molecules at the level of up to 2\%. The primordial molecules allow the gas to cool, contract and fragment, that is very important for estimation of mass function of the first luminous objects. Since the cosmological perturbations exist and evolve, the concentrations of the molecules change over time in different places in different ways. They are determined by the dynamics of change of baryonic density, temperature, radiation spectrum and intensity and by the dependence of effective cross sections of molecular formation/destruction reactions on the physical state of the baryonic matter. The number densities of molecules decrease on a uniformly expanding background (further cosmological background or c.b.) and increase in the clouds which contract and virialize. We can compute the evolution of molecular number density on the cosmological background with accuracy which is defined by accuracies of cosmological parameters (a few percents, as it was mentioned above) and accuracies of cross-sections, which are known a bit worse. At the same time the computation of molecular fractions in the forming halos is less certain, since it is dependent on models of halo formation, which are mostly phenomenological in different aspects (see \citet{Lepp1998,Barkana2001,Padmanabhan2002,Lepp2002,Bromm2011,Galli2013} and citing therein). In this paper, we study formation of the first molecules in the halos which virialize at the end of the Dark Ages and compare molecular number densities with ones on the cosmological background. We describe the evolution of spherical perturbations in the multicomponent medium from the early stage when the corresponding peaks in the Gaussian field of cosmological perturbations as their seeds were super-horizon up to virialization of dark matter halos. Two models of baryonic gas behavior are considered: (i) when it is adiabatic at all stages and (ii) when it reaches the virial temperature after virialization of dark matter halo. In the second section, we describe the model of spherical scalar perturbations in the four-component medium (dark matter, dark energy, baryonic gas and thermal relic radiation), equations, initial conditions, method of integration and evolution of density and velocity perturbation amplitudes in the central part of spherical overdensities with baryonic mass $\sim10^8-10^9$ $M_{\odot}$. In the third section, we analyze the formation of molecules in the halo and on the cosmological background during the Dark Ages before re-ionization by first stars and compare them. Discussions and conclusions are presented in the fourth section. | We have analyzed the formation of spherical halos with $M\sim10^8-10^9\,\mathrm{M_{\odot}}$ which are virialized in the Dark Ages at $10\le z\le30$ in the four-component medium: dark matter, dark energy, baryons and radiation. It is shown that dark matter halos can virialize at $z\approx30$ if they are forming from high density peaks in the Gaussian field of initial density perturbations with $\delta_{\rm m}^{\rm init}\sim5-6\sigma_{\rm m}$, where $\sigma_{\rm m}$ is rms density fluctuations computed for $\Lambda$CDM model with Planck2015 parameters. The dark matter halos which are forming from peaks with $\delta_{\rm m}^{\rm init}\sim2-3\sigma_{\rm m}$ are virialized at $z\approx10$, their number density (in units Mpc$^{-3}$) are close to the number density of bright galaxies estimated on the base of galaxy redshift catalogues. The radiation component is important at the linear stage of evolution of precursors at the radiation-dominated epoch and at decoupling time. The Silk damping effect depresses the amplitude of perturbations in the baryon-photon plasma before and during cosmological recombination (Figs. \ref{vir}-\ref{vir_st}). The density and velocity perturbations in the dark energy component oscillate after entering the particle horizon when its effective sound speed is equal to the speed of light as it is in the case when the classical scalar field is dark energy. In this case, the dark energy perturbations are not important for dynamic of halo formation after its entering into the particle horizon (Fig. \ref{vir}). In the case when the effective sound speed is small, comparable with the effective sound speed in baryonic matter at the end of the Dark Ages for example, then such dark energy can infall into forming halo of dark matter, reaching the state of hydrostatic equilibrium (Fig. \ref{vir_st}). Such dark energy can be important also at the late stage of halo formation. The evolution of the baryonic component is most important since it can be observable. At the end of cosmological recombination the baryonic matter on the interested here scales have been smoothed, but immediately after recombination it starts to free fall into the potential wells of dark matter halo seeds, thus the amplitudes of density and velocity perturbations in both components become practically the same at $z\approx200$ (Fig. \ref{vir}). Further evolution of the dark matter and baryonic matter in halo is the same up to the beginning of virialization when the heating of gas makes its behavior again different. The gas in halos and on the cosmological background had different dynamical and thermal history, so, the molecular fractions can be different too. To estimate such difference we have analyzed the kinetics of formation and dissociation of molecules and molecular ions in all stages of the dark matter halo formation: the linear and quasi-linear stages when dark matter overdensity expanded and non-linear stages when it turnarounds, collapses and virializes. We have simplified the description of the last phase by stopping infall at the virial density $\rho_{\rm m}^{\rm (vir)}=\Delta_{\rm v}\rho_{\rm m}(a_{\rm v})$ (see details in \citet{Novosyadlyj2016}). The temperature of gas was equal to the temperature of radiation up to $z\approx800$, later it has been defined by adiabatic expansion before the turn around and adiabatic compression after it, that is shown in the left panel of Fig. \ref{rho-t-sj} by dark solid lines for halos virialized at $10\le z\le30$. Thin solid red lines show the virial temperatures (\ref{Tvir}) of these halos, which we set by smoothly transition from the adiabatic value to the virial one for short time corresponded $\Delta a=0.1a_{\rm v}$ immediately after $a_{\rm v}$. One can see that in any case the key parameters of virialized halo - density and temperature, - are defined by the moment of virialization $a_{\rm v}$. The estimated number densities of atoms, molecules and their ions in halo are essentially larger than on the cosmological background. At the moment of halo turnaround the number densities of neutral atoms HI, DI and HeI are by 5.6 times larger than corresponding values on the cosmological background. Such ratio of number densities equals to the density contrast for top-hat halo which follows from the well known Tolman model of dust-like spherical cloud. For the number density of molecules H$_2$ and HD, which are important coolers of Dark Ages protostar clouds, these ratios are 13 and 21 accordingly (Table \ref{h2f}, 3rd column). For the moment when the density contrast of collapsing halo reaches the contrast $\Delta_{\rm v}\approx178$, following from the virial theorem, the ratios $n_{\rm i}^{\rm halo}/n_{\rm i}^{\rm (c.b.)}$ for H$_2$ and HD $\approx1000$ and $\approx400$ accordingly, while for the neutral atoms HI, DI and HeI they are equal 178. This effect is explained by crucial dependence of chemical reaction chains, which lead to formation of these molecules, on the local density and temperature of Hydrogen-Deuterium slightly ionized gas. Taking into account this effect for computation of cooling/heating processes in the Dark Ages halos can help us to resolve the problem of fragmentation of primordial medium into protostar clouds with mass $\lesssim10^3\,\mathrm{M_{\odot}}$. We plan to do that in the next work. | 18 | 8 | 1808.00375 |
1808 | 1808.06848_arXiv.txt | Coalescing \nsbh binaries are promising sources of \acp{GW} that are predicted to be detected within the next few years by current \ac{GW} observatories. If the \ac{NS} is tidally disrupted outside the \ac{BH} innermost stable circular orbit, an accretion torus may form, and this could eventually power a \ac{SGRB}. The observation of an \ac{SGRB} in coincidence with gravitational radiation from an \nsbh coalescence would confirm the association between the two phenomena and also give us new insights into \ac{NS} physics. We present here a new method to measure \ac{NS} radii and thus constrain the \ac{NS} \acl{EOS} using joint \ac{SGRB} and \ac{GW} observations of \nsbh mergers. We show that in the event of a joint detection with a realistic \ac{GW} \ac{SNR} of $10$, the \ac{NS} radius can be constrained to $\lesssim 20$\% accuracy at 90\% confidence. | \acresetall The first observation of a binary \ac{BH} merger in \acp{GW} made by Advanced LIGO, GW150914, marked the dawn of the \ac{GW} astronomy era~\citep{GW150914}. Subsequently, the LIGO-Virgo Collaboration reported other nine binary \ac{BH} merger observations~\citep{GW151226, GW170104, GW170608, GW170814, LIGOScientific:2018mvr}, and the detection of GW170817, a signal that is consistent with a binary \ac{NS} inspiral~\citep{GW170817}. % \citet{Hinderer2018} showed that \nsbh systems with certain parameter combinations are also consistent with the \ac{GW} and \ac{EM} observations of GW170817. Second-generation \ac{GW} detectors --- {\it i.e.}, Advanced LIGO~\citep{AdvLIGO}, Virgo~\citep{AdvVirgo}, KAGRA~\citep{KAGRA}, and LIGO-India~\citep{M1100296, LigoIndia} --- will also be able to detect the \ac{GW} radiation emitted by \nsbh coalescing binaries, a category of compact binary that remains to be observed. In addition to \acp{GW}, among the reasons of interest in coalescing \nsbh binaries is the possibility that if the \ac{NS} is tidally disrupted outside the \ac{ISCO} of its \ac{BH} companion, matter can be accreted onto the \ac{BH}, powering a \ac{SGRB}~\citep{Nakar:2007yr}. We now know that a binary \ac{NS} merger can power an \ac{SGRB}~\citep{GW170817GRB}, and future joint \ac{GW}-\ac{EM} observations will be able to determine whether this is true for \nsbh systems too. Naturally, such observations are intrinsically challenging due to the low expected \joint joint detection rate for \nsbh binaries. This is predicted by \citet{C15} to be $0.4$--$10\,\mathrm{yr^{-1}}$ for LIGO-Virgo at design sensitivity and an idealized \ac{SGRB} observing facility with all-sky coverage, in line with earlier results from \citet{Nissanke:2012dj} (up to $3\,\mathrm{yr^{-1}}$ with a three detector network when ignoring source inclination requirements). The estimate drops to $0.03$--$0.7\,\mathrm{yr^{-1}}$ when considering the \emph{Swift} field of view. For comparison, \citet{Wanderman:2014eza} calculated joint detection rates with {\it Swift} and {\it Fermi} of $0.3$--$1.4\,\mathrm{yr^{-1}}$ and $3$--$10\,\mathrm{yr^{-1}}$, respectively, while \citet{Regimbau:2014nxa} determined $0.001$--$0.16\,\mathrm{yr^{-1}}$ in the case of {\it Swift}. The assumptions behind these frameworks are different and we refer the interested reader to the original articles for details. The upcoming third generation of \ac{GW} detectors, however, will have a much larger observational horizon (up to $z\simeq 4$ for \nsbh binaries) which automatically increases the joint detection rate considerably \citep{Punturo:2010zz, ETdesignstudy, Kalogera:2019sui, Sathyaprakash:2019rom}. Further interest in \nsbh binaries is due to the possibility that the tidally disrupted material is ejected away from the \nsbh system, generating an \ac{EM} transient powered by the decay of $r$-process ions (macronova)~\citep{LP98, Kulkarni:2005jw, Metzger10, MB12, Fernandez:2015use, Metzger2017}. Similar to the \ac{SGRB} case, recent \ac{GW}-\ac{EM} observations of GW170817 have confirmed that binary \acp{NS} are sites that host $r$-processes~\citep{GW170817MMA, GW170817kn}, but whether this holds for \nsbh binaries as well, remains to be proven observationally. Whether the \ac{NS} in an \nsbh binary undergoes tidal disruption or not, and the amount of matter that is available for accretion (or to feed into the ejecta) in the event of a tidal disruption, both depend on the physical properties of the \ac{BH} (mass and spin) and of the \ac{NS}, including the currently unknown \ac{EOS} that regulates the microphysics of the \ac{NS}~\citep{Pannarale2010, Foucart2012, Foucart2018}. The \ac{GW} radiation of coalescing \nsbh systems also depends on the source properties, and among them is the \ac{NS} \ac{EOS}~\citep{BC92, KS95, V00, Shibata:2009cn, DFKOT10, Kyutoku2010, KOST11, Lackey2014, LK14, Foucart:2012vn, Foucart:2014nda, Pannarale2013a, Pannarale2015a, Pannarale2015b, Kawaguchi:2015bwa, HTF16, KPP17, DK18}, but it may be hard to constrain the \ac{NS} \ac{EOS} with \nsbh \ac{GW} inspiral signals only~\citep{PannaraleRezzolla2011}. Therefore, the \ac{GW} and \ac{EM} emission of \nsbh binaries that undergo tidal disruption will carry information about all the properties of the progenitor system, and hence about the \ac{NS} \ac{EOS}. \citet{Pannarale:2014rea} showed how joint \ac{GW} and \ac{SGRB} observations of \nsbh coalescences may provide invaluable information about the \ac{NS} \ac{EOS}. On the basis of this observation, we propose a method to exploit such observations in order to constrain the \ac{NS} radius, and thus the \ac{NS} \ac{EOS}. In the scenario in which \nsbh systems are progenitors of \ac{SGRB} central engines, it is reasonable to expect the \ac{SGRB} energy to be proportional to the rest mass of the torus that accretes onto the remnant \ac{BH}. In turn, this mass can be expressed as a function of the mass and spin of the \ac{BH} initially present in the binary, and the \ac{NS} mass and radius~\citep{Foucart2012, Foucart2018}. Our method explores the portion of parameter space that is pinpointed by the \ac{GW} observation --- \ac{GW} Bayesian inference provides posterior distributions for the two masses and the \ac{BH} spin --- and thus determines a posterior distribution for the \ac{NS} radius by imposing the condition that the merger yields a torus sufficiently massive to power the observed \ac{SGRB} energy. Assuming an \ac{SGRB} isotropic energy of $\EISO = 10^{51}\,$erg, we expect to be able to measure the \ac{NS} radius (at 90\% confidence) with $\lesssim 20$\% accuracy, given a \ac{GW} detection with a \ac{SNR} of $10$. This measure is expected to improve for less energetic \acp{SGRB} and \acp{GW} with higher \ac{SNR}. We show that the poorly known parameters that our analysis marginalizes over --- such as the mass-energy conversion efficiency for the \ac{SGRB} --- have a negligible impact on our results, provided the \ac{SGRB} energy is sufficiently low. Our method is well behaved even for (non-isotropic) energies as high as $E_\gamma = 10^{50}\,$erg, thus the restriction is not very limiting. The paper is organized as follows. In Sec.\,\ref{sec:methodology} we describe our method in detail, discussing the poorly constrained parameters involved in the analysis. In Sec.\,\ref{sec:results} we test the method and present the results we obtained by simulating joint \joint observations. Finally, in Sec.\,\ref{sec:discussion} we draw our conclusions. Throughout the paper, we assume geometric units ($G=c=1$), unless otherwise explicitly noted. | \label{sec:discussion} The joint observation of GW170817 and GRB 170817A has unambiguously associated \bns coalescences and \acp{SGRB}~\citep{GW170817GRB} confirming the long-standing hypothesis that \bns binaries are \ac{SGRB} progenitors~\citep{Blinnikov1984, ELPS89, Paczynski1986, ELPS89, P91, NPP92}. While the rate of \bns mergers can accommodate for the rate of observed \ac{SGRB} events~\citep{GW170817GRB}, the question of whether \acp{SGRB} have more than one kind of progenitor remains an open one, and one that future observing runs of current and upcoming \ac{GW} detection facilities will help answer. \nsbh systems, in particular, remain a viable \ac{SGRB} progenitor candidate (see, {\it e.g.}, \citet{Nakar:2007yr}). \citet{C15} determine a projected joint \joint detection rate for \nsbh coalescences of $0.1$--$2\,$yr$^{-1}$ for Advanced LIGO and Virgo at design sensitivity and the {\it Fermi} Gamma-Ray Burst Monitor, which decreases to $0.03$--$0.7\,\mathrm{yr}^{-1}$ with \emph{Swift}. Similarly, \citet{Regimbau:2014nxa} found a joint \joint detection rate with \emph{Swift} of $0.05$--$0.06\,\mathrm{yr^{-1}}$ while \citet{Wanderman:2014eza} found $0.4$--$1\,\mathrm{yr^{-1}}$ ($3$--$6\,\mathrm{yr^{-1}}$ with {\it Fermi} Gamma-Ray Burst Monitor). The next generation of \ac{GW} interferometers will extend the \nsbh detection horizon up to $z\simeq4$~\citep{ETdesignstudy} therefore boosting such detection rates. In this paper, we presented a method based on \citet{Pannarale:2014rea} to exploit joint \joint observations of \nsbh coalescences in order to measure the \ac{NS} radius, and hence, constrain the \ac{EOS} of matter at supranuclear densities. We sample the \ac{GW} posterior distribution of the component masses and the \ac{BH} spin along with uniform prior distributions on other unknown physical parameters involved in the problem --- among which is the \ac{NS} radius (see Sec.\,\ref{sec:methodology} for details) --- and determine a distribution of isotropic gamma-ray energies. This is then combined with the \ac{EM} measurement of the isotropic gamma-ray energy to yield a constraint on the \ac{NS} radius, after marginalizing over all other sampled quantities. \citet{Hinderer2018} performed a similar analysis on GW170817, also using \citet{Foucart2018} and working under the assumption that the event originated from a \nsbh coalescence, but exploiting the \ac{EM} constraints from the kilonova light curve, rather than the \ac{SGRB} energy. In order to test the performance and the robustness of our method, we simulated six joint \joint \nsbh detection scenarios (see Table \ref{tab:cases}). In each case, we compared the injected $\rNS$ value to the posterior distribution recovered by our analysis. While this setup does not allow us to assess systematics in our methodology (see the discussion at the end of Sec.\,\ref{sec:errors}), it is currently the only possible benchmark and it allows us to draw the following first, important conclusions about our method: \begin{itemize} \item The 90\% credible regions we determine always contains the injected value of $\rNS$, regardless of the mass and/or spin of the \ac{BH} in the \nsbh system under consideration. \item With the exception of case \texttt{m100chi070H}, the median of the $\rNS$ posterior distribution is usually higher than the injected \ac{NS} value and it is narrower for lower-energy \acp{SGRB} ({\it i.e.}, $\EISO\lesssim 10^{50}\,$erg). \item We can constrain the \ac{NS} radius with an uncertainty (quantified from a 90\% of credible interval) below 20\% even for low \ac{SNR} events. \item The $\rNS$ lower bound is rather solid and depends mostly on the \ac{SNR} of the \ac{GW} signal through the informed prior for the \ac{GW} parameters. \item By directly sampling the posterior distributions of \ac{GW} parameter estimation analyses, our method inherits any uncertainty that is present in such distributions. This component of the overall error on the recovered $\rNS$ reduces as the \ac{SNR} of the \ac{GW} increases. However, in Sec.\,\ref{sec:errors} we showed that the \ac{SGRB} energy determines a hard lower limit for the uncertainty on $\rNS$. The value of this contribution to the overall error is clearly \ac{SNR} independent, but it decreases with the \ac{SGRB} energy. For example, for the source configuration considered in Fig.\,\ref{fig:RelError}, this lower limit varies from $\sim 3\%$ to $\sim15\%$ as $\EISO$ goes from $10^{50} \, \rm erg/s$ to $5\times 10^{51} \, \rm erg/s$. \end{itemize} A central ingredient of our method is the fitting formula that predicts the mass of the matter that remains in the surroundings of the remnant \ac{BH} immediately after the merger as a function of the \nsbh initial parameters~\citep{Foucart2018}. This can be replaced as improved or different versions of such formula are published. However, as long as it remains the only available option in the literature, a study of systematics continues to be a time and resource intensive task that would essentially require a campaign of numerical-relativity simulations (see discussion at the end of Sec.\,\ref{sec:errors}). Furthermore, for such a study to be fully self-consistent, one would require simulations that evolve the \nsbh system all the way from inspiral to the ignition of the \ac{SGRB}. For the time being, the tolerance we introduce in Eq.\,(\ref{eq:tolerance}) when comparing our inferred $\EISO$ values to the observed $\EISO$ accounts for systematic uncertainties in the fit of \citet{Foucart2018}, but also for possible differences between the remnant mass that it models and the disk mass that actually accretes onto the central \ac{BH}. These two quantities may differ, for instance, if a non-negligible fraction of remnant mass were to be lost in form of dynamical ejecta or disk winds~\citep{Kawaguchi:2016ana}. Although our method is therefore model-dependent, we note that this is a shared feature of all other existing methods to measure \ac{NS} radii (for a recent review, see \citet{OF16}). For example, $\rNS$ constraints from low-mass X-ray binary observations that are based on spectroscopic measurements of such sources in a quiescent state~\citep{HRNG06, WB07, GRB11, BHOG16} or after a thermonuclear burst~\citep{V79, OGP09, GWCO10, GOCW10, OGG12, GO13} require, among other things, introducing assumptions about the \ac{NS} atmosphere composition and magnetic field. Other methods that involve timing measurements of oscillations in accretion-powered pulsars \citep{PG03, LMC08, LMCC09, LMC11, ML11} require modeling the pulsed waveform and therefore depend on assumptions about \ac{NS} spacetimes and other geometrical factors, such as the shape and location of the surface hotspots. Finally, \ac{EOS} constraints that rely on the analysis of \ac{GW} data, including our method, intrinsically depend on the waveform models used to process the \ac{GW} data and on how these treat tidal effects~\citep{GW170817EOS, GW170817PE}. These examples illustrate that a model dependency is unavoidable when addressing the task of measuring \ac{NS} radii. However, the availability of a number of methods each one of which relies on different assumptions and on the observation of different astrophysical systems is crucial: the combination of results that stem from various approaches can provide a more solid, final result. On the basis of the work carried out in this paper, there are a number of lines of investigation that we plan to explore. Firstly, in the event of an \nsbh detection, a detailed analysis of the \ac{GW} that constrains the \ac{NS} tidal deformability would be carried out, as was the case for the \bns coalescence event GW170817~\citep{GW170817, GW170817EOS, GW170817PE}. In turn, this information and the so-called ``universal relations'' (see, {\it e.g.}, \cite{YY17}{ for a review}) could be exploited to build a less agnostic sampling of the \ac{NS} radius to be used within our approach (currently a uniform prior between $9\,$ and $15\,$km): upper limits on the tidal deformability would result in a narrower interval to be sampled. Moreover, this informed prior on $\rNS$ would also ensure a more consistent sampling of the \ac{NS} mass and radius, with more massive objects associated with higher compactnesses. Furthermore, in the event of an \nsbh merger observation in which the \ac{NS} is disrupted by the \ac{BH} tidal field, the \ac{GW} signal is expected to shut off at a characteristic frequency which depends, among other things, on the \ac{NS} \ac{EOS}~\citep{Shibata:2009cn, KOST11, Pannarale2015a}. The measurement of this frequency would yield constraints on $\rNS$ with a $10$--$40$\% accuracy~\citep{Lackey2014, LK14}, and we want to assess the impact of including such information into our analysis. This scenario is particularly relevant for third-generation \ac{GW} detectors because the shutoff of \nsbh signals happens in the $\sim$kHz \ac{GW} frequency regime. The projected \nsbh detection rate for the Einstein Telescope is $\mathcal{O}(10^3$--$10^7\,$yr$^{-1})$~\citep{ETdesignstudy}. In order to guarantee a high-joint \joint detection rate of such events and to unleash the full potential they have to constrain the \ac{NS} \ac{EOS}, it will be of paramount importance to have functioning high-energy gamma-ray observing facilities during the lifespan of third-generation \ac{GW} detectors. Finally, other independent constraints that would reduce our prior on $\rNS$ are expected to result from ongoing and future missions such as NICER~\citep{2014SPIE.9144E..20A}, ATHENA~\citep{Motch:2013wfn}, and eXTP~\citep{Zhang:2016ach}. | 18 | 8 | 1808.06848 |
1808 | 1808.03140_arXiv.txt | In this paper, we report the line-locking phenomenon of the blended narrow absorption lines (NALs) within trough-like broad absorption lines (BALs) in quasar SDSS J021740.96--085447.9 (hereafter J0217--0854). Utilizing the two-epoch spectroscopic observations of J0217--0854 from the Sloan Digital Sky Survey, we find that each of its C\,{\footnotesize IV}, Si\,{\footnotesize IV}, N\,{\footnotesize V}, and Ly$\alpha$ BAL troughs actually contain at least seven NAL systems. By splitting these BAL troughs into multiple NAL systems, we find that the velocity separations between the NAL systems are similar to their doublet splittings, with some of them matching perfectly. Cases like J0217--0854, showing line-locking signatures of NALs within BAL troughs, offer a direct observational evidence for the idea that radiative forces play a significant role in driving BAL (at least for Type N BAL) outflows. | \label{sec:intro} {The} line-locking, observational signature of the velocity separation between distinct absorption components, similar to the velocity splitting of an absorption doublet and/or multiplet, was reported as early as the 1970s, not long after the discovery of quasars (e.g., \citealp{Williams1975,Coleman1976,Adams1978,Perry1978}). Since then, the line-locking signatures in narrow absorption lines (NALs; absorption widths of less than 500\,$\rm km\,s^{-1}$) have been largely identified in both broad absorption line (BAL) quasars (absorption widths of at least 2000\,$\rm km\,s^{-1}$) and non-BAL quasars (e.g., \citealp{Foltz1987,Borra1996,Tripp1997,Srianand2000,SrianandandPetitjean2000,Vilkoviskij2001,Srianand2002, Ganguly2003,Fechner2004,Gallagher2004,Benn2005,Misawa2007,Misawa2013,Misawa2014, Misawa2016,Misawa2018,Simon2010,Hamann2011,Bowler2014}). Of particular note is a systematic study by \citet{Bowler2014} that showed evidence for line-locking signatures in almost two-thirds of quasars possesses multiple C\,{\footnotesize IV} absorption systems, based on a sample of about 34,000 quasar spectra \citep{Schneider2010} from the Sloan Digital Sky Survey (SDSS; \citealp{York2000}). This study indicates that line-locked C\,{\footnotesize IV} doublets are a very common feature of NAL outflows. Previous studies have also shown that the probability of a line-locking signature accidentally occurring over a relatively small redshift path is negligible \citep{Foltz1987,Srianand2000,Srianand2002,Ganguly2003,Benn2005}, and line-locking is usually interpreted as an observational signature of radiative acceleration (e.g., \citealp{Mushotzky1972,Scargle1973,Braun1989}). Therefore, the line-locking signature can serve as a reliable criterion to distinguish intrinsic NALs from intervening NALs (e.g., \citealp{Misawa2018}, and references therein). \begin{figure*} \includegraphics[width=2.1\columnwidth]{f1.eps} \caption{Spectra of quasar J0217--0854. The SDSS MJDs of the spectra are labeled. The flux density is in units of $\rm 10^{-17}~erg~s^{-1}~cm^{-2}$. The blue vertical dashed lines mark out the main emission lines. The blue horizontal bars {on top of} the spectra are the regions used to fit the power-law continua. The purple and blue solid lines along the two spectra profiles are the pseudo-continuum fits for the spectra. The purple dashed line in the lower panel is the final pseudo-continuum fit for the MJD 52162 spectrum. The dotted lines near the bottom of each panel are the formal 1$\sigma$ errors. Fits to the emission lines are given as green Gaussian profiles in the bottom of each panel. Both the longitudinal and transverse axes are logarithmic.} \label{fig.1} \end{figure*} Though the radiative line-driving in NAL outflows has been proven through confirming the line-locking phenomenon of intrinsic NALs in both BAL and non-BAL quasars, there is no convincing evidence to confirm the line-driven radiative BAL outflows. One piece of evidences for line-driven radiative BAL outflows {is the double} trough signature of Ly$\alpha$--N\,{\footnotesize V} line-locking (the so-called ``ghost of Ly$\alpha$") seen in the mean profile of BALs \citep{Weymann1991,Korista1993,Arav1994, Arav1995,Arav1996,North2006,Cottis2010}. \citet{Arav1995} suggested that the Ly$\alpha$--N\,{\footnotesize V} line-locking signature was due to the increased radiation pressure of Ly$\alpha$ photons on outflowing N\,{\footnotesize V} ions, which can serve as evidence for radiative acceleration in quasars {(see also \citealp{Arav1994,Arav1996})}. However, observational evidence based on a large sample of SDSS quasars \citep{Schneider2010} proved that the presence of the ``ghost of Ly$\alpha$" in the objects that meet all five physical criteria suggested by \citet{Arav1996} is elusive \citep{Cottis2010}. \defcitealias{Lu2018b}{Paper~I} Recently, Lu \& Lin (\citeyear{Lu2018b}, hereafter \citetalias{Lu2018b}) have found that some BALs (hereafter Type N BALs\footnote{It is the same as the ``Type II BALs" in Paper I. It is now changed in order to avoid confusion with the narrow-line AGN that exhibits BAL troughs. Corresponding to the Type N BALs, we rename the other relatively smooth BAL troughs that cannot be decomposed into multiple NALs as ``Type S BALs” (which were called as ``Type I BALs" in Paper I).}) are complexes of NALs in reality and that the splitting of a Type N BAL into multiple NALs can serve as a useful way for probing quasar outflows. In this paper, we report the discovery of the line-locking signatures of NALs within BALs in quasar SDSS J021740.96--085447.9 (hereafter J0217--0854) as a warm-up of our systematic study program on Type N BALs (Lu, W.-J. et al., in preparation). This interesting discovery may offer a convincing evidence for line-driven radiative BAL outflows. The paper is structured as follows. Section \ref{sec:spectro} presents the quasar spectra and describes how we identified the NALs and their line-locking signatures. Section \ref{sec:disscu} contains a discussion and a brief conclusion. Throughout this paper, a $\Lambda$CDM cosmology with parameters $H_0=70\,\rm km\,s^{-1}\,Mpc^{-1}$, $\Omega_{\rm M}=0.3$, and $\Omega_{\Lambda}=0.7$ \citep{Spergel2003} is adopted. \begin{figure} \includegraphics[width=1.01\columnwidth]{f2.eps} \caption{Portions of the normalized spectra of J0217--0854, showing the NAL systems detected within the BALs in different ions. The black and red lines show the normalized spectra from observations on MJD 52162 and 55828, respectively. Normalized fluxes are plotted versus the radial velocity with respect to an emission-line redshift of 2.568. The red, blue, purple, green, orange, dark blue, and dark green vertical lines in each panel mark out the {seven} identified NAL systems. {Gaussian fits with corresponding colors were applied to the NALs within Si\,{\footnotesize IV} and Ly$\alpha$ BALs.} The brown dotted lines in the Si\,{\footnotesize IV} panel represent the total fit model. ``G" marks the strong Si\,{\footnotesize II} $\lambda$1260 Galactic absorption line at redshift of 2.455. The vertical axis is logarithmic. \label{fig.2}} \end{figure} | \label{sec:disscu}% The results in Section \ref{sec:spectro} provide important information on the quasar outflows. First, the phenomenon of multiple line-locked NALs within a BAL trough strongly favors the ideas that the BAL outflow (at least in J0217--0854) is made up of several physically separated clumpy structures with similar locations, kinematics, and physical conditions and that the radiative line-driving plays an important role in the acceleration of these clumpy clouds. The line-locking signatures in J0217--0854 also indicate that our line of sight is approximately parallel to the wind streamlines (e.g., \citealp{Hamann2011}). We can make a rough estimate of the inclination of the wind streamlines with respect to our line of sight based on the Si4--Si7 line-locked NAL pair (we choose this line-locked pair because it retains relatively complete Gaussian profiles, which are useful for more correctly estimating the outflow velocities; see Figure \ref{fig.2}). The velocity-split difference with respect to the laboratory splitting of the Si4--Si7 pair is $\thicksim31\,\rm\,km\,s^{-1}$ (see Table \ref{tab.1} and Figure \ref{fig.2}), which implies the outflow trajectory is $\thicksim10^\circ$ of the line of sight. Second, anticorrelation between the fractional variation of the ionizing continuum and ultraviolet (UV) outflow lines has recently been proved (\citealp{Lu2017,Lu2018a}), revealing the ubiquitous effect of the ionizing continuum variability on the UV outflow lines. In the case of J0217--0854, although the pseudo-continuum shows a fractional weakening of $0.129\pm0.029$ between the two observations (Figure \ref{fig.1}), the BALs show no significant {($\Delta$EW\textless1$\sigma^{'}_{\rm EW}$)\footnote{{The definition is $\rm \frac{\Delta EW}{\sigma^{'}_{\rm EW}}=\frac{|EW_2-EW_1|}{\sqrt{\sigma^{2}_{\rm EW_1}+\sigma^{2}_{\rm EW_2}}}$, where $\rm EW_1$ and $\rm EW_2$ are EWs of a BAL for two epochs and $\sigma^{2}_{\rm EW_1}$ and $\sigma^{2}_{\rm EW_2}$ are their uncertainties. EW and $\sigma_{\rm EW}$ are calculated using equations (2) and (3) in \citetalias{Lu2018b}.}}} variability on rest-frame timescales of about 2.8 years. The lack of variability in these outflow lines is probably due to their line saturation \citep{Lu2018c}. However, in this case, the line-locking signatures {are} still visible, indicating that the line-locking signature can remain steady for at least a few years (e.g., \citealp{Vilkoviskij2001}). Third, the absorption depth of C\,{\footnotesize IV} and Si\,{\footnotesize IV} BAL troughs of J0217--0854 is deeper than their corresponding broad emission lines (Figure \ref{fig.1}), which indicates that the absorption regions should be extended enough to occult the broad emission line region (BELR). Besides, it can be inferred from the C\,{\footnotesize IV} BAL that the NAL systems $1\thicksim4$ are very close to zero intensity while the NAL systems $5\thicksim7$ {show residual light at their bottom}, suggesting that the systems $1\thicksim4$ almost completely occult the BELR while the systems $5\thicksim7$ only partially cover the BELR. Such a situation, the complete and partial coverage {coexisting} in a single BAL system, is reported for the first time {in the framework of BALs formed by blended NAL complexes}. Finally, as pointed out by \citet{Hamann2011}, it is still unclear whether radiation pressure can actually lock the UV absorption doublets together and, if this phenomenon is really feasible, then how the line-locking is performed in quasar outflows. Our discovery, the multiple line-locked NALs within BALs, may offer more observational materials on the study of these problems. Systematic investigation of the line-locking signatures in Type N BALs will be presented in the future work. | 18 | 8 | 1808.03140 |
1808 | 1808.08855_arXiv.txt | Radio jets in active galaxies have been expected to interact with circumnuclear environments in their early phase evolutions. By performing the multi-epoch monitoring observation with the KVN and VERA Array (KaVA) at 43~GHz, we investigate the kinematics of the notable newborn bright component C3 located at the tip of the recurrent jet of 3C~84. During 2015 August-September, we discover the flip of C3 and the amount of the flip is about 0.4~milli-arcsecond in angular scale, which corresponds to 0.14 parsec in physical scale. After the flip of C3, it wobbled at the same location for a few months and then it restarted to propagate towards the southern direction. The flux density of C3 coherently showed the monotonic increase during the observation period. The flip is in good agreement with hydrodynamical simulations of jets in clumpy ambient medium. We estimate the number density of the putative clump based on the momentum balance between the jet thrust and the ram pressure from the clump and it is about $10^{3-5}~{\rm cm^{-3}}$. We briefly discuss possible origins of the clump. | Active galactic nuclei (AGNs) are powered by surrounding gas accretion onto supermassive black holes (BHs) at the center of each galaxy. Since the circumnuclear material plays important roles of a gas reservoir for the accretion flow, its nature, such as the structure, the size, and the mass, has been intensively investigated (e.g., Ramos Almeida and Ricci 2017 for review). While AGNs themselves radiate across the entire electromagnetic spectrum, from the radio and up to $\gamma$-rays, circumnuclear material do not emit significant electromagnetic signal. Hence, circumnuclear matter distributions in particular at the central parsec scale regions in AGNs are highly uncertain. Observationally, there are some indications of inhomogeneous matter distribution. VLBI observations of young radio sources reveal that a large fraction of young radio lobes with a two-sided structure show the asymmetry in arm-length ratio and their flux density from a pair of the lobes and the pair of the lobes satisfies a brighter-when-farther behavior (Dallacasa and Orienti 2013; Orienti and Dallacasa 2014; Orienti 2016). This asymmetry can be naturally explained by the interaction between the jet and an inhomogeneous environment. At X-ray energy band, time variations in absorbing column density on timescales from months to years have been known, which indicate highly non-uniform circumnuclear material such as clumps on a sub parsec scale (Ives et al. 1976; Malizia et al. 1997; Risaliti et al. 2002). Theoretically, jets are thought to have a strong impact on the interstellar medium of a host radio galaxy. The deposition of jet mechanical energy may affect formation of stars in the host galaxy and the accretion of matter down the gravitational potential onto the central BH, which is so called AGN feedback (e.g., Silk and Rees 1998; Bicknell et al. 2000; King 2003; Croton et al. 2006; Fabian 2012). In the context of AGN feedback, Wagner and Bicknell (2011) ran numerical simulations of AGN jets interacting with a non-uniform medium containing dense clumps, focusing in the effects of the AGN jet on the cold dense clumps in which stars were formed. Intriguingly, Wagner and Bicknell (2011) indicate flips of the jet head caused by the interaction with the clumps, although such a phenomenon has not yet been observed so far. Similar behavior have been also reported in several other studies (e.g., Fragile et al. 2017). However, no direct evidence for such dramatic impact is yet reported so far. NGC~1275 is a notable nearby giant elliptical galaxy at the core of the Perseus cluster, with an optically luminous nucleus, currently classified as a Seyfert 1.5/LINER (Sosa-Brito et al. 2001). Interestingly, Nagai et al. (2010) found the emergence of newborn bright component so-called C3 during multi-epoch monitoring of 3C~84 using Very Long Baseline Interferometer (VLBI). The component C3 showed a proper motion towards the southern direction with the apparent velocity of $0.2-0.3~c$ (Nagai et al. 2010; Suzuki et al. 2012; Hiura et al. 2018). Since various observations of 3C~84 indicate the gas-rich environment such as molecular gas (Krabbe et al. 2000; Salome et a. 2006; Lim et al 2008), warm [H$_{2}$] gas (Wilman et al. 2005; Scharwachter et al. 2013) and dense ionized plasma surrounding the 3C~84 jet (O'Dea 1984; Walker et al. 2000), 3C~84 is regarded as a quite unique laboratory to explore interactions between the jet and the circumnuclear environment on parsec scale. In this work, we will report the KaVA monitoring observation of 3C~84 during 2015-2016 , in which we indeed discover a theoretically predicted a flip of the jet head accompanying the clear increase of the C3 flux density. In this work, we define the radio spectral index $\alpha_{R}$ as $S_{\nu}\propto \nu^{-\alpha_{R}}$. The cosmological parameters used here are as follows; $H_{0} = 71~{\rm km/s/Mpc}$, $\Omega_{\lambda} = 0.73$ and $\Omega_{m} = 0.27$ (e.g., Komatsu et al. 2011). The redshift of 3C~84 ($z=0.018$) is located at the distance $75$~Mpc and it corresponds to 0.35~pc mas$^{-1}$. The mass of the black hole in NGC~1275 is estimated to be around $M_{\bullet} \approx 8 \times 10^{8}M_{\odot}$ (Schawachter et al. 2013) and we adopt this value. Corresponding Eddington luminosity is $L_{\rm Edd}\approx 1 \times 10^{47}~{\rm erg~s^{-1}}$. Normalized physical quantity is denoted as $Q=10^{x}Q_{x}$, otherwise stated. | \subsection{Origin of the clump} In the previous section, we constrain the number density of the clump located at ~1~pc from the central engine of 3C~84 via the simple dynamical argument of jet-clump interaction. Here, we will discuss possible origins of the clump based on its location and number density. Since typical density and location of a cloud in a broad line region (BLR) (e.g., Osterbrock 1989; Perterson 1997; Maiolino et al. 2010; Bentz et al. 2013; Czerny et al. 2017) are obviously different from those for the clump in 3C~84, we will not further discuss the possibility of clouds in BLR below. \subsubsection{A cloud in narrow line region?} Let us discuss a possibility of a cloud in narrow line region (NLR) as the origin of the clump. A typical number density is known as $\sim 10^{2-5}~{\rm cm^{-3}}$ (e.g., Osterbrock1989) and thus it is obvious that a typical number density of cloud in NLR well agree with the estimated number density of the clump ($n_{\rm cl}$) in 3C~84. In this sub-section, we further discuss whether the redial dependence of $n_{\rm cl}$ in 3C~84 has a similar trend with those in other AGN sources. Typically, NLR clouds in moderately luminous AGN such as Seyfert galaxies are expected to be located in the broad radial range of $10^{1-3}$~pc distance from the central engine (e.g., Ramos Almeida and Ricci 2017). Some of NLRs expand large enough to be partially resolved on the sky (e.g., Pogge 1988; Tadhunter and Tsvetanov 1989; Schmitt et al. 1994; Fischer et al. 2013). First, let us begin with an estimate the size of NLR ($R_{\rm NLR}$) for NGC~1275. The $L_{H\beta}$ luminosity is estimated by the luminosity of the $H\alpha$ and $H\beta$ emission line luminosities has the empirical relation of $3 L_{H\beta}\approx L_{H\alpha}$ (Figure~5 in Greene and Ho 2005) and we have $L_{H\alpha}\sim 1\times 10^{42}~{\rm erg~s^{-1}}$ in NGC~1275 by KANATA observation (Yamazaki et al. 2013; Kino et al. 2016). Therefore, the size of NRL in NGC~1275 can be estimated as $R_{\rm NLR}\approx 0.6\times 10^{2}~{\rm pc}~ L_{H\beta,41}^{1/3} \epsilon_{\rm fill, -2}^{-1/3} n_{4}^{-2/3}$ where the filling factor of the NRL, and the number density of electrons are $\epsilon_{\rm fill}$, and $n$, respectively. The derived $R_{\rm NLR}\ $value is comparable to typical values in other sources (e.g., Osterbrock 1989). Next, let us compare the location and $n_{\rm cl}$ of the clump in 3C~84 with those in other AGN sources. Interestingly, Walsh et al. (2008) conducted the imaging spectrograph observations of nearby low-luminosity AGNs with the Space Telescope Imaging Spectrograph (STIS) aboard the the Hubble Space Telescope (HST) and and they find the electron number density measurements are characterized by a similar slope with the normalization of the electron number density $5\times 10^{3}~{\rm cm^{-3}}$ at $\sim 1$~pc distance from the core for five low-luminosity AGNs of NGC~1052, NGC~3227, NGC~3998, NGC~4278, and NGC~4579 (see Figure~7 in Walsh et al. 2008). Murayama and Taniguchi (1998) also indicate the radial dependence of the electron number density distribution in NLR via detection of coronal lines, which is known as an indicator of denser clump. Regarding the location of the clump, i.e., 1~pc distance from the central engine, the Figure~7 of Walsh et al. (2008) indicates the electron number density as $n_{\rm cl}\sim 10^{3-5}~{\rm cm^{-3}}$, which well agrees with our estimate. Therefore we conclude that the clump in 3C~84 can be undersood as a cloud located deep inside (i.e., $\sim 1$~pc) the NLR with a normal number density. \subsubsection{Self-gravitating molecular cloud?} As alternative case which also agrees with the estimated $n_{\rm cl}$, here we discuss the case when an intergalactic cloud such as a dark cloud, a circum-stellar cloud, with its typical sizes, masses, and temperatures $0.1-3$~pc, $10^{2-7}~{\rm cm^{-3}}$, and $10-100~{\rm K}$ (Goldsmith 1987) is the origin of the clump. Interactions between galactic jet sources and such cold clumps are indeed suggested in some observations (e.g., Fukui et al. 2009). Here we briefly discuss when the clump can be a self-gravitating one since intergalactic molecular clouds are quite important in the context of star formations. The radius and mass of a self-gravitating clump in the context of AGNs have been estimated in literatures (e.g., Krolik and Begelman 1988; Honig and Berkert 2007, Kawaguchi and Mori 2011) and we follow those arguments. The two conditions required for self-gravitational clumps are as follows. The first requirement is (1) the free-fall time for spherical clump ($t_{\rm ff}=\sqrt{3\pi/(32\rho_{\rm cl} G)}$) is shorter than the sound crossing time ($t_{\rm crs}=R_{\rm cl}/c_{s}$). The second requirement is (2) these clump should be stable against the tidal force in the gravitational field of the central black hole ($F_{\rm tidal} \approx 2GM_{\bullet} M_{\rm cl}R_{\rm cl}/r^{3}$) by the self-gravity force ($F_{\rm sg} \approx GM_{\rm cl}^{2}/R_{\rm cl}^{2}$) where $r$ is the distance from the central black hole to the clump. The second requirement comes down to the relation $(R_{\rm cl}/r)^{3} \lesssim M_{\rm cl}/2M_{\bullet}$. From these conditions, we have $R_{\rm cl} \lesssim \frac{\pi}{\sqrt{16 G}}\frac{c_{s}r^{3/2}}{M_{\bullet}^{1/2}} \approx 1.6\times 10^{-2} c_{s,5} (r/1~{\rm pc})^{3/2} (M_{\bullet}/10^{8}~M_{\odot})^{-1/2} ~{\rm pc}$ and $M_{\rm cl, crit} = \frac{\pi^{2} c_{s}^{2}}{8G}R_{\rm cl} \approx 0.5~M_{\odot} c_{s,5}^{3} (r/1~{\rm pc})^{3/2} (M_{\bullet}/10^{8}~M_{\odot})^{-1/2}$. From this, it is clear that the clump can become self-gravitating one only when the clump is cold (i.e., slow sound speed of order of $c_{s}\lesssim 10^{5}~{\rm cm/s}$), although there are no direct evidence for cold clump in 3C~84 so far. Therefore, observational explorations of cold clumps are essential to test this scenario. Further discussions on cold gas will be made in the sub-section 5.3.2. \subsubsection{A giant cloud?} Here we briefly discuss a possibility of the existence of a clump with its cross sectional area $A_{\rm cl}$ larger than that of $A_{\rm cross}$ (i.e., $\eta >1$). If a giant molecular cloud with its size larger than a few parsec scale (Goldsmith 1987) known in our Galaxy exists in NGC~1275, then it may be one of the possible candidates for it. A remaining part of such a large cloud may probably hold back on the western side of the radio lobe made of the shocked material escaped from C3 and produce a remnant structure (see details in Sec 5.2). Do we find such a remnant feature? Here we speculate that the diffuse component C2 may be identical to the remnant of the exhausted jet produced by the interaction of the jet and a large clump. Although little attention has been paid to C2, its existence was well known in literatures (e.g., Venturi et al. 1993; Dhawan et al. 1998) and C2 does not show a systematic motion but shows random wobbling behavior. Such behaviors could be explained by the collision with the large clump, which may cause a back-flows of the exhausted jet (Asada et al. 2006; Mizuta et al. 2010). This scenario could be investigated through high energy $\gamma$-ray emissions caused by strong shocks (e.g., Kino et al. 2011, 2017), although detailed investigations of such high energy $\gamma$-ray spectra is beyond the scope of this paper. \subsection{Comparison with hydrodynamical simulations} In this sub-section, we will make comparisons of the observational results with hydrodynamical simulations of jet propagations in surrounding ambient matter. \subsubsection{Jet confinement condition in a uniform ambient matter} First of all, it is important to check the availability of Eq.~(1) based on hydrodynamical simulations of jet propagations. To this end, simulations of jet propagations in a uniform ambient matter provide us a criterion for the availability of Eq.~(1). A propagation of a jet in a uniform ambient matter is essentially governed by the momentum balance between the jet thrust and the ram pressure from the ambient matter. Hence, the physical quantity $L_{j}/n_{a}$ is the key to understand the jet propagations where $n_{a}$ is the number density of the ambient matter. It practically corresponds to the case of $\eta \gg 1$. Hereafter, we discuss the comparison of the obtained $n_{\rm cl}$ in 3C~84 with hydrodynamical simulation of De Young (1993) by focusing on the quantity $L_{j}/n_{a}$. Let us begin with a brief summary of the hydrodynamical simulation of De Young (1993), which thoroughly examine the various combinations of $L_{j}$ and $n_{a}$ and investigated the condition of jet-confinement by uniform ambient matter. They examined the case of jet propagations with the total kinetic power $L_{j} \approx 1\times 10^{45}$ erg/s and changing $n_{a}$ and they found no jet confinement below $n_{a}\lesssim 3~{\rm cm^{-3}}$ (Fig. 9 of De Young 1993). They also tested the case of $L_{j}\approx 1\times 10^{44}~{\rm erg/s}$and the jet is still "confined" when $n_{a} \gtrsim1.6~{\rm cm^{-3}}$ (Fig. 9 of De Young 1993). Therefore, the condition for realizing the jet confinement by the uniform ambient medium based on the simulation of De Young (1993) can be written as \begin{eqnarray}\label{eq:confinement} \frac{L_{j}}{n_{a}} \lesssim (6\times 10^{43} - 3\times 10^{44}) ~ {\rm erg~cm^{3}/s}. \end{eqnarray} The estimate of $n_{\rm cl}$ together with the adopted $L_{j}$ shown in Eq.~(\ref{eq:power}) in 3C~84 in the present work shows a good agreement with the result of De Young (1993), i.e., the estimated upper limit of $L_{j}/n_{\rm cl}$ is almost the same as the criterion value inferred from the numerical result of De Young (1993). Thus, the flip of the jet head of 3C~84 is well justified as a consequence in this jet clump collision. It is worth to add that the condition of confinement shown in Eq.~(\ref{eq:confinement}) is almost consistent also with the previous work of Kawakatu et al. (2009), which shows that a hot spot, which corresponds to the reverse shock at the tip of the jet, is generated when \begin{eqnarray} \frac{L_{j}}{n_{a}} \gtrsim 1\times 10^{44} ~ {\rm erg~cm^{3}/s}. \end{eqnarray} based on a Mach number value at the hot spot (termination shock) region. \subsubsection{More realistic case: inhomogeneous ambient matter} Next, we discuss the case of an inhomogeneous ambient matter, which would provide us with an insight for more realistic situation. The work of Wagner et al. (2012) indeed made a comprehensive work of jet propagation in two-phase interstellar medium (ISM), which consists of a warm and hot phases. The warm phase has a density perturbation and it produces the clumps with the upper limit temperature $\sim 10^{4}$~K. This situation qualitatively corresponds to the case of $\eta \sim O(1)$. In Wagner et al. (2012), no lower temperature limit is enforced and temperatures in the core of cloud (i.e. clump) may initially be less than 100~K. They have found that the acceleration of the dense embedded clouds (clumps) is realized by the ram pressure of the high-velocity flow through the porous channels of the warm phase. This process transfers $10\%$ to $40\%$ of the jet kinetic energy to the cold and warm gas, accelerating/blowing it to velocities that match those observed radio jets in AGNs. That is so-call AGN feedback in mechanical energy form. The key feature seen in the simulation of is a mixture of the flip of the jet head and blown out of clumps (Figure 9 of Wagner et al. 2012). Because of the significant non-uniformity of ambient matter, dynamics of clumps cannot be purely determined by the head-on collision between the jet and a single clump alone. Perhaps, such a hybrid case of the jet flip and AGN feedback may be more realistic in actual AGN jet sources. Wagner et al. (2012) show that the AGN Feedback is efficient for $ 10^{43}~{\rm erg~s^{-1}}\lesssim L_{j} \lesssim10^{46}~{\rm erg~s^{-1}}$, with the number density of the clump as $30~{\rm cm^{-3}}\lesssim n_{\rm cl} \lesssim 10^{3}~{\rm cm^{-3}}$. Since there is an overlap in these parameter ranges, we can expect a hybrid phenomena in 3C~84. Indeed, as mentioned in \S 3.1, we have detected a short-lived extended structure at the West of C3 from 2015 December to 2016 January. This structure is similar with a substructure at the collision site between the jet and a clump demonstrated in the Figure 9 of Wagner et al. (2012) . As for the cloud/clump velocity, Wagner et al. (2012) predicted a typical velocity of the clumps after the collision as from a few $100$ km/s to 1000~km/s. Therefore, this clump velocity will be one of the key observational quantities towards testing AGN feedback process in 3C~84 in the future. \subsection{Other signatures of clumpy medium} Lastly, we briefly discuss other signatures of clumpy medium in 3C~84. \subsubsection{Enhancement of linear polarization emission} Nagai et al. (2017) recently reported the detection of a significant polarized emission at the C3 component by VLBA observation data in the BU blazar monitoring program. Faraday rotation is also detected within an entire bandwidth of the 43~GHz band. The obtained rotation measure is $\sim 6\times 10^{5}~{\rm rad~m^{-2}}$ at a maximum. Similar RM values were also reported at 210-345~GHz by Plambeck et al. (2014) using CARMA and SMA while the 210-345~GHz emissions are likely to originate in the compact region at the close vicinity of the black hole. Nagai et al. (2017) claimed that a simple spherical accretion flow cannot explain the RM observed with the VLBA and SMA/CARMA consistently. To reconcile it, Nagai et al. (2017) proposed a local clumpy/inhomogeneous ambient medium being responsible for the observed RM. When an equipartition condition between magnetic field and accreting gas holds, the electron density is estimated as $\sim 10^{4}~{\rm cm^{-3}}$, which is comparable to the estimate of $n_{\rm cl}$ in this work. These results also strongly support the existence of the clump near the C3 component. \subsubsection{Searching for cold gas} Observationally, direct detections of cold gas within ~10 pc scale from central BHs are challenging and such explorations have just begun for other nearby active galaxies. In this context, Imanishi et al. (2018) made a first detection of cold gas on this scale (what they detected was a rotating dusty molecular torus) in radio band by using the benefit of ALMA's high spatial resolution. First VLBI detection of HCN molecular absorption line features in NGC~1052 has been reported by Sawada-Satoh et al. (2016). We add to note that there are some detections of absorption lines by such clumps for ultra-luminous IR galaxies (e.g., Geballe et al. 2006, Sajina et al. 2009). We further add to note that a large fraction of X-ray selected Seyfert 2 galaxies indeed show significant variations in the X-ray absorbing column density of $N_{\rm H}$ on the typical timescale less than 1~yr, which clearly suggests the presence of clumpy circumnuclear material on a scale below a parsec (Risaliti et al. 2002). Although a cold gas has not yet detected at the circumnuclear region of NGC 1275, ALMA can potentially probe the cold molecular gas in the region. We proposed ALMA observations to study the morphological and kinematical properties of the cold molecular gas in the central a few ten parsec (PI: H. Nagai), and the observations were partially completed. We also performs KVN observations to look for absorption line caused by the cold molecular gas in the central a few ten parsec (PI: K. Wajima). Such high resolution observations towards NGC~1275 will definitely accelerate our understanding on a cold gas at the center of nearby radio galaxies. | 18 | 8 | 1808.08855 |
1808 | 1808.06623_arXiv.txt | \noindent Assuming that the Universe at higher redshifts ($z \sim 4$ and beyond) is consistent with \lcdm model as constrained by the Planck measurements, we reanalyze the low redshift cosmological data to reconstruct the Hubble parameter as a function of redshift. This enables us to address the $H_0$ and other tensions between low $z$ observations and high $z$ Planck measurement from CMB. From the reconstructed $H(z)$, we compute the energy density for the ``dark energy'' sector of the Universe as a function of redshift {without assuming a specific model for dark energy}. We find that the dark energy density has a minimum for certain redshift range and that the value of dark energy at this minimum $\rde^{\text{min}}$ is negative. This behavior can most simply be described by a {negative cosmological constant} plus an evolving dark energy component. We discuss possible theoretical and observational implications of such a scenario. | {Thanks to various sets of cosmological data, we can now talk about ``{\it the standard model of cosmology}'', the $\Lambda$CDM Universe \cite{Ade:2015xua, Ade:2015rim}. It provides the simplest paradigm that fits remarkably well to most of the current cosmological observations. As the precision of the low redshift data increases, there are however emerging tensions in \lcdm model which is otherwise consistent with high redshift CMB observations by Planck. The major tension is between the model independent measurement of $H_{0}$ parameter ({$\sim 73 ~\text{km/s/Mpc}$}) by SH0ES collaboration \cite{Riess:2016jrr, Riess:2017lxs, Riess:2018byc} and that by Planck assuming \lcdm model \cite{Ade:2015xua, Ade:2015rim, Akrami:2018vks,Aghanim:2018eyx}. The latest Planck-2018 data shows, {$H_0 = 67.8 \pm 0.9~ \text{km/s/Mpc}$} \cite{Akrami:2018vks} and this is at tension over $3.5\sigma$ with the SH0ES measurement which is $H_0 = 73.52 \pm 1.62~\text{km/s/Mpc}$. A similar mild inconsistency in $H_{0}$ for $\Lambda$CDM model is also observed by Strong Lensing experiments like H0LiCow using time delay measurements \cite{Bonvin:2016crt} which measured $H_0 = 71.9^{+2.4}_{-3.0}~\text{km/s/Mpc}$ for \lcdm Universe. Moreover the BOSS survey for baryon acoustic oscillations measurements using Lyman-$\alpha$ forest \cite{Delubac:2014aqe} has also measured the expansion rate of the Universe at $z=2.34$. This measured expansion rate of the Universe at $z=2.34$ is also at tension over $2\sigma$ with Planck result for \lcdm \cite{Sahni:2014ooa}. The important consequences of these tensions are the prediction of the dark energy density evolution with time\footnote{{That evolving and dynamical dark energy is necessitated by the data has also been discussed in \cite{Sola:2018sjf, Sola:2016ecz,Ryan:2018aif, Ooba:2018dzf, DiValentino:2017zyq, Rivera:2016zzr, Zhao:2017cud, Zhang:2017idq}.}} {and more importantly for us,} the possibility of having negative dark energy density at higher redshifts as discussed by \cite{ Delubac:2014aqe, Sahni:2014ooa, Poulin:2018zxs}. Similar conclusion has also been obtained recently by \cite{Wang:2018fng} using a dark energy model independent approach. In their study, Wang et al. attributed the evolution of dark energy density as well as its negative values at high redshifts to non-minimally coupled scalar field theory like Brans-Dicke theory. Interestingly in all these studies, the dark energy density is not only negative at higher redshifts but it is unbounded from below. This poses a serious problem as in a spatially flat universe, with such a negative dark energy density, the matter energy density parameter will be more than one and grow quickly without any upper bound at higher redshifts. This will have catastrophic effect on the structure formation scenario.This surely needs to be resolved. In this work, we revisit the process of constraining the dark energy behavior using cosmological observations to find the likely sources of tensions between low and high redshifts cosmological observations. For this, }we should be careful that although Planck constrains the high redshift Universe using CMB with unprecedented accuracy, it may not be sensitive enough to any new physics beyond \lcdm at low redshifts. Fitting the entire background evolution of the Universe from $z\sim1100$ till $z=0$ using $\Lambda$CDM in which the dark energy density is a redshift independent quantity, can be the source of aforementioned tensions. {Therefore we reanalyze the low-redshift data involving background cosmology assuming that for a certain higher redshift $z=z_{\text{match}}$ and beyond, one recovers the background evolution as constrained by Planck-2018 for \lcdm Universe. {For} this, we assume that $z_{\text{match}}$ has to be larger than $z\sim 2.4$, as {around this redshift}, we have the constraint on expansion rate of Universe from BOSS survey using Lyman-$\alpha$ forest. This takes care of any new physics for dark energy evolution (beyond \lcdm Universe) that is predicted by low-redshift observations for $z < z_{\text{match}}$ including the SH0ES measurement for $H_{0}$. {With these assumptions, we study the dark energy behaviour which is consistent with the low-redshift data for $z\leq z_{match}$ {while smoothly matching} the Planck-constrained $H(z)$ behaviour for $\Lambda$CDM model for $z>z_{match}$}. In this process, we do not assume any particular dark energy model. We directly reconstruct the Hubble parameter $H(z)$ as a function of redshift using cosmographic approach. Subsequently, we reconstruct the dark energy density evolution} {in a ``dark energy relaxed XCDM framework,'' in which we do not assume any specific dark energy model,}\footnote{{We note that similar reconstruction of dark energy properties have been extensively analyzed in the literature, e.g. see \cite{Shafieloo:2007cs, Seikel:2012uu, Gerardi:2019obr, Wang:2019ufm}. Our data analysis method and the data sets we consider here is different than these other works.}} {while still assume the Universe is spatially flat and contains pressure-less matter (including baryons and dark matter) plus another unknown component. This extra unknown component can be due to dark energy or it may arise as an effective dark energy component due to any sort of modification of gravity at large cosmological scales. We also ignore the contribution from radiation energy density as it is negligible compared to matter or dark energy at late times.} Our {dark energy model-independent} analysis within the above set of working assumptions and framework reveals that $\rde(z)$ has two specific features: it has a minimum and a phantom crossing at $z=z_{\text{min}}$. Moreover, the value of $\rde$ at $z=z_{\text{min}}$ is negative. The simplest explanation for $\rde^{\text{min}} < 0$, is the existence of a {\it negative Cosmological Constant}. | Based on two requirements that $H_0$-tension is {ameliorated} (in favor of \cite{Riess:2016jrr}) and \lcdm is still the best fit for Planck data at higher redshifts, we reanalyzed the low redshift data. {If we consider just the low-redshift data, as we did in section \ref{sec:2.1}, we get $H_0=72.56 \pm 1.55$, which is perfectly consistent with \cite{Riess:2016jrr}. Adding the $H(z)$ data for higher redshift from Planck chains for \lcdm as we did in section \ref{sec:2.2}, we get the following results: For $z_{\text{match}}= 4$, $H_0=68.7 \pm 1.3$, which is $2.3\sigma$ tension with \cite{Riess:2018byc}. For $z_{\text{match}}= 6$, $H_0=70.0 \pm 1.4$ which is at $1.65 \sigma$ tension with \cite{Riess:2018byc} and is at less than $2 \sigma$ tension with \cite{Riess:2019cxk}. The details of these analysis will appear in an upcoming paper. So the take away is: (1) adding the Planck constraint on background evolution at higher redshifts, the $H_0$ shifts towards lower values compared to only low-redshift analysis and (2) adding the data point from $H(z)$ reconstructed by Planck chains, prefers higher $z_{\text{match}}$. As we already pointed out with higher $z_{\text{match}}$ the likelihood of having negative minimum in $f(z)$ is greater. As we mention below, the exact value of $z_{\text{match}}$ should be obtained with a thorough analysis including low redshift and full CMB likelihood. } We reconstructed the $H(z)$ behavior {for $z\lesssim 8$ region}\footnote{{Note that we are using Pade $P_{2,2}$ parametrization only for $0<z_{\text{match}}\lesssim 8$ region and for higher $z$ {one should use higher order Pade parametrization for better fit to actual model}.}} and subsequently the $\rde(z)$ behavior. We stated and discussed three generic results of our % analysis in the previous sections. % It is important to highlight the differences of our work from some recent works along this line. In the work by \cite{Zhao:2017cud}, the equation of state parameter for the dark energy was reconstructed with the assumption of $\rde > 0$, and also no interaction between matter and the dark energy, therefore, setting aside a very large class of dark energy models including modified gravity models. In a subsequent paper by \cite{Wang:2018fng}, $\rde$ was reconstructed directly, and it was found to be unbounded from below at higher redshifts. Almost similar inference was drawn in \cite{Poulin:2018zxs}. {In our analysis, we directly reconstruct the $H(z)$ from low redshift data using Pade approximation and get the similar result. Our analysis reproduces the result of \cite{Wang:2018fng} that $\rde < 0$ at higher redshift. The fact that two different reconstructions yield similar results, supports the validity of {our $H(z)$ reconstruction process which is based on Pade parametrization}. Subsequently we incorporated Planck constraint on background universe, directly through $H(z)$ as measured by Planck for \lcdm at intermediate redshifts. This makes sure that our reconstructed $H(z)$ from low redshift data is consistent with Planck measured $H(z)$ for \lcdm at intermediate redshifts. This results in a minimum for $\rde$ and this minimum is negative. {The notable features of our approach for low-redshift data analysis are direct $H(z)$ reconstruction (using Pade approximant/parametrization) and imposing Planck constraint on $H(z)$ at intermediate redshifts.} } Below, we add some further comments and things to be worked out and studied in future: \begin{itemize} \item[(I)]\textbf{{Vetting the working assumptions and robustness of our results:}} \vskip 2mm This is probably the first study where we assume two different $H(z)$ behavior at low and high redshifts and match them around some $z=z_{\text{match}}$, {for which we chose some reasonable values. To justify this working assumption,} we have tested different $z_{\text{match}}$ and overall behavior of our results remain the same. This is also shown in Fig.\ref{fig:fplot}, where we plot $f(z)$ for two different choices of redshift range to generate $H(z)$ data from Planck \lcdm model. Of course, to get an actual estimate of $z_{\text{match}}$, one needs to do a full combined analysis using all the low redshift data together with Planck Likelihood assuming that for $z \leq z_{\text{match}}$, $H(z)$ is given by \eqref{H-Pade} and for $ z> z_{\text{match}}$, $H(z)$ is given by \lcdm model and get a estimate of $z_{\text{match}}$. This is beyond the scope of present study and will be reported in a separate publication. \vskip 1.5mm Our other working assumption was the use of Pade Approximation. Pade approximation has been extensively used earlier for low-redshift reconstruction purposes, more detailed references can be found in \cite{Saini:1999ba} and more recent usage of Pade parametrization in \cite{Sahni:2014ooa, Aviles:2014rma, Gruber:2013wua, Wei:2013jya, Rezaei:2017yyj, Mehrabi:2018oke} . Probably the first work on reconstruction of quintessence potential using Type-Ia Supernova data used the rational function like Pade Approximation to model the Luminosity Distance \cite{Saini:1999ba}. In our case, we use the same for the Hubble Parameter with larger set of observational data. We emphasis that use of Pade Approximation to reconstruct of Hubble parameter does not bias the final result. The result that dark energy density can be negative at higher redshifts (primarily due to $H_0$ measurement by Riess et al and the Lyman-$\alpha$ measurement of BAO at $z \sim 2.4$) has been also confirmed by several earlier works. It is, however, useful to reanalyze the data using other parametrizations and verify that the final features and results do not depend on the parametrization used in any crucial way.\\ \item[(II)] \textbf{Consistency with Planck's measurement of the CMB anisotropy:} \vskip 2mm \begin{figure} \begin{center} \resizebox{220pt}{220pt}{\includegraphics{omega.pdf}} \end{center} \caption{The dotted red line is for $\Omega_{\text{m}}(z)$, the dashed green line is for $\Omega_{_{\text{DE}}}(z)$ and the solid blue line is for $\pde/(3H^2)$. This is for Planck H(z) data at $z=4,5,6$ and $\Omega^{(0)}_{m}=0.32$. }\label{fig:omplot} \end{figure} The simplest dark energy model we can suggest based on our results is a negative Cosmological Constant and a phantom crossing for the rest of the dark energy. To show that such a model is consistent with CMB temperature anisotropy as measured by Planck, we calculate $C_{l}^{TT}$ for CMB anisotropy spectra assuming that till $z\leq 6$, $H(z)$ is given by best fit of our reconstructed $H(z)$ and for $z>6$, it is given by Planck best fit \lcdm model \cite{Ade:2015xua, Ade:2015rim} . Note that at $z=6$, \lcdm model is well within matter dominated era. In Fig. \ref{fig:omplot}, we show the behaviours of density parameters for matter and dark energy. As one can see, around $z=6$, $\Omega_{m} = 1$ and $\Omega_{\text{DE}} = 0$, allowing us to match our reconstructed $H(z)$ with a matter dominated era. \vskip 1.5mm Using this $H(z)$, we use CLASS code \cite{class} to compute the $C_{l}^{TT}$ and compare with Planck data as well as Planck best fit \lcdm model. The result is shown in Fig.\ref{fig:CMB-ell}. As expected and one can see, $C_{l}^{TT}$ in our model is a good fit to Planck's measurement.\\ \item[(III)] \textbf{Effects on Structure Formation:} \vskip 2mm The existence of this small negative $\Lambda$ can have interesting effect on growth of structures. As one can see in Fig. \ref{fig:omplot}, $\Omega_{m}$ is slightly greater than $1$ for a certain redshift range depending on where we match the $H(z)$ with Planck's \lcdm model. This will give enhancement in growth of structures at higher redshifts and the nonlinear regime may start earlier than in \lcdm model. This may result in the presence of more massive galaxies at higher redshifts compared to $\Lambda$CDM model, effects on reionization process as well as on lensing. All these are potential observable signatures for our model that can be tested by present and next generation galaxy surveys and CMB experiments.\\ \item[(IV)] \textbf{Modeling the $\rde (z)$:} \vskip 2mm Within our data analysis framework, we have a clear indication that {dark energy sector cannot be described by {only} a cosmological constant}. Moreover, as discussed, $\rde, \pde$ cannot be obtained from a minimally coupled scalar field (with any potential); that is, quintessence models do not lead to our $\rde(z)$. Given that $\rde$ takes negative values for a range of $z$, the simplest model is to assume presence of a \emph{negative cosmological constant} with its value $\Lambda=\rho_{\text{min}}$ and then try to model $\rho=\rde-\rho_{\text{min}}$ within a non-minimally coupled scalar theory with a positive definite potential. This latter should be such that it provides crossing to phantom region $(\rde+\pde <0)$ for $z<\zm$. Such models can be constructed, e.g. within Brans-Dicke theory \cite{Wang:2018fng}. Seeking and exploring such models is postponed to future works. \\ \item[(V)] \textbf{Theoretical implications of our dark energy model:} \vskip 2mm A positive cosmological constant which is assumed to drive the current accelerated expansion of the Universe is a theoretical challenge: Getting a vacuum solution with a positive cosmological constant within moduli-fixed consistent and stable string theory compactifications has been a daunting task \cite{Maldacena:2000mw, Kachru:2003aw, Conlon:2007gk, Danielsson:2018ztv}. Moreover, formulating quantum field theory on the background of a de Sitter space has its own challenges, from the choice of the vacuum state to non-existence of a well-defined S-matrix (on global de Sitter space) \cite{Witten:2001kn, Goheer:2002vf}. Our findings here, lifts all those questions by simply removing the need for a positive cosmological constant. \vskip 1.5mm \begin{figure} \begin{center} \resizebox{220pt}{220pt}{\includegraphics{cmb.pdf}} \end{center} \caption{CMB TT spectra. Top one: for model considered in this work (see text) together with Planck data and error bars for TT spectra. Middle one: the difference in TT spectra for our model and Planck best fit $\Lambda$CDM model. Bottom one: The residual for our model with the Planck data. }\label{fig:CMB-ell} \end{figure} On the other hand, a negative cosmological constant is a theoretical sweet spot: it provides an anti-de Sitter (AdS) background, which is very much welcome due to AdS/CFT duality \cite{Maldacena:1997re}, providing a ``dual'' framework for cosmology. In addition, string theory clearly prefers AdS background to de Sitter, consistent AdS backgrounds are ubiquitous in string theory settings \cite{BP}. Interestingly, it has been argued that accelerated expansion of the Universe may be possible with a negative cosmological constant \cite{HHH}. \vskip 1.5mm Finally, we comment on the recently proposed ``swampland'' conjecture \cite{swampland} which favors quintessence models over $\Lambda$CDM. Our results here are in clear tension with the swampland conjecture. Similar statement has also been made in \cite{Eoin}. \\ \item[(VI)] \textbf{Our results and anthropic reasoning.} \vskip 2mm In mid 1980's and long before observational establishment of late-time cosmic acceleration, based on structure formation constraints which is necessary to yield existence of life (as it is usually perceived) it was argued that the value of cosmological constant, if positive, should not be much bigger than $H_0^2$ \cite{Weinberg:1987}. A negative value of the cosmological constant, too, is bounded by similar anthropic reasoning \cite{Barrow-Tipler}. Interestingly, the negative cosmological constant in our model which is within $1\%$ of the current total energy density of the Universe is certainly consistent with these bounds. \end{itemize} {We emphasize that we ascribe the tension between the inferred value of $H_0$ between the local measurements and the Planck data fully on the dynamics of dark energy. Given this is the true case for this tension, what we find is that a negative value of the cosmological constant is still allowed by the data.} To summarise, we discuss the prospects of solving the tension between low and high redshift cosmological observations in the presence of a small {negative cosmological constant}. This is probably the first time, that there are observational suggestions for presence of a {negative cosmological constant}. Its presence predicts definite observational signatures in large scale structure formation in the Universe and can be tested with present and future experiments. \textbf{Note added:} As we were updating and revising our paper, the paper \cite{Riess:2019cxk} appeared which has now a new measured value for $H_{0}$ which is $74.03 \pm 1.42$ km/s/Mpc. This is now in tension with Planck-2018 measurement for $H_{0}$ at $4.4\sigma$. For our case with PL2 ($z_{match} = 6$), the value for $H_{0}$ is $70^{+1.45}_{-1.50}$ Km/s/Mpc. The deviation of this value with \cite{Riess:2019cxk} is $1.98\sigma$ which is at less than $2\sigma$ tension. {See also \cite{Dutta:2019pio} for more discussions and analysis. } | 18 | 8 | 1808.06623 |
1808 | 1808.06415_arXiv.txt | The strength of neutron star crust is crucial for modelling magnetar flares, pulsar glitches and gravitational wave emission. We aim to shed some light on this problem by analysing uniaxial stretch deformation (elongation and contraction) of perfect body-centered cubic Coulomb crystals, paying special attention to the inherent anisotropy of this process. Our analysis is based on the semi-analytical approach of Baiko \& Kozhberov (2017), which, for any uniform deformation, allows one to calculate, in fully non-linear regime, critical deformation parameters beyond which the lattice loses its dynamic stability. We determine critical strain, pressure anisotropy and deformation energy for any stretch direction with respect to the crystallographic axes. These quantities are shown to be strongly anisotropic: they vary by a factor of almost 10 depending on the orientation of the deformation axis. For polycrystalline crust, we argue that the maximum strain for the stretch deformation sustainable elastically is 0.04. It is lower than the breaking strain of 0.1 obtained in molecular dynamic simulations of a shear deformation by Horowitz \& Kadau (2009). The maximum pressure anisotropy of polycrystalline matter is estimated to be in the range from 0.005 to 0.014 $nZ^2e^2/a$, where $n$ is the ion number density, $Ze$ is the ion charge, and $a$ is the ion-sphere radius. We discuss possible mechanisms of plastic motion and formation of large crystallites in neutron star crust as well as analyse energy release associated with breaking of such crystallites in the context of magnetic field evolution and magnetar flaring activity. | The crust of a neutron star (NS) is crystallised, i.e.\ ions (atomic nuclei) form a lattice (e.g., \citealt*{hpy07,ch08,ch17}). In most of the crust the groundstate lattice has the body-centered cubic (bcc) structure (see \citealt*{Baiko14} for discussion of screening effects). Crystalline properties of NS crust are invoked to explain a number of observed astrophysical phenomena. For instance, crustal elasticity is thought to be important for interpretation of quasi-periodic oscillations observed in magnetars (e.g., \citealt*{Gabler_etal18}). Solid crust can support asymmetric distributions of density, so-called, mountains, which can produce gravitational wave (GW) emission (\citealt*{ucb00,Horowitz10,jmo13}). Size of these mountains is limited by the strength of the crust (i.e.\ maximum stress, which the elastically deformed crustal material can withstand). If the crust is indeed as strong as it is suggested by molecular dynamic (MD) simulations (\citealt*{hk09,ch10,ch12}), the GW emission can be poweful enough to be detectable by existing GW observatories (\citealt*{jm13,ch17}). \cite{hp17} have recently suggested that this GW emission could be responsible for observed spin-down during the accreting phase of PSR J1023+0038. Crust breaking under magnetic stress is likely responsible for magnetar outbursts (e.g., \citealt*{llb16}), while plastic motion may be crucial for magnetic field evolution (e.g., \citealt*{Lander16}). \cite{Tsang_etal12} argued that crust breaking could produce an electromagnetic precursor of the GW signal from NS mergers. Thus, an accurate knowledge of the crust strength is fundamental for NS physics and in this paper we analyse it, paying special attention to its anisotropy. Previous works in this field explored a shear deformation by MD simulation (\citealt*{hk09,ch10,ch12,hh12}) and volume-conserving crystal stretching in two highly symmetric directions semi-analytically (\citealt*{BK17}; Paper 1). Here we apply the approach proposed in the latter work to study stretching in arbitrary directions. In Paper 1, the existence of a limit of a uniform Coulomb crystal deformation, above which the crystal loses dynamic stability, has been demonstrated using standard lattice dynamics. The maximum crystal elongation and contraction factors as well as the maximum pressure anisotropy at breaking has been predicted essentially analytically. The authors have discussed two particular stretch directions and have found that the limiting stretch factor and the pressure anisotropy were strongly dependent on the angle between the stretch direction and the crystallographic axes. In this paper, we generalise the results of Paper 1 and perform an extensive study of the dependence of the maximum deformation, pressure anisotropy and deformation energy on the stretch direction for a perfect bcc Coulomb crystal (Section \ref{Sec_res}). We find this dependence to be very strong (for instance, critical pressure anisotropy differs by a factor of almost 10 between the weakest and the strongest directions). We point out that the maximum deformation is typically smaller for a contraction of the lattice, than for an elongation. Also, typical maximum contraction deformation is about 5\%, which is a factor of two lower than the breaking strain for the shear deformation, obtained by \cite{hk09} and \cite{ch10,ch12}. Our results clearly demonstrate that the Tresca and von Misses failure criteria are not valid for perfect Coulomb lattice in NS crust. Our limits of the lattice stability are upper bounds for particular deformation types, because other factors, such as ion motion about lattice nodes (due to zero-point or finite temperature effects) or electron screening, will further degrade material strength. In Subsection \ref{Sec_elastEnerg} we calculate the elastic energy at the critical deformation for all stretch directions. In Section \ref{Sec_spec} we consider astrophysical applications of our results. In particular, Subsection \ref{Sec_poly} is devoted to breaking properties of polycrystalline matter, which is a possible microscopic state of NS crust. In Subsection \ref{Sec_Heat} we estimate heat sources associated with breaking events in magnetar crust. Our results can be applied to both outer and inner crust of neutron stars provided that the effect of dripped neutrons on the interionic interaction can be neglected, and excluding the exotic pasta phases, which may appear at mass densities exceeding $10^{14}$ g cm$^{-3}$. | \label{Sec_conc} In an attempt to deepen understanding of the strength of the NS crust, we have performed a detailed study of uniaxial stretches of crystallites comprising it. The crystallites are modelled as arbitrarily oriented perfect bcc Coulomb crystals. In Section \ref{Sec_res} we have demonstrated that the critical strain, above which the crystal lattice lost its stability, was highly anisotropic varying from 0.04 to 0.3 as a function of mutual orientation of the stretch direction and the crystallographic axes. The same holds true for the critical pressure anisotropy (which varies from 0.005 to $0.04 \, nZ^2e^2/a$) and the excess energy per ion at the critical strain (which may vary from 0.0001 to $0.0018 \, Z^2e^2/a$; note that this is insufficient to melt the whole crystallite even if it is already at melting temperature). The high degree of breaking anisotropy implies that the von Misses and Tresca failure criteria are invalid for bcc Coulomb monocrystals. Our numerical results are presented in Figs.\ \ref{xicrit_s}--\ref{dU_c} in dimensionless form. These data can be converted to physical units with the aid of the scaling relations (\ref{dP_scale}) and (\ref{U_scale}). All results of Section \ref{Sec_res} are based on standard lattice dynamics and are rather robust. However, they are obtained for a pure Coulomb static lattice. We expect that ion motion and electron screening effects would reduce critical deformation parameters by some 10--20 \%. The approach of Paper 1 applied here can be used to study the whole five-dimensional yield surface in the six-dimensional space of strains, while in this paper we constrain ourselves to uniaxial deformations only, which span a three-dimensional subspace\footnote{As discussed in Section \ref{Sec_res}, a uniform compression should not lead to breaking. Hence, two additional deformation parameters require further research. These can be, for instance, the orientation of the second principal strain axis and the strain along this axis.}. In Section \ref{Sec_spec}, our results for monocrystals are used to estimate the strength of the polycrystalline NS crust. We argue that polycrystalline crust breaking is determined by either the minimum critical strain or the minimum critical pressure anisotropy achievable in monocrystals. This implies the breaking strain for the uniaxial stretch to be $\epsilon_{\rm crit} \approx$ 0.03--0.04, which is a factor of 2.5--3 lower than the typically assumed value of $\epsilon^{\rm trad}_{\rm crit} \approx 0.1$ motivated by MD simulations of a shear deformation. Unlike the breaking strain, the breaking stress is found to be model dependent. The pressure anisotropy at breaking is predicted to lie in the range between 0.005 and $0.014 \, nZ^2e^2/a$. This range excludes all the estimates reported previously for the shear deformation: $\Delta P^{\rm pMD}_{\rm crit} \approx 0.016$ from the polycrystalline MD, $\Delta P^{\rm trad}_{\rm crit} \approx 0.0238$ used traditionally, and $\Delta P^{\rm MD}_{\rm crit} \approx 0.039$ from the monocrystalline MD. Alternatively, referring to the von Misses criterion, the critical parameter $\sigma_{\rm max}$ for stretches is predicted to lie in the range from 0.003 to 0.008. The upper bound here coincides with $\sigma^{\rm pMD}_{\rm max}$ for the shear deformation in the polycrystalline MD. The other two estimates for the shear deformation, $\sigma^{\rm trad}_{\rm max} \approx 0.0119$ (traditional) and $\sigma^{\rm MD}_{\rm max} \approx 0.0195$ (from the monocrystalline MD), fall outside our range. We also discuss a formation of large crystallites in the NS crust and raise a possibility of plastic flow driven by the stress-induced grain growth. Finally, we calculate heat release from crust breaking events in the context of magnetic field evolution and magnetar flaring activity. We would like to emphasise that Section \ref{Sec_spec} is of somewhat speculative character. However, its predictions can, in principle, be checked with carefully designed MD simulations. We plan to do this subsequently. | 18 | 8 | 1808.06415 |
1808 | 1808.00908_arXiv.txt | We examine how the various observable statistical properties of the FRB population relate back to their fundamental physical properties in a model independent manner. We analyse the flux density and fluence distributions of Fast Radio Bursts (FRBs) as a tool to investigate their luminosity distance distribution and the evolution of their prevalence throughout cosmic history. We examine in detail particular scenarios in which the burst population follows some power of the cosmic star formation rate. FRBs present an important additional measurable over source counts of existing cosmological populations, namely the dispersion measure. Based on the known redshift of FRB121102 (the repeater) we expect at least 50\% of the dispersion measure to be attributable to the inter-galactic medium and hence it can be used as a proxy for distance. We develop the framework to interpret the dispersion measure distribution, and investigate how the effect of Helium reionization in the intergalactic medium is evident in this distribution. Examination of existing data suggests that the FRB luminosity function is flatter than a critical slope, making FRBs easily detectable to large distances; in this regime the reduction in flux density with distance is outweighed by the increase in the number of bright bursts within the search volume. | The progenitors of the Fast Radio Burst (FRB) population are presently a subject of intense speculation. The lack of any definitive model for an FRB motivates an approach, adopted here, to examine how the FRB observables of flux density, fluence and dispersion measure (DM), are related to the intrinsic properties of an FRB in a model independent manner. This requires a few broad assumptions. The DMs of these millisecond-duration events place them outside the galaxy \citep{Thorntonetal2013}, and the localisation of FRB121102 to a galaxy at $z = 0.19$ \citep{Chatterjeeetal2017,Tendulkaretal2017} shows that at least 50 percent of the DM for this FRB is attributable to the intergalactic medium. Thus it is reasonable to proceed on the assumption that the larger DMs are ascribed primarily to the intergalactic medium \citep[see, e.g.,][]{Lorimeretal2007,Thorntonetal2013} and that, while the DM contributions from the host galaxies will increase the scatter, they will not destroy the DM-distance relationship. However almost nothing is known about the distribution of the population with distance. This is another important clue in unravelling their origin, since it would reveal how the abundance of FRBs has evolved throughout cosmic time. One obvious means of attacking this problem at present is through an understanding of the FRB event rate counts. The distribution of FRB fluences offers a means of decrypting the identity of the progenitor population because it is coupled to the distributions of the luminosities and event distances, and hence to the evolutionary history of the progenitors. In a companion paper \citep[][hereafter Paper I]{MacquartEkers18}, we describe the venerable history and the proven usefulness of source count statistics in the analysis of other astrophysical populations, such as quasars and gamma-ray bursts, and recount how they were employed to assess the distribution of these sources over cosmological distances. Paper I outlines in detail the motivation for investigating FRB source counts in particular. It discusses the treatment of various biases inherent to the current FRB sample, and it derives the present observational constraints on the event rate distribution. Application of a maximum likelihood technique to the Parkes data indicates that the index of the integral rate counts distribution, ${\cal R}(>F_\nu) \propto F_\nu^{\beta}$ as a function of limiting fluence, $F_\nu$, is steep, with $\beta = -2.6_{-1.3}^{+0.7}$ at $F_\nu > 2\,$Jy\,ms; this constraint invites interpretation in the context of the evolution of the FRB population. The purpose of the present paper is to elucidate how the various observable statistical properties of the FRB population relate back to their fundamental physical properties. Though the theory of source counts statistics is well understood in the context of radio galaxies, active galactic nuclei and gamma ray bursts, FRBs add a new dimension to the problem because each detection is accompanied by its dispersion measure (DM). The ability to measure the DM distribution for any extragalactic population represents a new and powerful diagnostic of its properties and, potentially, its distance and evolution. Analysis of the DM distribution of FRBs would be a potent cosmological tool providing that a considerable portion of each FRB dispersion measure is attributable to the intergalactic medium (IGM). This question provides the motivation to undertake the present study: the predicted characteristics of the DM distribution can be used to investigate the validity of this hypothesis and, if proven, would constitute a means of probing the distribution in detail. Such quantities are particularly useful in the present era, when FRB localisations are currently scarce \citep[the only localisation being that of][] {Tendulkaretal2017} and their distances are largely unknown. However, even when the burst redshifts are known it will be necessary to understand how the measured underlying redshift and DM distribution relates to the detection parameters of a given survey, especially through its sensitivity and spectral resolution. In this paper we have investigated the event rate distributions of the FRB population in terms of the observables; flux density, fluence and dispersion measure as determined by the underlying luminosity function and its redshift dependence. We place particular emphasis on properties involving the fluence. This is motivated by the fact that the observed flux density of an impulsive radio burst is affected by both the detector temporal resolution (as discussed in Paper I), and by temporal smearing of the pulse due to multipath propagation. Temporal smearing is known to be an important effect for FRBs \citep[e.g.][]{Lorimeretal2007,Thorntonetal2013,MacquartKoay2015}, but its effects are not well understood, there being no clear relation between the dispersion measure of an FRB and its scattering timescale, thus rendering its incorporation into the source counts formalism problematic. However, a treatment of the fluence distribution obviates the need to account for finite detector resolution, and thus the distribution of burst durations relative to it. The time-integrated pulse energy is also invariant to the scattering timescale for a statistically homogeneous scattering medium (but may deviate from this if the assumption of statistical homogeneity is violated on scales from which the scattered emission is received). If more sophisticated detection criteria (e.g. matched filtering) are used for the survey this will affect the completeness fluence, but one would expect an analysis of its effects to form a part of the survey completeness analysis, rather than an intrinsic component of the source counts theory. The paper is partitioned as follows. In \S\ref{sec:Definition} we introduce the event rate formalism for a flux-density or fluence limited survey. In \S\ref{sec:Distribution} we apply this theory to derive the behaviour of these distributions for various broadly-generic FRB evolutionary scenarios. In \S\ref{sec:RateandDMcompare} we introduce the formalism to derive the DM distribution of the population. In \S\ref{sec:Discussion} we illustrate the application of this formalism using a comparison with the DM distribution of of published events and discuss the implications of our findings. Our conclusions are presented in \S\ref{sec:Conclusions}. | \label{sec:Conclusions} In this paper we have investigated the event rate distributions of the FRB population in terms of flux density and fluence, and redshift and dispersion measure. We place particular emphasis on quantities involving the fluence because this quantity mitigates interpretational issues related to finite detector resolution and the effects of pulse temporal smearing due to multipath propagation. We summarise here the important points from each of these analyses, and detail their import for future studies of the population. Even for events that emit as standard candles or standard batteries (i.e. with constant spectral energy density), the slope of the event rate distribution is a continuously changing function of fluence and, equivalently, flux density. However, the energy and luminosity distributions of the FRB population are likely to be broad. If, after accounting for the contributions of the host galaxy and the Milky Way, we interpret the bulk of the DM as a measure of distance, and taking into account the range in burst flux densities, we would infer that FRB luminosities span a broad range. We remark that a model incorporating a broad luminosity function can explain both the observed DM distribution and a large scatter in the DM-fluence relation, under the assumption that the IGM dominates the observed DM, without having to appeal to significant host DM contributions. However, it presently remains an open question whether the IGM contribution does indeed dominate the total DM. From studies of other cosmological populations such as AGN, it is known that for such a broad luminosity distribution the event fluence distribution separates into three distinct regions. Normalised by the Euclidean source counts slope, $F_\nu^{-5/2}$ this then consists of a (i) low-fluence region that rises with fluence, (ii) a broad plateau and (iii) a high-fluence region in which the distribution declines with increasing fluence until, at very high fluence, it declines proportional to the Euclidean scaling of $F_\nu^{-5/2}$. The high fluence end of the rate distribution, region (iii), is dominated by the distribution of the population on cosmological scales, and not by the luminosity function of the bursts. The width of the maximum between regions (ii) and (iii) depends more on the luminosity function than the cosmology. For shallow energy (luminosity) distributions $E_\nu^{-\gamma}$ with $\gamma < 2$, there is a prominent break (evident as a maximum in the Euclidean normalised counts), which occurs at a fluence $F_\nu \approx E_{\rm max} (1+z_0)^{2-\alpha}/(4 \pi D_L(z_0)^2)$ (or, equivalently, $S_\nu \approx L_{\rm max} (1+z_0)^{1-\alpha}/(4 \pi D_L(z_0)^2)$ in the flux density distribution), where $z_0$ is a characteristic redshift beyond which the population density begins declining sharply. By contrast, the breadth of the plateau at lower fluences, region (ii), is determined by the width of the energy (luminosity) distribution. There is a large motivation to undertake large field-of-view surveys to characterise the high fluence tail of the FRB distribution. It is this region that encodes the most useful information about the cosmological evolution of the population; observations that probe below the turnover instead yield information on the luminosity distribution of the population. This presents an important distinction between the statistics of FRBs and non-transient phenomena such as AGNs. The source counts statistics of static populations can only be improved in regions (i) and (ii), since generally the whole sky is already characterised at high flux densities. However, continued observations of the bright FRB population are able to progressively refine knowledge of their evolutionary history. An important unresolved question is related to the existence or otherwise of a direct relationship between fluence and distance, for FRBs. This depends on the slope of the energy (or luminosity) distribution of events; distributions with power law indices flatter than $\gamma=2$ contain a large fraction of observed events at large distances, and with steeper distributions the observations will be dominated instead by an overwhelming fraction of nearby events. This critical value of the power law of the luminosity function was first recognised in early analyses of the quasar and radio galaxy populations; at the critical value the increasing numbers of fainter sources exactly cancel the decreasing volume in which they can be seen \citep{vonHoerner1973}. The fluence - distance relation will have a very large scatter for any luminosity function whose slope is near the critical value. This situation is nearly the case for radio galaxies and quasars, yielding no clear relation between flux density and distance in region (ii). For variations in the slope of the luminosity function either side of this critical value there is either a direct statistical relation between fluence and distance, or an inverse relation in which on average fainter objects are more likely to be closer. For extragalactic radio source surveys the brightest known sources are AGN at large distances while the faintest radio sources are more likely to be nearby starburst galaxies. It is possible that a similar situation applies to the FRB population; an energy function shallower than $\gamma \approx 2$ would enable even widefield surveys with sufficient sensitivity to access region (ii) to detect events over a large range of redshifts over which the population is distributed. Observational constraints on the slope of the event rate distribution place bounds on the slope of the FRB energy and luminosity function. In particular, it is just possible to explain the steep fluence distribution suggested by existing Parkes data, but requires contrived conditions. We have investigated this in the context of a cosmological population tied to some power of the star formation rate. Although the counts slope changes continuously past the peak of the distribution in these models, an average integral counts slope of $\beta \approx -2.5$ is possible over a decade in fluence, compatible with the value $\beta = -2.6_{-1.3}^{+0.7}$ derived from the Parkes FRB sample \citep{MacquartEkers18,Bhandarietal17} . However, such a steep slope, in which strong evolution is inherent to the population, would be expected to persist only over a factor $\sim 10$ range in fluence past the peak of the distribution, and requires that the luminosity function be flatter than $\gamma \lesssim 2$. For a population that evolves linearly with the star formation rate, only bursts whose spectra are either flat or rising with frequency exhibit this behaviour. However, bursts whose fluence declines with frequency also exhibit this behaviour if the population evolves faster with redshift, as exemplified by a scenario in which the density changes quadratically with the star formation rate, a scenario which also approximates the evolution of the AGN phenomenon. The event rate counts should revert to Euclidean at yet higher fluences as the horizon of observable events draws inwards, and the effects of population evolution and spacetime curvature become negligible over the volume of detectability. The generic power-law behaviour of the high-fluence region of the distribution holds implications for the influence of scattering and lensing effects on the event rate distribution. We examine the manner in which diffractive scintillation contributes to the behaviour of the FRB counts at high fluence. The exponential distribution of event amplifications which is normally expected from diffractive scintillation declines more sharply than the $F_\nu^{-5/2}$ behaviour intrinsic to the distribution, indicating that the limiting form of the source counts should be a power-law in nature. Qualitatively different models \citep[e.g. those based on caustics produced by plasma lenses,][]{Cordesetal17} may produce different observed source counts distributions in detail, but whether FRBs are likely to be observed very far from their de-magnified (``intrinsic'') fluences still depends on the slope of the intrinsic counts distribution relative to the slope in the tail of the magnification probability distribution. The FRB dispersion measure distribution holds the potential to be both a new probe of cosmological physics, and a further measure of the burst energy and luminosity function, and the spectral index. The behaviour of the DM distribution depends on the slope of the FRB luminosity function. As with the event rate distributions with fluence, there is a qualitative change in the character of the DM distribution at at $\gamma=2$, with flatter distributions probing to high redshifts, and with the peak of the distribution turning over slowly to higher DM values. By contrast, steeper distributions exhibit a sharp peak in the DM distribution, and the mean DM moves progressively lower as the the luminosity function steepens. For this reason the evolution of the DM distribution with survey sensitivity potentially yields further information on the underlying FRB luminosity and cosmological distributions. We remark that an analysis of this type could be readily undertaken by comparing the DM distributions of surveys with different sensitivities, such as those samples acquired by Parkes and ASKAP. There is tentative evidence from the DM distribution of Parkes FRBs that favours a luminosity function flatter than $L_\nu^{-2}$. If confirmed by a rigorous analysis of the Parkes DM histogram and its associated selection biases, this would confirm that sensitivity is not the primary determinant to detect distant bursts. We remark that in this context that the recent report of a DM 2600\,pc\,cm$^{-3}$ event \citep{Bhandarietal17} would therefore be unsurprising. The DM distribution is potentially an extremely powerful probe of the baryonic content of the intergalactic medium and its cosmological development. Baryonic feedback processes may play a large role in the interpretation of FRB DMs \citep{McQuinn2014}. Another potential contributor to the shape of the DM distribution for events $z \gtrsim 2.5$ is He reionisation. We have shown that the effect of this phase transition is in principle observable in the DM distribution. However, the scenario we consider is optimistic: the DM signature is expected to be diluted if the transition is not abrupt. Random variations in the DM along individual lines of sight caused by feedback, resulting from inhomogeneity in the IGM destroying the direct relation between DM and $z$ along different sight lines, will further dilute this signal. Although FRBs have been heralded as potentially revolutionary cosmological tools \citep[e.g.][and references therein]{McQuinn2014,Macquartetal15}, it is pertinent to remember that GRBs were once similarly touted as cosmological tools, but that they largely failed to fulfil this expectation. This is because no independent distance indicator was available for each GRB, and the distribution of GRB redshifts was highly biased by the selection effects in the optical followup process. Many bursts were not followed up, and there large selection biases inherent in the GRB events for which the host galaxy redshift was obtained. FRBs offer renewed help in this regard, since the detection of events and the measurement of their associated DMs is not subject to the same strong selection biases inherent to the optical followup of GRBs. | 18 | 8 | 1808.00908 |
1808 | 1808.02620_arXiv.txt | We investigate the nature of nearby (10--15~kpc) high-speed stars in the \dr2\ archive identified on the basis of parallax, proper motion and radial velocity. Together with a consideration of their kinematic, orbital, and photometric properties, we develop a novel strategy for evaluating whether high speed stars are statistical outliers of the bound population or unbound stars capable of escaping the Galaxy. Out of roughly 1.5 million stars with radial velocities, proper motions, and \qual\ parallaxes, we identify just over 100 high-speed stars. Of these, only two have a nearly 100\% chance of being unbound, with indication that they are not just bound outliers; both are likely hyper-runaway stars. The rest of the high speed stars are likely statistical outliers. We use the sample of high-speed stars to demonstrate that radial velocity alone provides a poor discriminant of nearby, unbound stars. However, these stars are efficiently identified from the tangential velocity, using just parallax and proper motion. Within the full \dr2\ archive of stars with \qual\ parallax and proper motion but no radial velocity, we identify a sample of 19 with speeds significantly larger than the local escape speed of the Milky Way based on tangential motion alone. | The recent release of the \dr2\ catalog \citep{gaia2018a} has renewed interest in the highest velocity stars in the Galaxy. In addition to contributing new samples of candidates \citep[e.g.,][]{marchetti2018b,shen2018,hattori2018b}, \gaia\ proper motions and radial velocities allow more robust assessments of the distances and space velocities of previously identified high velocity stars \citep[e.g.,][]{boubert2018,brown2018,raddi2018}. As a result of these analyses, some stars are clearly unbound. Others are just barely bound to the Milky Way. The 6-D positions and velocities available for over 7 million sources in the \dr2\ archive also enable new tests of theoretical models for the highest velocity stars. In the current paradigm, hyper-runaway stars (HRSs) result from dynamical interactions among groups of massive stars \citep[e.g.,][]{pov1967,leon1991,perets2012} or the explosion of a massive star in a close binary \citep[e.g.,][]{blaauw1961,dedonder1997, port2000,perets2012}. The supermassive black hole in the Galactic Center may also disrupt a close binary system, capture one component, and eject the other as a hypervelocity star \citep[HVS;][]{hills1988,yu2003}. Other physical mechanisms may also accelerate stars to high velocity \citep[e.g.,][]{sesana2006,yu2007,sesana2009,abadi2009,piffl2014, capuzzo2015,fragione2016,subr2016,hamers2017}. Comparisons between observed and predicted space motions yield constraints on the Galactic potential and the ejection mechanism \citep[e.g.,][]{bromley2006,kenyon2008,bromley2009, kenyon2014,rossi2014,rossi2017,hattori2018a,marchetti2018a,kenyon2018}. Here, we use the \dr2\ proper motion and radial velocity data for the brightest stars to test several aspects of theoretical models for HRSs and HVSs \citep[see also][]{marchetti2018a,marchetti2018b}. Based on existing samples of HVSs with B-type spectra in the outer halo \citep[e.g.,][]{brown2005, edel2005, hirsch2005, brown2006a, brown2006b, brown2007a, brown2007b, brown2009b, brown2012a, brown2013, brown2014, brown2015a, brown2015b}, the probability of detecting more than one B-type HVS within 10~kpc of the Sun is small \citep{kenyon2008, kenyon2014}. Finding nearby HRSs is much easier \citep{bromley2009, kenyon2014}. For either HRSs or HVSs, surveys using the Galactic rest-frame tangential velocity should return a higher proportion of nearby high velocity stars than the radial velocity \citep{kenyon2018}. Our goal is to test these predictions with \gaia\ data. Identifying true high-velocity outliers in the \gaia\ DR2 archive is challenging. Among the roughly 7 million stars with measured parallax, proper motion, and radial velocity, no more than a few hundred candidates emerge with Galactic rest-frame velocity close to or exceeding the local escape velocity \citep[e.g.,][]{marchetti2018b,hattori2018b}. Based on the quoted errors of the measured quantities, only a few outliers unambiguously exceed the escape velocity. Our goal is to consider the wealth of kinematic information available in the \gaia\ data to analyze the distribution of dynamical parameters and their errors and to make robust estimates of the number of true outliers in the velocity distribution. Aside from identifying potential HRSs or HVSs, our analysis is important for measuring the local escape velocity and the mass of the Milky Way \citep[e.g.,][]{patel2018, gaia2018mw, watkins2018, posti2018, monari2018}. All of the \gaia\ stars with radial velocity data lie within roughly 15~kpc of the Sun. Understanding which of these stars have velocities smaller than the local escape velocity helps to establish the mass of the Milky Way within 20~kpc of the Galactic Center \citep[see also][]{monari2018}. Our analysis provides a strategy to isolate unbound outliers from those bound to the Milky Way. We begin with a discussion of the sample selection in \S\ref{sec:sampsel} and the basic properties of candidate high velocity stars in \S\ref{sec:highspeed}. In \S\ref{sec:getvel}, we assess the effectiveness of using radial velocities and proper motion separately to identify high-speed sources, and in \S\ref{sec:newvtpm} we introduce a new set of candidate stars selected on the basis of proper motion only. Comparisons with theory follow in \S\ref{sec:theory}. We conclude with a brief summary in \S\ref{sec:conclusion}. | \label{sec:conclusion} We analyze a sample of approximately 1.5~M stars with measured radial velocity and 5-$\sigma$ parallaxes from \gaia\ DR2 using a fast and accurate Quasi-Monte Carlo algorithm. The code incorporates Bayesian distance estimation and accommodates correlated erros in \dr2\ basic source parameters. All of the stars lie within about 15~kpc of both the Sun and the Galactic Center. Using their total space motion in the Galactic rest-frame, we identify the most promising HVS and HRS candidates. Considering only the stars' radial velocity or proper motion, we conclude that the Galactic rest-frame radial velocity provides a poor measure of total space motion for the fastest stars. However, the tangential velocity alone is sufficient to identify unbound star candidates within $\sim$15~kpc of the Sun. We determine Galactocentric locations and speeds, along with uncertainties, to find the probability that each source in our sample is unbound to the Galaxy. This probability, $\pub$, depends on the choice of Galactic potential \citep[we use the model in][]{kenyon2018}, the quality of the astrometric data, and the method of distance estimation from parallax. To reduce the impact of prior assumptions about source location on heliocentric distance estimation, we work with sources that have relative parallax errors of 20\%\ or less. An analysis with a heliocentric distance prior based on the bulk of \gaia\ stars gives similar results to an analysis where all parallaxes in the error distribution out to 5-$\sigma$ give physically plausible distances. Other assumptions, including a constant distribution of sources in space, admit more possibilities. We are encouraged that all methods, even the more restrictive ones, yield the same set of stars that have a high probability of being unbound. However, even when a star has $\pub$ near unity, it is only one of over 1.5 M stars with \qual\ astrometric and radial velocity data. For stars with large measurement errors, we expect to find statistical outliers drawn from the enormous bound population. Thus, we introduce an analysis to address quantitatively whether a star is truly unbound or whether its observed kinematics are consistent with a bound statistical outlier of a large sample. This analysis suggests that most high speed stars in the 6-D sample are bound outliers. Other features of the highest-speed stars support the case against unbound orbits. They have large errors in Galactocentric speed and are probably late-type giants with lifetimes rather short compared to the time scale for unbound stars to escape the Galaxy ($\sim 100$~Myr). While there may be some physical explanation for the coincidence in timing, the idea that these stars are outliers due to the large velocity errors is compelling. We suspect that many of the objects identified by \citet{marchetti2018b} and \citet{hattori2018b}, also predominantly late-type giants, are bound outliers as well. There is at least one promising object in our high-speed sample, (\gid{5932173855446728064}), first identified by \citet{marchetti2018b}, with the orbital elements of a star that is unbound to the Galaxy at a high level of confidence (\S\ref{subsec:really}). With colors (albeit reddened) that suggest an A-type main sequence star, and an orbit that runs close to the Galactic plane, this object is a hyper-runaway star candidate \citep{marchetti2018b}. However, a \dr2\ error flag is set, so we emphasize the need for observational confirmation of the source's orbital parameters. Twenty four other high-speed sources have trajectories and colors consistent with late-type giants that make them improbable HVS or HRS candidates. Our analysis of the likelihood that these objects are unbound suggests these stars are statistical outliers of the Milky Way's bound population. Nonetheless, these stars are excellent candidates for programs to obtain high quality ground-based spectra. One of these stars, \gid{1383279090527227264}, stands out, with the lowest probability that it is just an outlier. This object and another star in this group (\gid{6492391900301222656}) have orbits that passed near the LMC. Subsequent \gaia\ data releases with improved astrometry will allow refined orbit calculations and inferences about the origin of these high-speed stars. Whether bound outliers or unbound stars, some of our highest-speed stars probably have a Galactic disk origin. A significant majority show angular momentum aligned with the Galaxy's disk (Fig.~\ref{fig:gcvelgc}, lower right panel). Most of this majority are also on trajectories that are outbound from the Galactic Center. An analysis of the type introduced here, to determine whether a source is actually an unbound star or an outlier, may be adapted to constrain the mass of the Milky Way inside 10-20 kpc as in \citet{gnedin2005}. Motivated by our confirmation that proper motion alone can efficiently select nearby high-speed stars (\S\ref{sec:getvel}; see also \citealt{kenyon2018}), we identify new candidates selected from 5-D \gaia\ data. Even without radial velocities, 19 stars have unbound probabilities of 95\%\ or more, with inferred speeds between about 600~kms\ and 900~\kms. Their colors and magnitudes suggest that this sample includes both main sequence stars as well as evolved giants. Better astrometry and radial velocity measurements will help us learn if these intriguing objects are among the fastest moving stars in the Galaxy. | 18 | 8 | 1808.02620 |
1808 | 1808.10459_arXiv.txt | The kilonova emission observed following the binary neutron star merger event GW170817 provided the first direct evidence for the synthesis of heavy nuclei through the rapid neutron capture process ($r$-process). The late-time transition in the spectral energy distribution to near-infrared wavelengths was interpreted as indicating the production of lanthanide nuclei, with atomic mass number $A \gtrsim 140$. However, compelling evidence for the presence of even heavier third-peak ($A \approx 195$) $r$-process elements (e.g., gold, platinum) or translead nuclei remains elusive. At early times ($\sim$ days) most of the $r$-process heating arises from a large statistical ensemble of $\beta$-decays, which thermalize efficiently while the ejecta is still dense, generating a heating rate that is reasonably approximated by a single power-law. However, at later times of weeks to months, the decay energy input can also be dominated by a discrete number of $\alpha$-decays, $^{223}$Ra (half-life $t_{1/2} = 11.43$~d), $^{225}$Ac ($t_{1/2} = 10.0$~d, following the $\beta$-decay of $^{225}$Ra with $t_{1/2} =14.9$~d), and the fissioning isotope $^{254}$Cf ($t_{1/2} = 60.5$~d), which liberate more energy per decay and thermalize with greater efficiency than beta-decay products. Late-time nebular observations of kilonovae which constrain the radioactive power provide the potential to identify signatures of these individual isotopes, thus confirming the production of heavy nuclei. In order to constrain the bolometric light to the required accuracy, multi-epoch and wide-band observations are required with sensitive instruments like the James Webb Space Telescope. In addition, we show how a precise determination of the $r$-process contribution to the $^{72}$Ge abundance in the Solar System sheds light on whether neutron star mergers can account for the full range of Solar $r$-process abundances. | 18 | 8 | 1808.10459 |
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1808 | 1808.05693_arXiv.txt | Recent observations of several protoplanetary discs have found evidence of departures from flat, circular motion in the inner regions of the disc. One possible explanation for these observations is a disc warp, which could be induced by a planet on a misaligned orbit. We present three-dimensional numerical simulations of the tidal interaction between a protoplanetary disc and a misaligned planet. For low planet masses we show that our simulations accurately model the evolution of inclined planet orbit (up to moderate inclinations). For a planet massive enough to carve a gap, the disc is separated into two components and the gas interior and exterior to the planet orbit evolve separately, forming an inner and outer disc. Due to the inclination of the planet, a warp develops across the planet orbit such that there is a relative tilt and twist between these discs. We show that when other parameters are held constant, the relative inclination that develops between the inner and outer disc depends on the outer radius of the total disc modelled. For a given disc mass, our results suggest that the observational relevance of the warp depends more strongly on the mass of the planet rather than the inclination of the orbit.% | Planets form in cold discs of dust and gas orbiting young stars, and the gravitational interaction between forming planets and their parent disc plays a critical role in shaping young planetary systems \citep[e.g.][]{Kley:2012of,Baruteau:2014hb}. Planet-disc interactions may also create observable features, shown by high-resolution observations that have revealed a wealth of structures in many protoplanetary discs. These include spirals \citep[e.g.][]{Benisty:2015na,Stolker:2016ck,Stolker:2017of}, concentric rings \citep[e.g. HL Tau,][]{ALMA:2015ng} and large-scale asymmetries \citep[e.g.][]{vanderMarel:2018ve,Isella:2013ng,Perez:2014bq}. Such observations offer important new insights into the processes that shape the formation and early evolution of planetary systems. Recent observations have also identified a number of protoplanetary discs with asymmetric emission that is strongly suggestive of warped or tilted disc structures. Near-IR observations show non-axisymmetric moving shadows in scattered light, as observed in TW Hya \citep{Debes:2017fk} and HD 135344B \citep{Stolker:2016ck}. In the case of TW Hya, this feature may be explained by a warped or tilted inner disc casting a shadow on the outer disc, such that the shadow moves with the precession of the warped inner disc \citep{Debes:2017fk,Poteet:2018be}. Molecular line profiles in several protoplanetary discs have also been shown to be inconsistent with flat, circular discs, and instead require significant vertical or radial gas motion (e.g., HD 100546, \citealt{Walsh:2017ic,Booth:2018ng}; RY Lup, \citealt{Arulanantham:2018og}), while (sub-)mm continuum observations suggest significant misalignments between the inner ($\lesssim1$~au) and outer ($\gtrsim 10$~au) disc in a number of different objects \citep[e.g.][]{vanderMarel:2015ne,vanderMarel:2018ve,Ansdell:2016gh}. In some cases the relative inclination between the inner and outer disc components may be quite large; $\sim$72$^{\circ}$ for HD 100453 \citep{Benisty:2017kq,Min:2017od}, $\sim$45$^{\circ}$ for AA Tau, $\sim$80$^{\circ}$ for HD 100546 \citep{Walsh:2017ic} and $\sim$30$^{\circ}$ for DoAr 44 \citep{Casassus:2018te}. The deviation from planar, Keplerian motions observed in these discs is most readily explained by a gravitational perturbation from either a stellar or planetary companion. The asymmetric nature of the observed features suggests that the gravitational interaction is likely to be complicated by inclined or eccentric orbits \citep[e.g.][]{Ragusa:2017nv,Price:2018pf}. Stellar-mass binary companions are ruled out by current observations in most of these systems, but sub-stellar or planetary-mass perturbers on eccentric or inclined orbits may offer a self-consistent explanation for the observed disc structures \citep{Ruiz:2016ne}. Indeed, there is already circumstantial evidence for planetary companions in some of these systems. HD100546 has long been thought to host an embedded giant planet \citep[e.g.][]{Grady:2005vq,Acke:2006vs,Pinilla:2015ci}, perhaps on an inclined orbit \citep{Quillen:2006bv}. Recent observations of shocks in SO line emission support this hypothesis \citep{Booth:2018ng}. Similarly, in TW Hya the inner disc cavity is suggestive of dynamical clearing by a planetary companion \citep{Calvet:2002vr,Andrews:2016bw}, while recent observations also point towards the presence of one or more planets in the outer disc \citep{Tsukagoshi:2016hf,Uyama:2016ja}. Measurements of the disc inclination in TW Hya also hint at a small warp or misalignment, as observations of the inner disc ($\ll1$~au) typically infer inclinations that differ by a few degrees from those measured at large radii \citep[$\gtrsim 10$~au;][]{Qi:2004bw,Pontoppidan:2008ge,Hughes:2011gw}. Although current planet formation theory assumes that protoplanetary discs are flat, a co-planar planet may become inclined or eccentric during or after formation. For example, planet-planet interactions can move a planet to an inclined orbit \citep{Nagasawa:2008pj}. Eccentricity growth can be driven through similar mechanisms \citep[e.g.][]{Papaloizou:2001ng} as well as planet-disc interactions \citep{Goldreich:2003er,Dangelo:2006ud,Teyssandier:2017uh}. Using an analytical prescription, \citet{Thommes:2003hb} have further demonstrated that exchanges between eccentricity and inclination between two planets in resonance can increase the inclination of both planets. As interactions energetic enough to drive strong misalignments are also violent enough to eject a planet, we note that modest inclinations are likely to be favoured by scattering events. If planet formation occurs early \citep[e.g.][]{Nixon:2018hf}, the disc may still be accreting material with potentially varying angular momentum directions \citep{Bate:2010nh}, and thus could be misaligned to the average disc plane. Additionally, protoplanetary discs are relatively thick (with aspect ratio $H/R \simeq $ 0.05--0.1, where $H$ is the disc scaleheight and $R$ the cylindrical radius) such that modest planet inclinations (up to several degrees) may naturally occur during the formation process. \vspace{1cm} Early theoretical work demonstrated that the evolution of the planet's orbit is determined by its interaction with the disc \citep{Goldreich:1979ir,Goldreich:1908nf}. Numerical simulations of planet-disc interactions have primarily focused on the planet's motion, with N-body calculations of embedded planets \citep[e.g.][]{Papaloizou:2000ks} paving the way for three-dimensional simulations. For planets slightly misaligned to the disc mid-plane, the motion of the planet is characterised by radial migration and inclination damping \citep{Tanaka:2002of,Tanaka:2004od}. \citet{Bitsch:2011ne} and \citet{Ayliffe:2010ow} demonstrated that the direction of migration is constrained by the choice of thermodynamics (i.e. whether or not radiative effects are included). \citet{Ayliffe:2010ow} showed that different numerical modelling of planetary accretion (e.g. as a sink particle, with softening or with a surface) led to different migration rates, where a more accurate migration rate was achieved with the smallest accretion radius as this permitted better modelling of the flow around the planet. \citet{Bitsch:2013pf} investigated the effect of viscosity on the migration rate, demonstrating that as low viscosity discs have low temperatures, they behaved in a similar way to isothermal discs, promoting inward migration. Independent of the direction of radial migration, the inclination of these planets was always damped \citep{Bitsch:2011ne}.% More recently, \citet{Bitsch:2013hg} and \citet{Xiang-Gruess:2013fg} studied the response of a disc driven by a misaligned planet. They found that planets massive enough to carve a gap can separate the disc into two components, both of which tilted away from the initial mid-plane of the disc toward the orbit of the planet. \citet{Xiang-Gruess:2013fg} considered a wide range of planet masses and inclinations, ranging from $1 \rm M_J$ to $6 \rm M_J$ (where $\rm M_J$ is the mass of Jupiter) and 10$^{\circ}$ to 80$^{\circ}$ respectively. They demonstrated that when the planet was massive enough to open a gap, the inclination took longer to damp. In their simulations the inner disc tilted faster than the outer disc, generating relative misalignments of up to 15$^{\circ}$ after 200 planet orbits. Similarly, isothermal simulations by \citet{Bitsch:2013hg} considered the evolution of the tilt between the inner and outer disc; after 400 planet orbits larger relative misalignments were observed for smaller initial planet inclinations. \citet{Bitsch:2013hg} interpreted that lower inclination planets have a stronger effect on the disc, leading to larger disc tilts for a fixed simulation time. Importantly, their simulations demonstrated that the disc can achieve an inclination greater than the planet. The evolution of higher mass planets at inclinations of up to 20$^{\circ}$ have also been studied by \citet{Marzari:2009of}, motivated by planets that may have undergone scattering events. In agreement with \citet{Xiang-Gruess:2013fg} their results suggest that strongly inclined planets can only carve a gap once their inclination has damped to a threshold determined by the planet mass \citep[see fig. 5 of][]{Xiang-Gruess:2013fg}. \citet{Cresswell:2007ss} further demonstrated that the eccentricity of an inclined planet is generally damped, in agreement with linear theory. \citet{Arzamasskiy:2017ic} extended these studies by considering higher inclination planets and generating synthetic scattered light observations from a simulation of a disc with an inclined planet on a fixed orbit. At higher inclinations the planet spends a significant portion of its orbit outside of the disc, reducing planet-disc interactions to the instant where the planet passes through the disc. This process is usually analytically treated by a dynamical friction or aerodynamic drag approach \citep{Rein:2012so}. The synthetic observations generated by \citet{Arzamasskiy:2017ic} found little difference between the features formed by a co-planar or misaligned planet \citep[fig. 9,][]{Arzamasskiy:2017ic}. Common to all the numerical simulations that have been conducted to date is a restricted radial extent of $\lesssim 4$ times the semi-major axis of the planet (in many cases, this is equivalent to $\lesssim20$~au). However, protoplanetary discs are typically much more extended, with typical sizes of hundreds of au \citep{Andrews:2007pr,Ansdell:2018vi}. As we shall show, the choice of outer disc radius can have a significant impact on the response of the disc to the inclined planet and on the relative inclination that develops between the inner and outer disc. In this paper we use three-dimensional numerical simulations to extend previous investigations of inclined planet-disc interactions, focusing on the disc structure that can be formed by a planet on a modestly inclined orbit (i.e. $i < 3\times H/R$, where $i$ is the inclination of the planet). While asymmetric disc features may be formed in discs around binary stars \citep[e.g.][]{lubow_ogilvie_2000,Price:2018pf} or planets in systems with misaligned binaries \citep{Lubow:2016nw}, our observational motivations constrain us to exclusively consider single star systems. We make use of the smoothed particle hydrodynamics code \textsc{Phantom} \citep{Phantom}. In Section~\ref{section:nm} we describe our numerical methods and the common features in our simulations. In Section~\ref{section:linear} we benchmark our numerical code against two tests outlined by \citet{Arzamasskiy:2017ic}, demonstrating that \textsc{Phantom} can accurately model the radial migration and inclination damping of a low mass planet. For higher mass planets, Section~\ref{section:warped} begins with a comparison to a previous simulation by \citet{Xiang-Gruess:2013fg}, subsequently demonstrating the importance of a large radial extent for such simulations. We then focus more broadly on the response of a disc to an inclined, massive planet by measuring the averaged tilt and twist of disc segments carved by the planet. In Section~\ref{section:disc} we interpret these results by considering the relevant time-scales that describe the behaviour of the disc, and Section~\ref{section:concs} summarises our findings. \begin{table*} \caption{Summary of parameters used for our simulations. Multiple values indicate multiple simulations. $N$ is the number of particles, $i$ the initial inclination of the planet with respect to the mid-plane of the disc (in degrees), $m$ is the mass of the planet (in Jupiter masses), $L_{\rm p}/L_{\rm out}$ is the ratio of the angular momentum of the planet to the angular momentum of the gas exterior to the planet orbit and $R_{\rm out}$ the outer disc radius (in~au). The final time of the simulation, $t_f$, is measured in orbits of the planet at 5 au, $q$ determines the power-law slope for the sound speed (Equation~\ref{equation:sound_speed}), $R_{\rm acc}$ is the accretion radius of the planet where the Hill radius is defined in Equation~\ref{equation:hill_radius}. $\langle h \rangle/H$ is the shell averaged smoothing length to scaleheight ratio measured at the location of the planet, an indication of the resolution in each simulation (with a lower value indicating higher resolution).} \begin{tabular}{c|c|c|c|c|c|c|c|c|c} \hline Name & $N$ & $i$ & $m$ & ${L}_{\rm p}/{L}_{\rm out}$ & $R_{\rm out}$ & $t_f$ & $q$ & $R_{\rm acc}$ & $\langle h \rangle/H$ \\ \hline G Low & 1.25$\times10^5$ & 1.62-19.42 & 0.40 & 0.18 & 20 & 500 & 0 & 0.25 $R_{\rm Hill}$ & 0.50\\ G Med & 1.0$\times 10^6$ & 1.62-19.42 & 0.40 & 0.18 & 20 & 100 & 0 & 0.25 $R_{\rm Hill}$ & 0.23\\ G High & 8.0$\times 10^6$ & 1.62-19.42 & 0.40 & 0.18 & 20 & 20 & 0 & 0.25 $R_{\rm Hill}$ & 0.12\\ G2 & 1.0$\times 10^6$ & 1.62 & 0.40 & 0.18 & 20 & 100 & 0 & 0.125 $R_{\rm Hill}$ & 0.23\\ G3 & 1.0$\times 10^6$ & 1.62 & 0.40 & 0.18 & 20 & 100 & 0 & 0.40 $R_{\rm Hill}$ & 0.23\\ \hline L1 Low & 1.25$\times 10^5$ & 20.0 & 6.5 & 2.69 & 20 & 200 & 0.25 & 0.1 au & 0.60\\ L1 Med & 1.0$\times 10^6$ & 20.0 & 6.5 & 2.69 & 20 & 200 & 0.25 & 0.1 au & 0.23\\ L2 & 3.0$\times 10^5$ & 20.0 & 6.5 & 0.60 & 50 & 200 & 0.25 & 0.1 au & 0.62\\ L3 & 7.3$\times 10^5$ & 20.0 & 6.5 & 0.20 & 100 & 200 & 0.25 & 0.1 au & 0.60\\ L Low & 4.3$\times 10^5$ & 2.15, 4.30, 12.89 & 0.13, 1.3, 6.5 & 0.01 - 0.63 & 50 & 500 & 0.25 & 0.1 au & 0.50\\ L Med & 3.4$\times 10^6$ & 2.15, 4.30, 12.89 & 0.13, 1.3, 6.5 & 0.01 - 0.63 & 50 & 200 & 0.25 & 0.1 au & 0.28\\ \hline \end{tabular} \label{tab:sims_summary} \end{table*} | \label{section:concs} In this paper we use three-dimensional numerical simulations to investigate the response of a protoplanetary disc to a misaligned planet. We begin by studying a planet in the linear mass regime where the planet has a negligible effect on the disc, demonstrating that our numerical code of choice \textsc{Phantom} is able to accurately model the motion of a planet in this regime. We then consider more massive planets that are able to affect the structure of the disc. We confirm that a planet massive enough to carve a gap is able to separate the disc into two distinct discs, where the inclination of the planet drives a relative tilt and twist between the inner and outer disc. We demonstrate that altering the outer radius of the total disc alters the magnitude of the warp that develops. This is due to the angular momentum balance between the inner disc, outer disc and the planet. In the case that the outer radius is much larger than the planet orbit, the disc holds a larger fraction of the total angular momentum than the planet and does not respond strongly to the influence of the planet. The planet is still able to tilt the disc interior to its orbit (as this has less angular momentum than the planet), and so a larger relative tilt develops between the inner and outer discs than in simulations with a smaller outer radius. These results demonstrate that the outer radius is critical to the evolution of the warp in the inner regions of discs warped by misaligned planets. We examine the warp generated by planets with different masses and inclinations. To estimate which of these causes the largest (and most observationally relevant) warp we consider the difference between the unit angular momentum vectors of the inner and outer disc in each case. Importantly, this measure takes into account both the tilt and twist between each of the discs, either of which may lead to shadowing on the outer disc. We find that the mass of the planet is more relevant for generating large warps than the inclination (in the case that the planet is able to carve a gap). This suggests that the ability to carve a gap and separate the disc is the most important feature to generate a warp, with massive planets at moderate inclinations ($i \lesssim 3 \times H/R$) potentially driving significant warps. | 18 | 8 | 1808.05693 |
1808 | 1808.06373_arXiv.txt | \noindent The emission-line dwarf galaxy NGC 3413 is known to host a bright X-ray source near its optical center. The 0.3-10 keV luminosity of this source is estimated to be approximately 10$^{39}$ erg$s^{-1}$ potentially qualifying it as an ultra-luminous X-ray (ULX) source. A recent XMM-Newton observation suggests that the source is not point-like, and instead, is more likely a composite of point-like sources with extended and/or diffuse emission. The spectral and temporal features of the bright region are similar to those associated with the so-called broadened disk state of ULXs. Based on a multi-color blackbody spectral fit, we estimate the mass of the bright source to be in the range 3 - 20M$_{\odot}$. Potential optical counterparts are also explored with the aid of available SDSS and PanStars data. | \label{sec:intro} \noindent Many point-like X-ray sources with luminosities above a threshold luminosity of L$_x$ $>$ 10$^{39}$ erg$s^{-1}$, have been found in nearby galaxies \citep{astro-ph/0307077, 2004ApJS..154..519S, 2005ApJS..157...59L, 2015ApJ...805...12L}. Assuming isotropic emission, some of these sources have luminosities well in excess of 10$^{40}$ erg$s^{-1}$, surpassing emission beyond the Eddington limit for a stellar mass BH with a mass in the range 3 - 20 $M_{\odot}$. These ultra-luminous X-ray sources (ULXs) are non-nuclear i.e., located off-center from the nucleus of the host galaxy and therefore unlikely to be powered by accretion onto the central super massive black hole (SMBH). Several possibilities as to the nature of these intriguing objects continue to be discussed in the literature: a) an early model, though now seemingly less likely, poses the existence of BHs in the intermediate mass range of M$\sim$10$^{2}$ - 10$^{5}$M$_{\odot}$, accreting at sub-Eddington rates \citep{1999ApJ...519...89C, 2004ApJ...607..931M}; b) current models, based on recent data, tend to lean toward stellar-mass BHs (sMBHs) with a possible combination of effects such as relativistic beaming, and/or accretion at super-Eddington limits (\citep{2001ApJ...552L.109K, 2002ApJ...568L..97B, 2007Ap&SS.311..203R} and references therein). Indeed, very recent evidence, the detection of pulsations in a handful of sources \citep{2014Natur.514..202B, 2016ApJ...831L..14F, 2017Sci...355..817I}, strongly argues in favor of at least a fraction of these sources hosting neutron stars, implying an overall heterogeneous underlying population as opposed to a single class of objects.\\ % \\ ULX candidates have been found in diverse environments, including star-forming regions of spiral galaxies, elliptical galaxies, as well as dwarf galaxies. Dwarf galaxies are of special interest because of the existence of luminous X-ray sources in these less massive systems may have had a more significant impact on their early evolution in comparison with their more massive counterparts. \citet{2015ApJ...805...12L} with a 1.7-ks Chandra snapshot of NGC 3413 at 8.6 Mpc detected the presence of a bright X-ray source approximately 3$^{\prime\prime}$ from the optical center. Although \citet{2015ApJ...805...12L} were able to determine a luminosity of L$_{x}$ $\sim$1 x 10$^{39}$ erg$s^{-1}$, thus confirming the possible ULX candidate, the limited exposure lacked sufficient statistics to further elucidate the nature of the detected source. Of course, luminosities of the order of few $\times$ 10$^{38}$ erg$s^{-1}$ are also possible in XRBs containing (ordinary) stellar BHs and NS. The X-ray luminosity contributions from these type of sources can be estimated from the relations given by \citet{2010ApJ...724..559L}: the expected collective X-ray binary (XRB) luminosity can be parametrized in terms of the star formation rate and the contributions from LMXBs and HMXBs. For NGC 3413, the X-ray luminosity expected from the candidate ULX is in excess of the XRB contributions. Thus the source in NGC 3413, an emission-line dwarf galaxy with a mass of $\sim$10$^{8}$ M$_{\odot}$, relatively low SFR (0.06 $\pm$ 0.02 M$_{\odot}$$yr^{-1}$), and a low hydrogen column density (N$_H$$\sim$ 2$\times$10$^{20}$cm$^{-2}$), is potentially of high value as a prime ULX candidate. \\ \\ Recent studies indicate that ULXs exhibit different spectral behavior in comparison with low-luminosity galactic BHs, featuring two-component spectra with soft excess and a turnover at energies near 5 keV. Indeed, \citet{2013MNRAS.435.1758S} extracted fractional variability and constructed variability-hardness diagrams to distinguish three main states i.e., the broadened disk, hard ultraluminous, and the soft ultraluminous. One of the conclusions they reached was that ULXs with an X-ray luminosity of $<$ 3 $\times$ 10$^{39}$ erg s$^{-1}$ are dominated by broadened disk spectra. \\ \\ In this work, we present the results of a spectral analysis of a recent $\sim$ 100ks $XMM-Newton$ observation of the ULX candidate in NGC 3413. Our results suggest that of the three recently identified ULX transition states \citep{2009MNRAS.397.1836G, 2013MNRAS.435.1758S}, the source in NGC 3413 falls into the broadened disk category. The paper is organized as follows: Section 2 reports the details of the observations and the data reduction; Section 3 presents the results of the spectral and temporal analysis of $XMM-Newton$ data, as well as, our findings into the inquiry of potential optical counterparts probed via a color-magnitude diagram (based on SDSS and PanStars data) and mass estimates for the secondary. In section 4, we summarize our findings. | \noindent We have performed a detailed spectral analysis of deep \emph{XMM-Newton} observations of a bright X-ray source in the nearby dwarf emission-line galaxy NGC 3413. In addition, we have explored possible optical counterparts with the available SDSS and PanStars data. Our main findings are as follows: \begin{itemize} \item The bright X-ray source near the optical center of NGC 3413 is not point-like, and instead, is more likely a composite of point-like sources with extended and/or diffuse emission. This is consistent with \citet{2011ApJS..192...10L} catalog where it is flagged as a source with possible extended emission \item Assuming the source is composed of a brighter central component (src1) and a dimmer extended region (src2), our spectral and temporal results indicate that the bright component (src1) exhibits features similar to those associated with the so-called broadened disk state of ULXs \item The central bright region (src1) is best fitted with a combination of PL + diskbb models. The disk temperature is relatively high ($\sim$2 keV), a feature that is observed for other broadened disk states in ULXs. Furthermore, the fractional variability is also relatively high and is consistent with that observed for other ULXs in the broadened disk state. The turnover typically associated with ULX spectra is not immediately obvious from a visual inspection but a hint of its presence is obtained if one fits the spectrum with a broken power law; the break, while not tightly constrained, is implied around $\sim$ 3 keV. Hint of some diffuse emission is indicated with a fit using the mekal model but the extracted gas temperature is unusually low \item Based on the CMD for NGC 3413 and PARSEC isochrones, we estimate the age of possible optical counterparts to be $<$ 12 Myr, although we note that this is a rather crude estimate given the large error circles associated with the X-ray sources. Using the SDSS magnitudes and with the assumption of a main-sequence counterpart, we estimate its mass to be in the range 8 - 18M$_{\odot}$, making src1 consistent with a high-mass X-ray binary \item We follow the procedure of \citet{2000ApJ...535..632M} and use the normalization of the multi-color blackbody spectral fit to a determine a mass range of 3 - 20 M$_{\odot}$ for the compact object associated with src1 \item The dimmer region (src2) is best fitted with a combination of mekal + Raymond models, suggesting the presence of an extended source with diffuse emission. \item The bright optical center, spanning a region $\sim$120pc (assuming SDSS resolution), is consistent with multiple unresolved sources with a total mass in the range $\sim$(130 - 670)M$_{\odot}$. \end{itemize} \noindent Finally, we note that the sub-arcsecond resolution of the HST would be very helpful in resolving the multiplicty of the optical sources in the center of NGC 3413. Similarly, a deep Chandra observation, with its superior spatial resolution, would definitely settle the issue whether the bright X-ray source is point like or a composite of several sources. \begin{deluxetable*}{crrrrrrrr} \tablecaption{Possible counterparts for X-ray sources src1 and src2: Optical data from $sdss$ and $panstarrs$ source catalogs.} \tablehead{ \colhead{ObjID} & \colhead{RA} & \colhead{DEC} &\colhead{u-mag} & \colhead{g-mag} \\ \colhead{} & \colhead{degree} & \colhead{degree} & \colhead{AB Mag.} & \colhead{AB Mag.} } \startdata {\bf sdss sources} \\ J105121.00+324549.0\tablenotemark{1} & 162.838 & 32.764 & 23.29$\pm$0.68 & 21.13$\pm$0.06 \\ J105120.53+324613.2\tablenotemark{2} & 162.836 & 32.770 & 22.35$\pm$0.29 & 22.23$\pm$0.14 \\ J105120.73+324558.9\tablenotemark{3} &162.836 & 32.766 & 14.24$\pm$0.003 & 13.17$\pm$0.002 \\ {\bf panstarrs sources} \\ 147311628360796000\tablenotemark{1} & 162.836 & 32.763 & \nodata & \nodata \\ 147311628382496000\tablenotemark{1} & 162.838 & 32.763 & \nodata & \nodata \\ 147321628360082000\tablenotemark{2} & 162.836 & 32.768 & 19.83 & \nodata \\ 147321628359231000\tablenotemark{2} & 162.836 & 32.767 & 18.21$\pm$0.08 & \nodata \\ 147321628354416000\tablenotemark{2} & 162.835 & 32.771 & \nodata & \nodata \\ 147311628362049000\tablenotemark{3} & 162.836 & 32.766 & 17.10$\pm$0.01 & \nodata \\ 147321628364750000\tablenotemark{3} & 162.836 & 32.766 & 16.71$\pm$0.12 & \nodata \\ 147311628365499000\tablenotemark{3} & 162.836 & 32.766 & \nodata & \nodata \\ \enddata \label{table:nonlin} % \tablenotetext{1}{sdss/panstarrs potential counterpart for src 1.} \tablenotetext{2}{sdss/panstarrs potential counterpart for src 2.} \tablenotetext{3}{sdss/panstarrs optical source near center.} \end{deluxetable*} | 18 | 8 | 1808.06373 |
1808 | 1808.06659_arXiv.txt | We propose and test a method for applying statistical photometric parallax to main sequence turn off (MSTO) stars in the Sloan Digital Sky Survey (SDSS). Using simulated data, we show that if our density model is similar to the actual density distribution of our data, we can reliably determine the density model parameters of three major substructures in the Milky Way halo using the computational resources available on MilkyWay@home (a twenty parameter fit). We fit the stellar density in SDSS stripe 19 with a smooth stellar spheroid component and three major streams. One of these streams is consistent with the Sagittarius tidal stream at $21.1$ kpc away, one is consistent with the trailing tail of the Sagittarius tidal stream in the north Galactic cap at $48$ kpc away, and one is possibly part of the Virgo Overdensity at $6$ kpc away. We find the one sigma widths of these three streams to be $1.0$ kpc, $17.6$ kpc, and $6.1$ kpc, respectively. The width of the trailing tail is extremely wide ($41$ kpc full width at half maximum). This large width could have implications for the shape of the Milky Way dark matter halo. The width of the Virgo Overdensity-like structure is consistent with what we might expect for a ``cloud"-like structure; analysis of additional stripes of data are needed to outline the full extent of this structure and confirm its association with the Virgo Overdensity. | \subsection{Milky Way halo substructure} The distribution of stars in the Galactic halo is dominated by dwarf galaxies and tidal streams of stars that have been stripped from them [see Fig. 1 of \cite{newberg2002}, and the ``Field of Streams" from \citep{Belokurov2006}]. These substructures represent the recent minor merger history of the Milky Way \citep{BullockJohnston2005}, and contribute to the build-up of stars in the Milky Way stellar halo. Two or three dozen tidal debris streams, most of which extend tens of degrees or more across the sky, have been identified; the exact number cannot be determined due to controversy over the identity of tidal streams, particularly those discovered near the Galactic plane, and because some streams are detected at low enough significance that they are considered stream ``candidates." In addition to streams, other substructures of ambiguous origin (most notably ``clouds") continue to be discovered in the Galactic spheroid. For a review of tidal streams and clouds, see \cite{GrillmairCarlin2016}. A more recent list of streams and clouds included in the `GALSTREAMS' Python Package can be found in Table 4 of \cite{Mateu2018}. More recent halo substructures are identified in \cite{LiBalbinotMondrik2016}, \cite{Sohn2016}, \cite{Grillmair2017b,Grillmair2017a}, \cite{Jethwa2017}, and \cite{Shipp2018}. In particular, \cite{Shipp2018} identify eleven new substructures in the Milky Way halo using data from the Dark Energy Survey \citep[DES;][]{DESCollaboration2005, DESCollaboration2016}. Identification and measurement of tidal debris in the Milky Way halo is useful for understanding structure formation and galaxy assembly, and it has the potential to constrain the density distribution of the Milky Way's stellar halo. Methods for measuring the halo shape from tidal streams have been, and continue to be, developed \citep[e.g.][for a review]{Law2010, Koposov2013, Kupper2015, Bovy2016, DierickxLoeb2017a, Sanderson2017, JohnstonCarlberg2016}. In addition, the distribution of dark subhalos can be measured by looking for stars ejected from tidal streams \citep{SiegalGaskinsValluri2008}, stream heating \citep{Johnston2002} or stream gaps \citep{Carlberg2012}. \cite{Pearson2017} show that streams can also be used to constrain the rotation rate of the Galactic bar. These techniques to determine the dark matter distribution in the Milky Way from tidal streams rely on accurate measurements of the tidal debris itself, but as we discover that halo tidal streams are more numerous and complex than originally thought, the association of particular stars with particular tidal streams becomes more ambiguous. For example, \cite{Newberg2009} discovered that the blue horizontal branch stars (BHBs) thought to be associated with the southern portion of the Sagittarius (Sgr) dwarf tidal stream in \cite{Yanny2000} are actually part of the Cetus Polar Stream. The Sgr dwarf tidal stream, which is the most prominent tidal stream in the sky, and the so-called ``bifurcated" Sgr stream that appears to split off from it, have also caused confusion; for example \cite{newby2013} suggested that the southern Sgr stream could be associated with the ``bifurcated" stream in the north, and the northern Sgr stream could be associated with the ``bifurcated" stream in the south. These misidentifications and possible misidentifications of stars in the most prominent halo streams underscore the difficulties in counting and characterizing tidal streams. \subsection{Statistical photometric parallax} In this paper we present an improved {\it statistical photometric parallax} \citep{cole2008, newberg2013} method to measure the spatial density of stars in the Milky Way stellar halo, using turnoff stars from the Sloan Digital Sky Survey \citep[SDSS;][]{SDSSYork}. Statistical photometric parallax is the use of statistical knowledge of the distribution of the absolute magnitudes of stellar populations to determine the underlying density distributions of those stars. This differs from photometric parallax in that the distance to each individual star is not determined. The idea of using turnoff stars to trace Milky Way halo substructure was introduced by \cite{newberg2002}. They observed density substructure in the SDSS turnoff stars on the Celestial equator, and fit an absolute magnitude distribution to their blue turnoff star tracers. In \cite{cole2008}, these tracers and a simplified absolute magnitude distribution from \cite{newberg2002} were used to build a model of the Milky Way halo and its substructure and complete preliminary fits to the stellar density of the halo and the Sagittarius dwarf galaxy tidal stream. Several years later, \cite{newby2013} continued working with this model and showed that the massive distributed computing network, MilkyWay@home, could be effective in constraining the parameters in the density models of tidal streams. Taking advantage of this new computational power, several optimizations were run on each stripe. Initially, the fitting algorithms were allowed great freedom in selecting the parameters. Later, the parameters were constrained based on the results from neighboring stripes. These previous studies successfully used statistical photometric parallax to study the structure of the halo using SDSS turnoff stars. SDSS turnoff stars, detected to a limiting magnitude of $g=22.5$, can be used to trace the structure of the Milky Way to 45 kpc from the Sun. However, the turnoff stars in a single stellar population, with the same color, can differ in absolute magnitude by two magnitudes (producing a distance error of a factor of 2.5). Photometric parallax \citep[e.g.][]{Juric2008} is unusable with turnoff stars because astronomers do not have a way to determine the distance to individual stars with reasonable accuracy using photometry alone. It has been shown that the absolute magnitude distribution of turnoff stars in halo globular clusters are surprisingly similar to each other, over a metallicity range -2.3$<$[Fe/H]$<$-1.2 dex and over ages ranging from 9 to 13.5 Gyr \citep{newby2011}. \cite{Grabowski2013} showed that this similarity holds even for the globular cluster Whiting 1, which is only 6 Gyrs old and has a metallicity of approximately [Fe/H]$\sim$0.6 dex \citep{Carraro2007, Valcheva2015}. This surprising result, which comes about due to the age-metallicity relation for Milky Way stars, makes turnoff stars very useful for tracing the density of the stellar spheroid and outer disk. In our work, we improve on the statistical photometric parallax methods by implementing a better model for the absolute magnitude of the tracer stars and their detection efficiency, and by using better fitting methods on MilkyWay@home. In our implementation of statistical photometric parallax, we find the parameters in a density model that make the apparent magnitudes and angular positions of the observed stars most likely, using a maximum likelihood estimator (MLE) \citep{MLinAstronomy}. The statistical description of the absolute magnitudes of the stellar tracers, and of the selection effects in the data, make statistical photometric parallax somewhat complex to apply. Because we are able to take all of these effects into account, we can reliably measure density distributions in real data. There are four parts to statistical photometric parallax: data, a density model, an algorithm for measuring how well the model fits the data, and an algorithm for optimizing parameters. The algorithm that measures how well the model fits the data includes the MSTO absolute magnitude distribution, as well as any observational biases. It has taken us many years to perfect the algorithm that can simultaneously fit the spatial density of several tidal streams plus a smooth distribution to the SDSS MSTO stars; the smooth distribution represents the sum of: streams from small satellites, old streams that they are well mixed in density, and stars that were created during the collapse of the Milky Way, if any. In this paper, we describe an improved algorithm for characterizing the spacial characteristics of stellar streams in the Milky Way halo using turnoff stars, and show that it is capable of simultaneously recovering the characteristics from three tidal streams plus a smooth halo component, using simulated data designed to mimic the stellar density in the actual Milky Way halo. \subsection{The big three halo substructures: the Sgr tidal stream, the ``bifurcated" stream, and the Virgo Overdensity} We will present preliminary results for one $2.5^\circ$-wide SDSS stripe (stripe 19) of data that cuts across the northern Galactic hemisphere. Figure \ref{SDSSNorth} shows the position of stripe 19 in the SDSS northern footprint. The results from this stripe provide measurements of the largest known substructures in the Milky Way halo: the Sgr dwarf tidal stream, the so-called ``bifurcated" stream, and the Virgo Overdensity. Stripe 19 crosses the Sgr dwarf tidal stream and the bifurcated stream, in a region of the sky in which they are clearly separated. Since stripe 19 is more than 20 degrees from the densest portion of the Virgo Overdensity, it is uncertain whether a third substructure measured here is in the tails of the Virgo Overdensity, or whether it is associated with a new halo substructure. In addition to three substructures, we fit smooth Milky Way halo and thick disk distributions. The Sgr dwarf galaxy was first discovered by \cite{Ibata1995}, who found evidence of a dwarf galaxy within 16 kpc of the Galactic center, on the far side of the Milky Way, that was thought to be in the process of tidally disrupting. The tidal stream of stars stripped from this dwarf galaxy have since been found to dominate the substructure of the Galactic halo \citep[e.g.][]{newberg2002,Majewski2003,Belokurov2006,Hernitschek2017}. Though the Sgr dwarf galaxy and the stream of stars that have been tidally stripped from its gravitational grasp have been studied extensively \citep[see][for a recent review]{LawMajewski2016}; we are only starting to understand the dynamical history of this present-day merger. It has been a challenge to find a disruption model that simultaneously fits the positions of the leading and trailing tidal streams in the sky, the line-of-sight velocities of the stream stars, and the observed extension of the trailing tidal tail to $\sim$100 kpc from the Galactic center \citep{Newberg2003,Belokurov2014}. \cite{DierickxLoeb2017b} present a recent simulation of the tidal debris that reproduces most of the measurements of the position of the leading and trailing tidal debris, including the distant stars in the trailing tidal tail and the observed ``spurs" at apogalacticon \citep{Sesar2017}, but still doesn't reproduce the line-of-sight velocities of the leading tail. A previous model by \citet{Law2010} was able to fit the velocities of the leading tidal tail, using a triaxial dark halo model in which the disks rotate around the intermediate axis. However, this Milky Way configuration is very unlikely \citep{Debattista2013}. Refining the spatial distribution of the Sgr dwarf tidal stream using the algorithm described in this paper will help constrain N-body simulations of the Sgr dwarf tidal disruption, and lead to a better understanding of the shape of the Milky Way's dark matter halo. The ``bifurcated" stream can be seen clearly in the ``Field of Streams" as a lower surface brightness companion stream to the Sgr dwarf tidal stream \citep{Belokurov2006}. Belokurov identifies the Sgr dwarf tidal stream as ``Stream A," the ``bifurcated stream" as Stream B, and tentatively identifies a more distant ``Stream C" behind Stream A, which is now generally associated with an extension of the Sgr trailing tidal tail \citep{LiSmith2016}. \citet{Koposov2012} shows the analogous bifurcated stream in the south Galactic cap. Although the origin of the second, lower surface brightness stream close to the Sgr stream is not known, a leading possibility is that it could arise from multiple wraps of the stream around the Milky Way \citep{Fellhauer2006}. Since its discovery, this stream has remained relatively unstudied compared to its sibling. \cite{Newberg2007} derive distances that are slightly farther away than Sgr for the bifurcated stream. In contrast, \citet{NiedersteOstholt2010} says the bifurcated stream is slighly closer to the Sun than the Sgr dwarf tidal stream and \cite{Ruhland2011} finds the distances are basically the same. \citet{Slater2013} show that the southern bifurcated stream is closer to the Sun than the southern portion of the Sgr dwarf tidal stream. \citet{Yanny2009} show that the velocities and metallicities along the bifurcated stream are similar to those in Sgr. In \cite{Koposov2013}, there is evidence presented that the two streams may both pass through the progenitor, but due to the proximity of the progenitor to the Galactic bulge, it is difficult to see where exactly the two cross in reference to the progenitor. In \cite{newby2013} it is suggested the streams may be from two separate progenitors that accreted around the same time, but the evidence to support this is not strong. \citet{Hernitschek2017} give a possible fit to the bifurcated stream. Currently, the origin of this stream is still an open question that our results will help answer. Determining the origin of the ``bifurcated" stream is critically important, as it is useful for constraining the Milky Way potential \citep{Law2010, ViraCiro2013}. The Virgo Overdensity/Virgo Stellar Stream \citep{Vivas2001,Juric2008,Duffau2006,Newberg2007} is a third large halo overdensity in the northern Galactic hemisphere, at distances of $6-20$ kpc from the Sun. It is unclear whether this feature is a tidal stream, a ``cloud," or a combination of several different pieces. \cite{CarlinVirgo} fit an orbit to the puffy structure, and suggest that this overdensity is the result of a recently disrupted massive ($10^9 M_\odot$) dwarf galaxy. \cite{CarlinVirgo} also finds their orbit includes the Pisces Overdensity. \citet{LiBalbinotMondrik2016} suggest Virgo could instead be associated with the Hercules-Aquila Cloud and Eridanus-Phoenix overdensities, since they are on the same polar plane, have similar galactocentric distances (18 kpc), and are separated by 120 degrees. In \citet{Bonaca2012}, it is suggested that Virgo is ``cloud-like" and may have been the result of a minor merger that passed close to the Galactic center. \citet{Vivas2016} find several different, presumably unrelated, substructures of RR Lyrae stars at distances of 10-20 kpc in the Virgo region, and suggest there could be additional substructures at much larger distances. The evidence for a more distant Virgo substructure is amplified by \citet{Sesar2017}, who find an outer Virgo overdensity at a distance of 80 kpc from the Sun. The Milky Way stellar halo has traditionally been described by a smooth power-law distribution \citep[e.g.][]{Oort1975, Preston1991}. Since the discovery of significant substructure in the stellar halo \citep{newberg2002}, researchers have had to choose whether to include or exclude these substructures when fitting the overall spheroid density. The smooth density component of the halo includes smaller or more thoroughly mixed remnants of tidal stripping, as well as any stars that were created in the initial gravitational collapse of the Milky Way galaxy. The algorithms used in this project will fit the smooth component and streams simultaneously. Using simultaneous fitting for the background (as we will refer to the smooth component) and streams instead of subtracting a background to fit streams, we learn about the shape and density of this smooth component of the stellar halo without requiring a clean sky sample to fit it. | In this paper, we show the result of an updated version of the maximum likelihood technique developed by \cite{cole2008} and \cite{newby2013}, using updated MSTO absolute magnitude distributions from \cite{newby2011} and then fit both simulated data and real data from the SDSS. The major conclusions are: \begin{enumerate} \item Our new model returns parameters that are consistent with those used to create simulations, and is ready to use on real SDSS data. Clean separations of all of the simulated substructure in a simulated SDSS stripe 19 can be seen in Figures \ref{Stripe19Figure} and \ref{Sim19Figure}, and the comparisons of simulated to optimized parameter values can be found in Table \ref{ResultsTable}. \item With the inclusion of the absolute magnitude distribution and selection efficiency from \cite{newby2011} we fit the Sgr stream, the trailing tail of Sgr in the north, and a third stream, which is consistent with at least some measurements of the Virgo Overdensity. The fit density parameters for these structures in SDSS stripe 19 can be found in Table \ref{ResultsTable2}; the parameters from which the densities can be more easily derived using Equations \ref{PrelimBGDensity} and \ref{PrelimStreamDensity} are listed in Table \ref{ResultsTable3}. The large width could have implications for the shape of the dark matter halo. \item The presumed Sgr trailing tidal tail in north Galactic cap is much wider than the leading Sgr tidal stream, which has implications for the shape of the halo. This stream could possibly be what is seen as the bifurcation of the Sgr tidal stream in the \cite{Belokurov2006} ``Field of Streams" image. The trailing tidal tail has a width of $\sigma = 17.6$ kpc versus $1.0$ kpc for the Sgr leading tidal tail, in our preliminary SDSS stripe 19 data fits. The distance ($48$ kpc) to the trailing tidal tail is much father than we initially expected for the ``bifurcated" stream, which has previously been reported to be $32$ kpc in \cite{Newberg2007}, $27$ kpc in \cite{Belokurov2006}, and $17$ kpc in \cite{Hernitschek2017}. If there is a separate ``bifurcated" stream at the distance of the Sgr leading tidal tail, it was not fit by MilkyWay@home as one of the three streams. Fitting four or more streams will be attempted in future work. \end{enumerate} In summary, we have developed and tested an improved algorithm for fitting stellar substructure in the Milky Way halo and demonstrated it on the data available from SDSS stripe 19 as well as on simulated test data. We provide preliminary density information for the substructure fit in SDSS stripe 19. We demonstrate our ability to correct for biases introduced when streams are near the limiting magnitude of the survey, and to separate overlapping substructures from each other. Looking forward, this algorithm will be run on all of the available SDSS data in the north and south Galactic caps. In the north there are 24 stripes of data and in the South there are 5 stripes of data, each $2.5^{\circ}$ wide. Each of these stripes will be run through MilkyWay@home, through this effort, we will use SDSS turnoff stars to map the shape and density of the Milky Way stellar spheroid, including its major substructures. | 18 | 8 | 1808.06659 |
1808 | 1808.01286_arXiv.txt | In this work, we derive a spatially resolved map of the line-of-sight velocity of the interstellar medium and use it, along with a second map of line-of-sight velocity from Paper I of this series, to determine the nature of gaseous spiral structure in the Milky Way. This map is derived from measurements of the 1.527 $\mu$m diffuse interstellar band (DIB) in stellar spectra from the APOGEE survey and covers the nearest 4-5 kpc of the Northern Galactic plane. We cross-check this new DIB-based line-of-sight velocity map with the map derived in Paper I and find that they agree. We then compare these maps with line-of-sight velocity maps derived from simulations of quasi-stationary density wave spiral structure and dynamic, or material, spiral structure in a Milky Way-like galaxy. While none of the maps derived from these simulations is an exact match to the measured velocity field of the Milky Way, the measurements are more consistent with simulations of dynamic spiral structure. In the dynamic spiral structure simulation that best matches the measurements, the Perseus spiral arm is being disrupted. | \label{sec:intro} There are two main models for how a differentially rotating galaxy can have long-lived spiral structure. The first is the stationary density wave model \citep[SDW;][]{1964ApJ...140..646L,Shu:2016ij}. In the SDW model, spiral arms are global oscillatory modes of a stellar or gaseous disk. The group velocity of a wave does not have equal the velocity of the oscillating matter, so the arms can propagate without winding up. The second model for long-lived spiral structure is known as the dynamic, the transient and recurrent, or the material spiral structure model \citep{1984ApJ...282...61S}. Here, we will use the term ``dynamic'' to describe these models. In the dynamic spiral structure model, the pattern is corotating with the matter. Individual spiral arms form through a process such as swing amplification, wind up, and dissipate over one to a few Galactic rotation periods \citep{1984ApJ...282...61S,2013ApJ...766...34D}. If the arm formation process is efficient, these dissipating arms are rapidly replaced, meaning that although individual spiral arms are short-lived, spiral structure in general is long-lived. It is not known whether the type of spiral structure in most spiral galaxies is SDW or dynamic. Evidence has been found for both models, sometimes in the same galaxies; see \citet{Shu:2016ij} for examples of evidence in support of the SDW model and \citet{2014PASA...31...35D} in support of the dynamic model. In this paper, we focus on the spiral structure of the Milky Way. Since the 1950s, the consensus has been that the Milky Way has spiral arms in has spiral arms in gas, star formation, and young stars \citep{Morgan:1952gm,1954BAN....12..117V} The gaseous spiral arms are detected as contiguous features in $\ell-v$ diagrams and have been seen in \HI{}-traced neutral gas \citep{1954BAN....12..117V} and CO-traced molecular gas \citep{1980ApJ...239L..53C}. Arms traced by young stars \citep[e.g.][]{Morgan:1952gm,Xu:2018kg} and star formation regions \citep[e.g.,][]{2014ApJ...783..130R} are detected as contiguous overdensities in space. Emission from the gaseous arms can be detected out to large distances, including the far side of the Galaxy \citep{Dame_2011_outerspiralarm}, but their positions in space can only be inferred using indirect methods such as the kinematic distance method. The situation is reversed for the star forming arms -- their positions in space are known directly, allowing measurements of arm properties such as pitch angles to be made, but the necessary observations are not available at large distances from the Sun. The measurable distribution of stars and star formation in $\ell$, $b$, and $d$ and of gas in $\ell$, $b$, and $v_d$ can fit into the context of either model of spiral structure and cannot decisively distinguish between them. In this work, we investigate what can be determined about the Milky Way's spiral structure from the velocity field of its interstellar medium (ISM). These two theories of spiral structure make different predictions for large-scale streaming motions, i.e. spatially coherent deviations from simple rotation. Spiral structure induces, and is produced by, streaming motions. These streaming motions should be particularly clear in the velocity field of interstellar matter, which is collisional and hence dynamically cold. In the SDW model, interstellar matter flows \emph{through} a spiral arm or, equivalently, the overdensity of ISM that is an arm moves through the disk of the galaxy \citep{1969ApJ...158..123R}. An SDW arm is simultaneously accumulating matter from one side and losing it from the other in a flow that spans the entire length of the arm. In the dynamic model, gas converges on a spiral arm that is growing and is sheared or blown away (e.g., by stellar and supernova feedback) from a spiral arm that is winding up and dispersing \citep{2016MNRAS.460.2472B}. This convergence is thought to happen due to a combination of orbit crowding and the gravitational influence of the stellar component of the spiral arm. The SDW model predicts a global flow through each arm; the dynamic model predicts local flows converging or diverging from each arm. This distinction is why the velocity field of the ISM is a powerful discriminator. To have a quantity that can be directly compared to a gas velocity field measurement, we have collected ten simulations of spiral structure in Milky Way-like galaxies. Five of these are SDW simulations and five are dynamic simulations. These simulations were tuned by their authors to match certain observations of the Milky Way but are not considered to be perfect matches. Our primary observable, the line-of-sight velocity of interestellar matter as a function of position in the Galaxy (which we call $v_d(x, y)$), was not directly used to tune any of the simulations and so can be considered a prediction. We construct empirical maps of the Milky Way's $v_d(x, y)$ field and compare them with predictions for $v_d(x, y)$ from the simulations. The maps are made using a collection of techniques we call ``Kinetic Tomography'' (KT). In \citet[][henceforth TP17]{Tchernyshyov_2017_kt1}, we developed a method for combining measurements of \HI{} and CO emission in ($\ell,b,v_d$) space with the three-dimensional ($\ell, b, d$) reddening map of \citet{2015ApJ...810...25G} to produce a map of $v_d(\ell, b, d)$. We will call this method ``gas and dust KT'' (G\&D KT) and the resulting map the G\&D KT map. In TP17, we validated the G\&D KT map in regions containing very dense gas. To discriminate between theories of spiral structure, we also need to be sure that the $v_d$ map is correct in more diffuse regions. The main observational contribution of this work is a second map of the Milky Way ISM velocity field, which we will compare to the G\&D KT map and simulations of spiral structure. This map is based on ISM absorption lines in spectra of stars with known distances. An ISM absorption line provides the sightline-integrated distribution of its carrier species with respect to $v_d$. By taking differences between the optical depth profiles of the ISM along approximately the same sightline but with different terminal distances (i.e., different stellar distances), we can get a measurement of the average $v_d$ of the ISM between the endpoints of those sightlines. With enough background stars, a more sophisticated version of this procedure can be used to make a continuous map of $v_d$. The absorption line we use is the 1.527 $\mu$m diffuse interstellar band (DIB) \citep[][henceforth Z+15]{Geballe_2011_IRdibs,Zasowski:2015hi} in stellar spectra from the APOGEE survey \citep[Section~\ref{subsec:dib_in_apogee};][]{Majewski_2017_apogeeoverview}. APOGEE covers much of the Northern Galactic plane and, because it is a near infrared survey, can obtain high resolution, high signal-to-noise ratio spectra of distant, highly reddened stars. In the first half of this paper, we analyze DIB absorption in APOGEE spectra to produce a map of the local (i.e., non-integrated) line-of-sight velocity using a procedure we will call DIB KT. The rest of the paper is organized as follows: In Section \ref{sec:data}, we describe the datasets we use. In Section \ref{sec:dibkt}, we explain our map-making procedure, and in Section \ref{sec:comparison-models}, we list and describe the simulations to which we compare our $v_d$ maps. In Section \ref{sec:results}, we check the DIB KT map against the G\&D KT map. In Section \ref{sec:discussion}, we compare the DIB KT and G\&D KT maps with $v_d$ maps from simulations and argue that the Milky Way has dynamic, rather than SDW, spiral structure. Finally, in Section \ref{sec:conclusion}, we conclude. Throughout this work, we assume a Sun-Galactic center separation of 8.5 kpc, a Galactic rotation rate of 220~km~s$^{-1}$, and a motion of the Sun relative to the local standard of rest (LSR) of 12 km/s towards the Galactic center, 9 km/s in the direction of Galactic rotation at the Sun, and 7 km/s towards the North Galactic pole. \begin{figure} \includegraphics[width=\linewidth]{integrated-frame-data.pdf} \label{fig:integrated-quantitites} \caption{(Left) The equivalent width of the DIB feature towards stars in our subsample of the APOGEE survey. The Galactic center is to the right and the direction of Galactic rotation at the position of the Sun is upwards. (Middle) The first moment $v_{d,int}$ of the DIB feature towards stars in our dataset. The velocity $v_{d,int}$ is the DIB density-weighted average of the (local, un-integrated) velocity $v_d$ along the line of sight. (Right) The $v_{d,int}$ field of a uniform interstellar medium undergoing flat 220 km/s rotation. The measured and computed $v_{d,int}$ are both in the heliocentric rest frame. } \end{figure} | \label{sec:conclusion} In this work, we have been able to distinguish between different theories of spiral structure for the Milky Way using maps of the Milky Way ISM velocity field. In particular \begin{itemize} \item{In a process called \emph{Kinetic Tomography}, we have constructed a map of Milky Way's ISM velocity field using observations of a diffuse interstellar band ISM absorption line toward distant disk stars that is consistent with previous maps.} \item{Spiral density wave theory and dynamic spiral theory make significantly different predictions about this velocity field, especially in the neighborhood of spiral arms.} \item{We find that one simulation of each spiral theory has rough quantitative agreement with the maps of the Milky Way velocity field.} \item{We find that only dynamic spiral theory can account for the Milky Way's complex velocity field, and the divergence of the velocity field detected at the Perseus Arm.} \end{itemize} This work has shown that measuring the velocity field of the dynamically cold interstellar medium can provide a unique and powerful tool for distinguishing between theories of how our Milky Way is structured. There are a number of upcoming programs that will continue to enhance our view of the velocity field of the ISM. APOGEE-II \citep{Zasowski_2017_apogee2targeting} will explore the southern hemisphere in DIBs, and the DECaPS program \citep{2018ApJS..234...39S}, designed to fill in the fourth quadrant of the \citet{2015ApJ...810...25G} 3D dust map, will allow us to construct all-sky G\&D maps. These maps will allow us to study the complete Scutum-Centaurus arm in the inner galaxy, home of much of Milky Way's star formation. SDSS-V, expected to begin observations in 2020, will observe many millions of giants across the Milky Way disk at APOGEE resolution, and has a subprogram devoted to measuring DIBs and dust toward stars within 4 kpc of the Sun. This will give us both better reach to study velocity fields across the entire Milky Way, and a much more detailed picture locally. | 18 | 8 | 1808.01286 |
1808 | 1808.04376_arXiv.txt | We demonstrate the formation of gravitationally unstable discs in magnetized molecular cloud cores with initial mass-to-flux ratios of 5 times the critical value, effectively solving the magnetic braking catastrophe. We model the gravitational collapse through to the formation of the stellar core, using Ohmic resistivity, ambipolar diffusion and the Hall effect and using the canonical cosmic ray ionization rate of $\zeta_\text{cr} = 10^{-17}$ s$^{-1}$. When the magnetic field and rotation axis are initially aligned, a $\lesssim1$~au disc forms after the first core phase, whereas when they are anti-aligned, a gravitationally-unstable 25~au disc forms during the first core phase. The aligned model launches a 3~km~s$^{-1}$ first core outflow, while the anti-aligned model launches only a weak $\lesssim 0.3$~km~s$^{-1}$ first core outflow. Qualitatively, we find that models with $\zeta_\text{cr} = 10^{-17}$ s$^{-1}$ are similar to purely hydrodynamical models if the rotation axis and magnetic field are initially anti-aligned, whereas they are qualitatively similar to ideal magnetohydrodynamical models if initially aligned. | \label{intro} Molecular clouds are magnetized \citep[for a review, see][]{HeilesCrutcher2005} but with low ionization fractions \citep{MestelSpitzer1956,NakanoUmebayashi1986,UmebayashiNakano1990}. The canonical cosmic ray ionization rate in molecular clouds is $\zeta_\text{cr} \approx 10^{-17}$ s$^{-1} \exp\left(-\Sigma/\Sigma_\text{cr}\right)$ \citep{SpitzerTomasko1968,UmebayashiNakano1981}, where $\Sigma$ is the surface density of the gas, and $\Sigma_\text{cr}$ is the characteristic attenuation depth of cosmic rays. The dense regions ultimately collapse to form protostars \citep{Shu1977}, and observations have suggested the presence of large gas discs and outflows around these young (Class 0) objects \citep[e.g.][]{Dunham+2011,Lindberg+2014,Tobin+2015,Gerin+2017}. Despite the low ionization fractions, many recent simulations of magnetized star formation used ideal magnetohydrodynamics \citep[MHD; e.g.][]{PriceBate2007,HennebelleFromang2008,DuffinPudritz2009,HennebelleCiardi2009,Commercon+2010,Seifried+2011,BateTriccoPrice2014}, which assumes that the gas is sufficiently ionized such that the magnetic field is `frozen' into the gas. The simulations that included realistic magnetic field strengths (mass-to-flux ratios of 3--5 times critical) produced collimated outflows but no protostellar discs; the lack of discs is known as the magnetic braking catastrophe \citep[e.g.][]{AllenLiShu2003,PriceBate2007,MellonLi2008,HennebelleCiardi2009}. The simulations that included weak magnetic fields ($\gtrsim$10 times critical mass-to-flux ratio) produced weak outflows and large discs during the first hydrostatic core phase. If large discs rotated rapidly enough, then they could become dynamically unstable to a bar-mode instability, leading to the formation of trailing spiral arms, as seen in purely hydrodynamical simulations \citep[e.g.][]{Bate1998,SaigoTomisaka2006,SaigoTomisakaMatsumoto2008,MachidaInutsukaMatsumoto2010,Bate2010,Bate2011}. In attempts to form discs during the star forming process, recent three-dimensional simulations have accounted for the low ionization fractions by including a self-consistent treatment of non-ideal MHD \citep[e.g.][]{MachidaMatsumoto2011,Tomida+2013,TomidaOkuzumiMachida2015,Tsukamoto+2015oa,Tsukamoto+2015hall,WursterPriceBate2016,Tsukamoto+2017,Vaytet+2018,WursterBatePrice2018sd}. Rotationally supported discs have been found in simulations that include Ohmic resisitivity and/or ambipolar diffusion \citep[e.g.][]{TomidaOkuzumiMachida2015,Tsukamoto+2015oa,Vaytet+2018}, and 15-30~au discs were recovered when the Hall effect was included \citep[e.g.][]{Tsukamoto+2015hall,WursterPriceBate2016,Tsukamoto+2017} so long as the magnetic field was anti-aligned with the rotation axis, since this geometry promotes disc formation \citep{BraidingWardle2012accretion}. \newpage In this paper, which follows from the work presented in \citet{WursterBatePrice2018sd} (hereafter \citetalias{WursterBatePrice2018sd}), we model the gravitational collapse of a magnetised molecular cloud core using Ohmic resistivity, ambipolar diffusion and the Hall effect and the canonical cosmic ray ionization rate of \zetaeq{-17}. This is the first study to model the collapse to the stellar core phase \citep{Larson1969} that includes the three main non-ideal effects, uses the canonical cosmic ray ionisation rate of \zetaeq{-17}, and anti-aligns the initial magnetic field and rotation vectors. Previous studies have anti-aligned the vectors but stopped the evolution after the first core phase \citep{Tsukamoto+2015hall,Tsukamoto+2017}; aligned the vectors and evolved to the stellar core phase \citep{Tsukamoto+2015hall}; studied both alignments using a higher cosmic ray ionization rate \citepalias{WursterBatePrice2018sd}; excluded the Hall effect \citep{TomidaOkuzumiMachida2015,Tsukamoto+2015oa,Vaytet+2018}; or followed the long term evolution by forming sink particles \citep{WursterPriceBate2016}. This paper focuses on disc formation. We refer the reader to \citetalias{WursterBatePrice2018sd} for discussion of the stellar cores. In Section~\ref{sec:methods}, we summarise our methods and in Section~\ref{sec:ic} we present our initial conditions. Our results are presented in Section~\ref{sec:results} and we conclude in Section~\ref{sec:conc}. | \label{sec:conc} In this study, we followed the collapse of a molecular cloud core through to the formation of the stellar core in a magnetized medium. We used a self-consistent treatment of non-ideal MHD, and used the canonical cosmic ray ionization rate of \zetaeq{-17}. We presented models with the magnetic field aligned and anti-aligned to the rotation axis since the Hall effect depends on the magnetic field orientation. We compared these models to partially ionized models with higher ionization rates (i.e. \zetaeq{-16}), an ideal MHD model and a purely hydrodynamical model. Our primary conclusions are as follows: \begin{enumerate} \item The magnetic braking catastrophe can be solved by the Hall effect if the magnetic field and rotation axis are anti-aligned. During the first core phase, the anti-aligned model with \zetaeq{-17} led to the formation of a gravitationally unstable \sm25~au disc. The aligned model formed no disc during this phase. Increasing the cosmic ray ionization rate by a factor of ten yielded models without discs in the first core phase for both magnetic field orientations. \item After the second collapse to form a stellar core, the aligned model with \zetaeq{-17} and both models with \zetaeq{-16} formed rotationally supported $1-3$~au discs. No such discs were formed when using ideal MHD. \item The model with \zetaeq{-17} where the initial magnetic field and rotation vectors are anti-aligned launched a weak $\lesssim 0.3$~\kms first core outflow, while its aligned counterpart launched the fastest (\appx3~\kms) first core outflow amongst our six models. \end{enumerate} By including the Hall effect in non-ideal MHD models that use the canonical cosmic ray ionization rate of \zetaeq{-17}, drastically different results can be produced depending on the initial orientation of the magnetic field. The Hall effect can qualitatively change the outcome, such that protostars produced from magnetized clouds can resemble results from purely hydrodynamical models (if the initial magnetic field and rotation vectors are anti-aligned) or ideal MHD models (if the vectors are initially aligned). These results are in agreement with \citet{Tsukamoto+2015hall} who used different initial conditions than presented here, suggesting that our findings are robust and independent of initial conditions, as long as \zetaeq{-17} is used. Thus we have demonstrated that formation of gravitationally unstable discs with radii more than 25~au is possible despite the presence of magnetic fields. This implies that such discs should indeed exist in the Class 0 phase. | 18 | 8 | 1808.04376 |
1808 | 1808.08969_arXiv.txt | Theoretically long gamma-ray bursts (GRBs) are expected to happen in low-metallicity environments, because in a single massive star scenario, low iron abundance prevents loss of angular momentum through stellar wind, resulting in ultra-relativistic jets and the burst. In this sense, not just a simple metallicity measurement but also low iron abundance ([Fe/H]$\lesssim$-1.0) is essentially important. Observationally, however, oxygen abundance has been measured more often due to stronger emission. In terms of oxygen abundance, some GRBs have been reported to be hosted by high-metallicity star-forming galaxies, in tension with theoretical predictions. Here we compare iron and oxygen abundances for the first time for GRB host galaxies (GRB 980425 and 080517) based on the emission-line diagnostics. The estimated total iron abundances, including iron in both gas and dust, are well below the solar value. The total iron abundances can be explained by the typical value of theoretical predictions ([Fe/H]$\lesssim$-1.0), despite high oxygen abundance in one of them. According to our iron abundance measurements, the single massive star scenario still survives even if the oxygen abundance of the host is very high, such as the solar value. Relying only on oxygen abundance could mislead us on the origin of the GRBs. The measured oxygen-to-iron ratios, [O/Fe], can be comparable to the highest values among the iron-measured galaxies in the Sloan Digital Sky Survey. Possible theoretical explanations of such high [O/Fe] include the young age of the hosts, top-heavy initial mass function, and fallback mechanism of the iron element in supernova explosions. | \label{introduction} Observationally, there is a population of gamma-ray bursts (GRBs) that shows \lq long\rq\ durations ($>2$s) of bursts \citep{Kouveliotou1993}. It is widely accepted that \lq long\rq\ GRB (hereafter just referred to as GRB) emission is emitted from ultra-relativistic jets that are launched at the core collapse of a single massive star. This scenario requires a large angular momentum in the collapsing core to launch such jets \citep[e.g.,][]{Woosley2006}. In this scenario, the angular momentum is removed by the stellar wind driven by radiation pressure that depends on the iron abundance \citep{Vink2005}. Thus, low iron abundance ([Fe/H]$\lesssim$ -1.0) is favored in this single massive star scenario, because the progenitor with low iron abundance does not lose its angular momentum by the mass loss during its evolution \citep[e.g.,][]{Yoon2006}. Observationally, oxygen abundance has been used as an indicator of iron abundance rather than iron abundance itself, since oxygen has strong emission lines in the rest-frame optical wavelength. Indeed, optical emission-line diagnostics show that many of the GRB hosts have the subsolar oxygen abundances \citep[e.g.,][]{Stanek2006,Modjaz2008,Levesque2010a,Levesque2010b}. However, recent observational efforts to investigate unbiased host samples indicate that GRBs can occur in dusty massive star-forming galaxies \citep[e.g.,][]{Hashimoto2010,Perley2016a,Perley2016b}, implying high oxygen abundance. In fact, some of these host galaxies have been confirmed to show high oxygen abundance approaching the solar value based on the emission-line diagnostics \citep[e.g.,][]{Graham2013,Hashimoto2015,Kruhler2015,Stanway2015}. The possible high-metallicity environment of GRBs has also been reported from the absorption system in the optical afterglow \citep[e.g.,][]{Savaglio2012}. Why is the oxygen abundance high despite the theoretical expectation? There could be three possible explanations for such a GRB environment with high oxygen abundance. One is the metallicity dispersion of star-forming region within a host galaxy. The spatial resolutions of abundance measurements at the GRB positions (typically kpc scale) are still not enough to resolve each star-forming regions \citep{Niino2015}, except for particularly near GRB host galaxies such as 980425 \citep{Hammer2006}. In this sense, observed high oxygen abundances (regardless of averaged over the whole host galaxy or at the GRB position) do not exclude the hypothesis that the GRBs are born in environments with low oxygen abundances. Secondly, there may be another channel for massive stars to become GRBs, other than the single massive star scenario. For instance, a binary star merger explosion scenario may be important to GRBs that are hosted by galaxies with high oxygen abundance. In this scenario, the merging stars take in their orbital angular momentum during the merging process. The sufficient angular momentum required for a GRB explosion is maintained even in the high-metallicity environment \citep[e.g.,][]{Nomoto1995,Fryer1999}. The third possible explanation is the overabundance of oxygen, i.e., a high value of [O/Fe] in GRB host galaxies. In such a case, the oxygen abundance can be as high as the solar value, even if the iron abundance is very low, as expected from the single massive star scenario mentioned above. It is well known that the iron is mainly produced by Type Ia supernovae \citep[SNe Ia;][]{Tinsley1979}, whereas $\alpha$ elements such as oxygen, neon and sulfur are produced by Type II supernovae (SNe II). The time scale of SNe Ia ($\sim 10^{9}$ yr) is much longer than that of SNe II ($\sim 10^{7}$ yr). Therefore it is possible that a very young galaxy has no (or less) experience with chemical enrichment from SN Ia but has contribution from SN II, which results in a high value of [O/Fe] \citep[e.g.,][]{McWilliam1997}. In fact, the overabundance of oxygen has been confirmed for galaxies with strong emission lines \citep{Izotov2006}, which are probably dominated by very young stellar populations undergoing active star formation. Such may be the case with GRB host galaxies, since many GRBs are hosted by galaxies with young stellar populations \citep[e.g.,][]{Savaglio2009}. In addition, the top-heavy initial mass function (IMF) also can increase the ratio of $\alpha$ element to iron, because theoretical predictions of elemental yields of SNe II show that [$\alpha$/Fe] increases with increasing progenitor mass \citep[e.g.,][]{Wyse1992,Woosley1995}. The single massive star scenario might favor the top-heavy IMF of GRB host galaxies. Therefore, GRB host galaxies could have a high value of [O/Fe]. If so, the oxygen abundances of GRB hosts might not always be a good indicator of iron abundance. To test these scenarios, it is important to directly measure the iron abundance of the GRB host with high oxygen abundance. Some nearby GRB host galaxies enable us to measure the flux of the weak [Fe~{\sc iii}]$\lambda$4658 line that is necessary to estimate the iron abundance based on emission-line diagnostics. Therefore, we measured the iron abundance of the nearby host galaxy of GRB 080517 at $z$=0.089. The oxygen abundance of GRB 080517 is around the solar value \citep{Stanway2015}. This is the first attempt to measure the iron abundance of the GRB host with high oxygen abundance. In addition, we present the iron abundance measurements of the GRB 980425 host galaxy at $z$=0.0085. GRB 980425 has been reported to show low oxygen abundance \citep{Hammer2006} but is an ideal case to constrain the iron abundance at the position of the GRB, thanks to the close distance and high signal-to-noise ratio (S/N) of the spectrum. Often, iron abundance was measured using the GRB afterglow \citep[e.g.,][]{Prochaska2007}, resulting in the integrated abundance rather than at the GRB site. In this work, we measure iron abundance from the spectra of GRB hosts. As such, for GRB 980425, we are able to constrain the iron abundance of the GRB site. This is the first case in which the iron abundance was constrained at the GRB site without contamination from the other part of the host galaxy. The Wolf--Rayet (WR) region in GRB 980425 host, which is $\sim$ 800 kpc away from the GRB site, is also one candidate for the birthplace of the GRB \citep{Hammer2006}. Therefore, we include the WR spectrum in our analysis, as well as the GRB 980425 site. Throughout this paper, we use the solar abundances by \citet{Asplund2009}. | The widely accepted single massive star scenario predicts that GRBs occur in low-metallicity environments. In the scenario, a low iron abundance ([Fe/H]$\lesssim$-1.0) is required rather than oxygen for a massive star to launch the relativistic jet. However, oxygen has often been used to measure abundances of GRB host galaxies because its strong emission is easier to detect. Puzzlingly, the measured oxygen-based abundances of some GRB host galaxies have been higher than theoretically expected. In this work, we measured iron abundance, which is more directly connected to the theory. We obtained rest-frame optical spectra of two nearby GRB host galaxies (GRB 080517 and 980425). We successfully detected weak [Fe~{\sc iii}]$\lambda$4658 emission lines in the spectra of the GRB 080517 host and WR region of GRB 980425 and constrained the upper limit of the iron abundance at the explosion site of GRB 980425. The total iron abundances of the GRB 080517 host and explosion site of GRB 980425 are well below the solar value, even though the oxygen abundance of the GRB 080517 host is comparable to the solar value. Although the error bars of iron abundances are large, the [Fe/H]$_{total}$ of our sample can be explained by the theoretical predictions ([Fe/H]$\lesssim$-1.0) of the single massive star scenario. Relying only on oxygen abundance could mislead us on the origin of the GRBs. The oxygen-to-iron ratio, [O/Fe]$_{total}$, of the GRB 080517 host is comparable to the highest value of the iron-measured SDSS galaxies. The high [O/Fe] suggests that more oxygen has been produced in massive stars than iron from less massive stars. This situation can happen in (i) young galaxies where many type Ia SNe have not exploded yet or (ii) under a top-heavy IMF, where the fractions of massive stars are higher. The observed young stellar age ($\lesssim 10^{9}$ yr) of the GRB 080517 host supports scenario (i), though a conclusive argument on the importance of the scenario (ii) is difficult based on our small sample. Alternatively, the fallback scenario of the SNe could also explain high [O/Fe]. In any case, iron is more directly related to the single massive star scenario of GRBs than oxygen. Our work is just the first step in this direction. In the future, it is important to investigate iron abundances of more GRB host galaxies. | 18 | 8 | 1808.08969 |
1808 | 1808.03626_arXiv.txt | In this paper, we propose an enhanced CNN model for detecting supernovae (SNe). This is done by applying a new method for obtaining rotational invariance that exploits cyclic symmetry. In addition, we use a visualization approach, the layer-wise relevance propagation (LRP) method, which allows finding the relevant pixels in each image that contribute to discriminate between SN candidates and artifacts. We introduce a measure to assess quantitatively the effect of the rotational invariant methods on the LRP relevance heatmaps. This allows comparing the proposed method, CAP, with the original Deep-HiTS model. The results show that the enhanced method presents an augmented capacity for achieving rotational invariance with respect to the original model. An ensemble of CAP models obtained the best results so far on the HiTS dataset, reaching an average accuracy of 99.53\%. The improvement over Deep-HiTS is significant both statistically and in practice. | Astronomy is entering into a new era of big data due to the construction of very large scale facilities such as the Large Synoptic Survey Telescope (LSST), an 8.4 m telescope with a 3.2 Gigapixel camera, which will begin operations in northern Chile in 2022 \cite{huijse2014computational}. The LSST is a robotic telescope that will scan the entire southern hemisphere sky every 3 days, collecting information on 50 billion objects for 10 years \cite{ivezic2008lsst}. Time-domain astronomy studies stellar objects that change in time or position, e.g. supernovae (SNe), the explosive death of stars. The High Cadence Transient Survey (HiTS) \cite{forster2016high} aimed at detecting SNe in their early stages in order to study the astrophysics associated with these phenomena. HiTS has a custom-made pipeline to process the images captured by the telescope and detect transients. Basically, the pipeline subtracts reference images from new images, detects sources and classifies them. The farther away from Earth, the higher the chance of finding SNe events because there are more galaxies. But deeper objects are usually fainter with a low signal-to-noise ratio. For this reason, among others, it is relevant to reduce significantly the false negative rate (FNR) and the false positive rate (FPR) at the output of this pipeline. In our previous work, we introduced a convolutional neural network (CNN) for classifying sources detected by the HiTS pipeline as true transients (`SN candidates') or bogus (`artifacts') \cite{cabrera2016supernovae}. In 2017 the model was enhanced by introducing partial rotational invariances, as well as improving the architecture and training algorithms, to yield the so-called Deep-HiTS model (DH) \cite{cabrera2017deep}. In this paper, we enhance Deep-HiTS by applying a new method for obtaining rotational invariance that exploits cyclic symmetry in CNNs \cite{dieleman2016exploiting}. In addition, we use a visualization approach, the layer-wise relevance propagation (LRP) \cite{samek2017evaluating}, in order to find the relevant pixels in each image that helps to discriminate between SN candidates and artifacts. We assess both qualitatively and quantitatively the effect of the rotational invariant methods using LRP and compare the original Deep-HiTS model with its enhanced version. In addition, we introduce ensemble classifiers to improve the performance of Deep-HiTS. | In this work, we enhanced the rotational invariant capability of the Deep-HiTS model by adding a cyclic pooling average layer. The results are consistent with the hypothesis that astronomical objects do not depend on the angle on which the image is observed given the same conditions of observation. An ensemble of CAP models obtained the best results so far with the HiTS dataset, reaching an average accuracy of 99.53\%. The improvement over Deep-HiTS is significant both statistically and in practice. For example, for a standard operation point with FPR $\sim$10$^{-2}$, the proposed model achieves an FNR of 1.38x10$^{-3}$, which entails a $\sim$40\% reduction of missing transients with respect to Deep-HiTS. From the astronomer viewpoint, it is important not to miss positive samples of rare SNe events. We have used the LRP method to visualize and analyze the heatmaps showing the most relevant pixels for the discrimination task at hand. We defined a measure to assess quantitatively the rotational invariance capability of the different models. The results show that the proposed model is more rotational invariant than the original Deep-HiTS model. This is a novel application for the LRP method and the first time that it has been applied to astronomical data. LRP is a positive step towards understanding and visualizing what a CNN has learned. However, the tool may be improved to visualize intermediate layers, adding gradient information, as well as improving the interpretation of positive and negative relevances. Obtaining the best ensemble classifier can also be investigated by changing the size and the rule of the ensemble. | 18 | 8 | 1808.03626 |
1808 | 1808.03410_arXiv.txt | We present results from a pilot observation of nearby ($\sim20$ Mpc) galaxies with mass similar to that of the Milky Way (MW) to address the missing satellite problem. This is the first paper from an on-going project to address the problem with a statistical sample of galaxies outside of the Local Group (LG) without employing an assumption that the LG is a typical halo in the Universe. Thanks to the close distances of our targets, dwarf galaxies around them can be identified as extended, diffuse galaxies. By applying a surface brightness cut together with a careful visual screening to remove artifacts and background contamination, we construct a sample of dwarf galaxies. The luminosity function (LF) of one of the targets is broadly consistent with that of the MW, but the other has a more abundant dwarf population. Numerical simulations by Okamoto (2013) seem to overpredict the number of dwarfs on average, while more recent predictions from Copernicus Complexio are in a better agreement. In both observations and simulations, there is a large diversity in the LFs, demonstrating the importance of addressing the missing satellite problem with a statistically representative sample. We also characterize the projected spatial distributions of the satellites and do not observe strong evidence for alignments around the central galaxies. Based on this successful pilot observation, we are carrying out further observations to increase the sample of nearby galaxies, which we plan to report in our future paper. | The $\Lambda$-dominated cold dark matter model ($\Lambda$CDM) is widely accepted as the standard cosmological model. It has passed many stringent observational tests on the large-scale matter distribution in the Universe, but it has a few possible flaws on small scales such as the cusp-core problem \citep{mcgaugh01,gilmore07,kuzio08}, too-big-to-fail problem \citep{boylan11,parry12}, missing satellite problem \citep{kauffmann93,klypin99,moore99}, and satellite alignment problem \citep{ibata13,pawlowski13,pawlowski15}. We do not have a satisfactory solution to these problems and they may urge us to adopt other models such as warm dark matter and self-interacting dark matter. This is a pilot of a project that aims to address the missing satellite problem: more than an order of magnitude shortage of observed dwarf galaxies around the Milky Way (MW) and M31 compared to the number expected if every subhalo hosts a galaxy. The problem was first pointed out in 1999 based on (dark matter only) N-body simulations. Since then, there has been tremendous progress in hydro-dynamical simulations of galaxy formation in a cosmological context, and recent simulations show that, once baryonic effects such as star formation, SNe feedback, UV background due to cosmic reionization are incorporated, many subhalos do not actually host galaxies and the tension between the observed and expected numbers of dwarf galaxies is significantly reduced (e.g., Okamoto et al. 2010; Sawala et al. 2016a,b). While it is clear that baryonic astrophysics is a natural solution to the problem, all models are calibrated to reproduce the Local Group (LG). This is obviously not a fair test of the problem. Furthermore, different models that currently claim to solve the missing satellite problem use different assumptions about baryon physics. In order to distinguish between these solutions and see if any one of them can actually solve the problem, we need to test model predictions against a sample that has not been used in the calibration. Constraining the physics that governs the abundance of satellites is a necessary first step to addressing higher order problems like their density profiles ('cusp-core'), which in turn break degeneracies between baryonic astrophysics and alternate forms of dark matter. There is some recent work in this direction. \citet{geha17} presented the first results from their spectroscopic campaign around galaxies at $20-40$ Mpc. They targeted galaxies from shallow SDSS data and constructed a luminosity function (LF) of the confirmed dwarf galaxies. We also have initiated a project to observationally test the missing satellite problem beyond the LG. This project is made possible with Hyper Suprime-Cam (HSC) mounted on the Subaru Telescope \citep{miyazaki12}. With its large light-collecting aperture over a wide area, we can now search for faint dwarf galaxies outside of the LG. This paper presents first results from our pilot observation. Unless otherwise stated, magnitudes are given in the AB system. | \label{sec:discussion} We have presented the LFs of dwarf galaxies around two nearby (15--20 Mpc) galaxies with MW-like mass observed with HSC. At this distance, dwarf galaxies are spatially extended and we use this property to largely eliminate background sources and select a high purity sample of dwarf galaxy candidates by applying a surface brightness cut. Our data are of high quality and we achieve $\sim0.5$ arcsec seeing in the $g$-band, which turns out to be critically important to distinguish dwarf galaxies from background face-on spirals (which also have low surface brightness). We statistically subtract any remaining contamination using a control field sample, which is processed in exactly the same manner as the targets. We also carefully account for the detection incompleteness and biases in our magnitude and size measurements using the simulations. Our primary results are (1) the satellite LF of N2950 is broadly consistent with that of the MW, whereas N3245 has a more abundant dwarf population, (2) the observed LFs are on average about a factor of two smaller than prediction from the hydrodynamical simulations of \citet{okamoto13}, while COCO reproduces the observed LFs well and importantly (3) there is a large diversity in the LFs both in the observations and simulations. The last point highlights the importance of addressing the missing satellite problem with a statistically representative sample. We have also examined the size-luminosity relation and found that the dwarfs around our targets follow the same relation as the MW dwarfs. The $g-i$ color of the dwarf galaxies spans a wide range, but many of them have red colors consistent with the red sequence of massive galaxies extrapolated to faint magnitudes. Finally, we have looked at the spatial distribution of the dwarf galaxies, but we do not observe strong evidence for the alignment of satellites around the central galaxies. Given these promising results, our next step is to increase the sample size to fully constrain the satellite abundance of MW analogues and so address a number of related challenges to $\Lambda$CDM. We are carrying out further HSC observations of nearby galaxies and we plan to report on their LFs in a subsequent paper. Our final sample will comprise both early- and late-type galaxies and it will be interesting to examine the dependence of dwarf galaxy properties on the central galaxy properties. Also, it will be important to extend the mass range; there is no reason why we should stick with MW-like mass and a sample with a wider mass range will give us a wider leverage to test the models. In addition, we will explore a more sophisticated algorithm to identify faint dwarfs. We have examined the LFs down to $\sim-9.5$~mag in this paper, but as mentioned earlier, that is not a limit imposed by the data. A more sophisticated method should allow us to probe fainter dwarf galaxies. Finally, we remind ourselves that our selection of dwarf galaxies is based on the surface brightness selection. We need spectroscopic confirmations of the dwarf galaxies to derive more reliable LFs. It is not practical to follow-up all possible candidates with any of the existing spectroscopic facilities, but Prime Focus Spectrograph (PFS; \citealt{tamura16}) is an ideal instrument. PFS is a massively multiplexed fiber spectrograph ($\sim2400$ fibers) to be mounted on the Subaru telescope and its field of view is nearly as large as that of HSC, which makes it possible to follow-up all the possible candidates in one go. We plan to perform an intensive follow-up programme with PFS in order to confirm dwarf satellite candidates and derive more secure LFs in our future work. | 18 | 8 | 1808.03410 |
1808 | 1808.00448_arXiv.txt | We present direct spectroscopic modeling of five Type Iax supernovae (SNe) with the one dimensional Monte Carlo radiative transfer code \begin{small}TARDIS\end{small}. The abundance tomography technique is used to map the chemical structure and physical properties of the SN atmosphere. Through via fitting of multiple spectral epochs with self-consistent ejecta models, we can then constrain the location of some elements within the ejecta. The synthetic spectra of the best-fit models are able to reproduce the flux continuum and the main absorption features in the whole sample. We find that the mass fractions of IGEs and IMEs show a decreasing trend toward the outer regions of the atmospheres using density profiles similar to those of deflagration models in the literature. Oxygen is the only element, which could be dominant at higher velocities. The stratified abundance structure contradicts the well-mixed chemical profiles predicted by pure deflagration models. Based on the derived densities and abundances, a template model atmosphere is created for the SN Iax class and compared to the observed spectra. Free parameters are the scaling of the density profile, the velocity shift of the abundance template, and the peak luminosity. The results of this test support the idea that all SNe Iax can be described by a similar internal structure, which argues for a common origin of this class of explosions. | Most Type Ia supernovae (SNe Ia) form a one-parameter family often called ``Branch-normal'' \citep{Branch06}. The amount of synthesized $^{56}$Ni determines their peak luminosity, which correlates with the shape of their light curves, making these explosions standardizable candles and ideal for distance measurements on cosmological scales. However, certain thermonuclear white dwarf (WD) explosions do not follow this correlation, and some of their other observable properties are usually also peculiar compared to those of the normal SNe Ia. A special group of these objects, named after the prototypical member of them \citep{Li03}, are called `2002cx'-like SNe, or, as it has come into general use after \citet{Foley13}, Type Iax SNe. \begin{table*} \centering \caption{Absolute peak magnitudes in V-band (in r-band for SN 2015H), redshifts, distance moduli and reddening values (both in magnitudes) for each SNe in our sample.} \label{tab:sample} \begin{tabular}{ccccccc} \hline Object & $M_\rmn{peak}$ & z & $ \mu$ & $E(B-V)_\rmn{MW} $ & $E(B-V)_\rmn{host} $ & Paper\\ \hline SN 2011ay & -18.39 $\pm$ 0.18 & 0.021 & 34.69 $\pm$ 0.15 & 0.069 & 0.00 & \begin{tabular}{@{}c@{}}\cite{Szalai15} \end{tabular} \\ \hline SN 2012Z & -18.50 $\pm$ 0.09 & 0.007 & 32.59 $\pm$ 0.09 & 0.036 & 0.07 & \begin{tabular}{@{}c@{}}\cite{Stritzinger14} \end{tabular}\\ \hline SN 2005hk & -18.08 $\pm$ 0.29 & 0.012 & 33.46 $\pm$ 0.27 & 0.019 & 0.09 & \begin{tabular}{@{}c@{}}\cite{Phillips07} \end{tabular}\\ \hline SN 2002cx & -17.62 $\pm$ 0.35& 0.024 & 35.09 $\pm$ 0.32 & 0.034 & 0.00 & \begin{tabular}{@{}c@{}}\cite{Li03}\end{tabular}\\ \hline SN 2015H & -17.27 $\pm$ 0.07& 0.012 & 33.91 $\pm$ 0.07 & 0.048 & 0.00 & \begin{tabular}{@{}c@{}}\cite{Magee16}\end{tabular}\\ \hline \end{tabular} \end{table*} \begin{table*} \centering \caption{Log of the spectra of our sample; the phases are given with respect to B-maximum. The times since explosion ($t_\rmn{exp}$) were fitting parameters for our TARDIS models (see in Sec. \ref{fitting_method}) within $\pm$1.5 day to their estimated values in the referred papers.} \label{tab:log} \begin{tabular}{cccccc} \hline MJD & $t_\rmn{exp}$ [days] & Phase [days] & Telescope$\setminus$Instrument & Wavelength [\r{A}] & Paper\\ \hline \multicolumn{6}{c}{SN 2011ay}\\ \hline 55642.7 & 10.0 & -3.4 & HET$\setminus$LRS & 4100-10000 & \cite{Szalai15}\\ 55643.7 & 11.0 & -2.4 & HET$\setminus$LRS & 4100-10000 & \cite{Szalai15}\\ 55645.7 & 13.0 & -0.4 & HET$\setminus$LRS & 4100-10000 & \cite{Szalai15}\\ 55647.7 & 15.0 & +1.6 & HET$\setminus$LRS & 4100-10000 & \cite{Szalai15}\\ 55648.7 & 16.0 & +2.6 & Lick$\setminus$Kast & 3350-9850 & \cite{Silverman12}\\ 55650.7 & 18.0 & +4.6 & HET$\setminus$LRS & 4100-10000 & \cite{Szalai15}\\ 55652.7 & 20.0 & +6.6 & Lick$\setminus$Kast & 3400-9700 & \cite{Silverman12}\\ 55655.7 & 23.0 & +9.6 & Lick$\setminus$Kast & 3400-9700 & \cite{Silverman12}\\ 55661.7 & 29.0 & +15.6 & Lick$\setminus$Kast & 3350-10550 & \cite{Silverman12}\\ 55664.7 & 32.0 & +18.6 & HET$\setminus$LRS & 4100-10000 & \cite{Szalai15}\\ \hline \multicolumn{6}{c}{SN 2012Z}\\ \hline 55958.2 & 5.4 & -9.2 & Lick$\setminus$Kast & 3400-10000 & \cite{Stritzinger14} \\ 55959.2 & 6.4 & -8.2 & Lick$\setminus$Kast & 3400-10000 & \cite{Stritzinger14} \\ 55960.4 & 7.6 & -7.0 & KAO$\setminus$LOSA & 4100-7900 & \cite{Yamanaka15} \\ 55965.4 & 12.6 & -2.0 & KAO$\setminus$LOSA & 4100-7900 & \cite{Yamanaka15} \\ 55968.5 & 15.7 & +1.1 & KAO$\setminus$LOSA & 4100-7900 & \cite{Yamanaka15} \\ 55973.1 & 20.3 & +5.7 & FLWO$\setminus$FAST & 3400-10000 & \cite{Stritzinger14} \\ \hline \multicolumn{6}{c}{SN 2005hk}\\ \hline 53675.2 & 5.3 & -9.3 & FLWO$\setminus$FAST & 3500-7400 & \cite{Blondin12}\\ 53676.2 & 6.3 & -8.3 & Lick$\setminus$KAST & 3300-10400 & \cite{Phillips07}\\ 53678.2 & 8.3 & -6.3 & APO$\setminus$DIS & 3600-9600 & \cite{Phillips07}\\ 53679.4 & 9.5 & -5.1 & Keck$\setminus$LRIS & 3200-9200 & \cite{Phillips07}\\ 53680.1 & 10.2 & -4.4 & APO$\setminus$DIS & 3600-9600 & \cite{Phillips07}\\ 53681.2 & 11.3 & -3.3 & FLWO$\setminus$FAST & 3500-7400 & \cite{Blondin12}\\ 53683.2 & 13.3 & -1.3 & FLWO$\setminus$FAST & 3500-7400 & \cite{Blondin12}\\ 53688.2 & 18.3 & +3.7 & MDM$\setminus$CDSS & 3900-7300 & \cite{Phillips07}\\ \hline \multicolumn{6}{c}{SN 2002cx}\\ \hline 52411.2 & 7.2 & -4.0 & FLWO$\setminus$FAST & 3500-7500 & \cite{Li03}\\ 52414.2 & 10.2 & -1.0 & FLWO$\setminus$FAST & 3700-7500 &\cite{Li03}\\ 52427.2 & 23.2 & +12.0 & FLWO$\setminus$FAST & 3700-7500 &\cite{Li03}\\ \hline \multicolumn{6}{c}{SN 2015H}\\ \hline 57065.1 & 18.9 & +3.2* & EFOSC2 & 3650-9250 & \cite{Magee16}\\ 57068.2 & 22.0 & +6.3* & EFOSC2 & 3350-10000 & \cite{Magee16}\\ 57072.3 & 26.1 & +10.4* & EFOSC2 & 3350-10000 & \cite{Magee16}\\ \hline \multicolumn{6}{l}{* In case of SN 2015H, r-maximum was used instead of B-maximum.}\\ \end{tabular} \end{table*} The characteristic observational properties of SNe Iax are the low peak luminosities, which extend in a wide range from the extremely faint SN 2008ha \citep[$M_\rmn{V,peak}$ $\sim$ -14 mag; ][]{Valenti09} up to the relatively luminous SN 2011ay \citep[$M_\rmn{V,peak}$ $\sim$ -18.4 mag; ][]{Foley13,Szalai15}. Their photospheric velocities are also significantly lower than those of normal Type Ia SNe, falling typically between 5\,000 and 8\,000 km s$^{-1}$ at maximum light \citep{Foley13}, but objects with expansion velocities of 2\,500 km s$^{-1}$ have also been observed \citep{Foley09,Stritzinger14}. Although a general, but not tight, correlation can be noticed between the luminosity and velocity values, some SNe Iax do not fit into this sequence, like SN 2009ku \citep{Narayan11,Foley13} or SN 2014ck \citep{Tomasella16}. Beyond the wide scale of peak luminosities, the light curves show further diversity. The rise times of SNe Iax are shorter than the typical values for SNe Ia \citep[18.0 $\pm$ 2.0 days in B-band, as found by][]{Ganeshalingam11}; however, the typically faster decline rates do not seem to correlate with the peak luminosities. The near-infrared light curves do not display a second peak, suggesting strong mixing in the ejecta of SNe Iax \citep{Jha06,Phillips07}. The early spectra of SNe Iax are dominated by lines of iron-group elements (IGEs). Spectral lines of intermediate mass elements (IME), e.g. Si \begin{small}II\end{small} and Ca \begin{small}II\end{small} are always present, but their strength is far from that observed in normal SNe Ia. High-velocity features have not been observed in spectra of any SNe Iax. At later epochs, both permitted absorption features and forbidden emission lines can be found at optical wavelengths, which is unlike the spectra of other thermonuclear explosions \citep{Jha06}. Studying SNe Iax has the potential to answer some of the open questions related to the progenitor systems of thermonuclear SNe. While no progenitor system of normal Type Ia SNe has ever been discovered, \cite{McCully14b} reported a possible source coincident with Type Iax SN 2012Z on a pre-explosion image of HST. The detected luminous blue star could be a potential He star donor to the exploding white dwarf. The existence of such progenitor systems, suggested first by \citet{Foley13}, is supported by detailed binary evolution calculations \citep{Wang13}. However, \cite{Liu15} noted that this kind of system is unlikely to be the progenitor of the majority of SNe Iax, because the long delay time of the single degenerate Chandrasekhar mass ($M_\rmn{Ch}$) models seemingly does not explain the observed number of Type Iax SNe. The above mentioned observables (i.e. low peak luminosities and photospheric velocities; strong mixing) and the spectral footprints of chemical elements together indicate that deflagrations play a major role in the explosion of Type Iax SNe. Subsonic explosions of carbon-oxygen (CO) $M_\rmn{Ch}$ WDs leaving bound remnants (also referred to as ``failed'' SNe) have been investigated by 3D hydrodynamical simulations \citep{Jordan12,Kromer13}. According to these and other studies \citep{Long14,Magee16}, the synthetic observables of weak pure deflagrations are able to broadly reproduce both the photometric and spectroscopic properties of SNe Iax. \cite{Fink14} presented a set of 3D hydrodynamical calculations using a multi-spot ignition approach to scale the strength of the deflagration. The peak luminosities of their models N3def, N5def, N10def, and N20def are comparable with the more luminous SNe Iax \citep{Foley13}. The growing number of ignition spots produces more energetic and luminous explosions in accordance with the wide range of the observed luminosities and expansion velocities. In addition, the pure deflagration models of \cite{Fink14} show highly mixed abundance structures with nearly constant mass fractions for each elements. The main goal of this study is to carry out a comprehensive abundance tomography \citep{Stehle05} analysis involving several different SNe Iax. The sample of objects is chosen to broadly represent the properties of the class. Similar to our pilot study for SN 2011ay \citep{Barna17}, we compare our findings with the results of previous spectral models, as well as with the predictions of the pure deflagration of a Chandrasekhar-mass WD with a bound remnant (the most promising explosion scenario for SNe Iax). This paper is structured in the following way. In Section \ref{sample}, we give an overview of the target sample of our study. In Section \ref{method}, we show how we use the \begin{small}TARDIS\end{small}\footnote{The TARDIS software package available from: \url{https://zenodo.org/record/1292315} .} code \citep{Kerzendorf14} for abundance tomography to simulate the spectral evolution of the objects. In Section \ref{results}, we present the results of our spectral modeling, compare the resulting physical picture of the ejecta with other models and introduce a template for the subclass of SNe Iax. Finally, we summarize our main findings in Section \ref{conclusions}. | \label{conclusions} We have performed a comprehensive study of the chemical composition and the main physical properties of the diverse Type Iax SN subclass. We analyze the spectral time series of five Type Iax SNe with different peak luminosities and expansion velocities. The data are fit by self-consistent atmosphere models calculated with the spectrum synthesis code \begin{small}TARDIS\end{small}. We follow the modeling strategy introduced in \citet{Barna17} applying a few modifications. We fit the density of the SN ejecta within the frame of density profiles from hydrodynamic deflagration models introducing new free parameters. However, our fits in general prefer a steeper cut-off in the density profiles at higher velocities than in the deflagration models. The fitted synthetic spectra show a very good agreement with the observed spectral features. The continuum levels are mostly reproduced well; however, the relatively blue SN 2005hk shows lower continuum flux beyond $\sim$5\,000 \r{A} than our models. The \begin{small}TARDIS\end{small} model atmospheres are built varying the mass fractions of C, O, Na, Mg, Si, S, Ca, Ti, Cr, Fe and $^{56}$Ni for all five SNe. The starting points of our fitting process are the predicted abundance profiles of the hydrodynamic calculations of pure deflagration models \citep{Fink14}; we deviate from these values only if it is indicated by the fitting of the spectral features. The best-fit models of the two least luminous SNe, 2002ck and 2015H, also include vanadium with mass fractions of 0.01-0.02. {The most abundant element in our best-fit models is oxygen, just like in the previous abundance tomography made for the spectral series of SN 2011ay \citep{Barna17}. Here we fit the density profiles with a cut-off at higher velocities (Table \ref{tab:log_tardis}) instead of purely exponential profiles. We still have to use oxygen as a ``filler'' element \citep{Barna17}, despite the reduced mass in the outermost regions of the ejecta.} Broadly, a similar structure can be recognized in the abundance profiles of the five studied SNe. The trends of the abundance functions are monotonic for each element, no abundance clump is found in the observed sample. Both the mass fractions of IMEs and IGEs decrease towards higher velocities, meanwhile only oxygen and carbon show upward trends. We can place limits only on carbon, which does not appear in the inner ejecta volume (except in case of SN 2015H) studied by the spectral series, its presence is allowed at higher velocities. The IGEs are dominated by the products of the $^{56}$Ni decay chain, whose initial mass fraction is typically 0.40-0.50 in the inner regions, while the initial amount of Fe is always below 10 percent. Silicon is one of the best-constrained elements in our analysis because of the sharp Si\,\begin{small}II\end{small} and Si\,\begin{small}III\end{small} lines. The mass fraction reaches 0.10-0.15 at lower velocities and continuously decreases outward. Magnesium, sulfur and calcium show similar trends; their peak values in the innermost regions are $\sim$0.10, $\sim$0.02 and $\sim$0.01, respectively. {We have compared our findings to the abundance profiles of deflagration models of similar peak luminosity. We found that our stratified abundance structures deviate from the explosion models significantly at higher velocities and the uniform abundances of the deflagration models do not describe well the outermost part of the Type Iax SN ejecta.} The mass fractions of the most abundant IMEs and IGEs in our \begin{small}TARDIS\end{small} models are in relatively good agreement with the hydrodynamic calculations in the inner regions of the studied ejecta. However, we can tolerate carbon only in the outermost layers in our spectral fitting, while the deflagration calculations show a constant carbon abundance throughout the ejecta due to turbulent mixing. {Given that the derived abundance profiles from our model sample show very similar structures, we constructed a template abundance profile. For this purpose, we average the abundances of the individual models in each radial shell with the same relative velocity from the transition velocities (where X($^{56}$Ni) = X(O)). We generated a new set of synthetic spectra shifting the abundance template (Fig. \ref{fig:template_abundance}) in the velocity space according to the different transition velocities (see Sec. \ref{results_template}) of the five individual SN models, . Their fitting with the observed data are nearly on the same level as our original best-fit models. The result that the abundance profile of these objects can be effectively described by one parameter (transition velocity), leads to the conclusion that the origins of the members of the diverse Type Iax subclass may not differ sharply from each other. Moreover, the transition velocities seem to correlate with the peak luminosities and the expansion velocities in our sample. The loose correlation between the peak brightnesses and the expansion velocities has been reported in the literature; although, some outliers have been also observed. Note that, at this point, we cannot classify the whole group of Type Iax SNe as a one-parameter family as the normal SNe Ia. However, we are able to describe the five members of our Type Iax sample with only three parameters: the transition velocity of the abundance profile, the central density, and the location of the cut-off of the density profile. This result points in the direction that these peculiar transient objects can be described with only a few parameters.} | 18 | 8 | 1808.00448 |
1808 | 1808.05571_arXiv.txt | We present an analysis of archival observations with the Atacama Large (sub-)Millimetre Array (ALMA) of the gravitationally lensed quasar MG~J0414+0534, which show four compact images of the quasar and an Einstein ring from the dust associated with the quasar host galaxy. We confirm that the flux-ratio anomalies observed in the mid-infrared and radio persists into the sub-mm for the continuum images of the quasar. We report the detection of CO (11--10) spectral line emission, which traces a region of compact gas around the quasar nucleus. This line emission also shows evidence of a flux-ratio anomaly between the merging lensed images that is consistent with those observed at other wavelengths, suggesting high-excitation CO can also provide a useful probe of substructures that is unaffected by microlensing or dust extinction. However, we do not detect the candidate dusty dwarf galaxy that was previously reported with this dataset, which we conclude is due to a noise artefact. Thus, the cause of the flux-ratio anomaly between the merging lensed images is still unknown. The composite compact and diffuse emission in this system suggest lensed quasar-starbursts will make excellent targets for detecting dark sub-haloes and testing models for dark matter. | \label{section:intro} Almost all well-studied four-imaged gravitationally-lensed quasars have image flux ratios that are inconsistent with those expected from a smooth lensing mass distribution (e.g. \citealt{Koopmans:2003}). These so-called {\it flux-ratio anomalies} are primarily thought to be due to a local perturbation in the mass model that effects one or more of the image magnifications. These perturbations can be in the form of a population of low-mass sub-haloes either within the lensing galaxy or along the line-of-sight that are predicted by dark matter simulations (e.g. \citealt{Mao:1998,Dalal:2002,Xu:2012}), or due to massive companion satellite galaxies of the main lens (e.g. \citealt{McKean:2007,More:2009}), or from unaccounted for mass structure in the form of large-scale galactic discs (e.g. \citealt{Gilman:2017,Hsueh:2018}). In addition, disentangling these flux-ratio anomalies from microlensing by stars within the lensing galaxy, or extrinsic effects such as scintillation or extinction, can be difficult due to the compactness of the lensed images \citep{Sluse:2013}. The confirmation of a persistent flux-ratio anomaly has typically required high-resolution radio or mid-infrared observations, where the source is expected to be largely immune to microlensing and dust extinction \citep{Minezaki:2009,Jackson:2015}. Recently, the narrow-line emission from gravitationally-lensed quasars has provided an alternate strategy to measure reliable flux ratios, as this emission is expected to probe scales larger than is typically affected by microlensing and is not expected to be effected by intrinsic variability \citep{Moustakas:2003,Nierenberg:2016,Nierenberg:2017}. However, the flux-ratio anomaly method has thus far been limited by the very small statistical samples of around 7 quadruply-imaged lensed quasars that are suitable for constraining dark matter \citep{Dalal:2002,Xu:2015}. In the near and long-term, large-scale surveys at optical wavelengths through, for example, the Dark Energy Survey (DES; e.g. \citealt{Agnello:2017}) and {\it Euclid} \citep[e.g.][]{Serjeant:2014} will substantially increase the number of known gravitationally-lensed quasars with four images and provide a sample size that is large enough to rule out models for dark matter \citep{Gilman:2018}. These systems will need to be followed-up with either radio/mid-infrared continuum or narrow-line flux measurements to establish robust flux ratios, which themselves may have unknown systematics (for example, on the assumed source size). Therefore, new methods for robustly measuring the flux ratios of the lensed images are required to test for such systematics, provide independent measurements for the same objects, or to increase sample sizes in general. In this letter, we present the detection of a flux-ratio anomaly in the high-excitation CO (11--10) emission line from the gravitationally-lensed quasar MG~J0414+0534, which reveals a new and independent channel for quantifying low-mass dark matter haloes. MG~J0414+0534 is a radio-loud quasar at $z=2.64$ that is gravitationally-lensed into four images by a foreground galaxy at $z=0.96$. While the optical and near-infrared fluxes of this lensed quasar are effected by microlensing and extinction, the anomalous fluxes of images A2 and B, relative to image A1, persist at mid-infrared and radio wavelengths. The nature of the flux-ratio anomalies are quite complex; image B is thought to be affected by an optically-luminous companion galaxy to the main lens (object X; \citealt{Falco:1997,Ros:2000,Minezaki:2009}), whereas image A2 is thought to be perturbed by a dark sub-halo with a mass $\sim10^{7}$~M$_{\odot}$ (within the Einstein radius; \citealt{Macleod:2013}). A possible detection of mm-continuum emission from this optically dark sub-halo (object Y, located 0.6~arcsec East of image A2) was recently reported by \citet{Inoue:2017} from deep Atacama Large (sub-)Millimetre Array (ALMA) observations. If genuine, this would confirm that the flux-ratio anomaly between the two merging images is due to a single low mass sub-halo, as predicted from cold dark matter models. Here, we re-analyse the ALMA observations of MG~J0414+0534 to investigate the flux-ratio anomaly in the continuum and, for the first time, also in the molecular gas emission from a lensed quasar at mm-wavelengths. We confirm that there is a flux-ratio anomaly in the continuum emission from the quasar, but we do not find any evidence for the candidate dusty dwarf galaxy, object Y. We detect a flux-ratio anomaly in the mm molecular gas emission, which is consistent with that seen at radio and mid-infrared continuum measurements for this object. In Sections~\ref{section:obs} and \ref{section:results} we report the observations and results, respectively, and in Section~\ref{section:discussion} we discuss our results in the context of substructure searches and consider future prospects for a direct detection of a dark sub-halo. \begin{figure*} \includegraphics[width=0.49\textwidth]{continuum_nat.pdf} \includegraphics[width=0.49\textwidth]{continuum_spu.pdf} \caption{ALMA 340~GHz continuum image of MG~J0414+0534 using a natural (left) and a super-uniform (right) weighting of the visibilities. Clockwise from the brightest image A1 are images A2, B and C. The naturally weighted image has a beam-size of $0.30 \times 0.25$~arcsec at a positional angle of 37.6~deg East of North, and an rms map noise of $19~\upmu$Jy~beam$^{-1}$. The super-uniform weighted image has a beam-size of $0.12 \times 0.10$~arcsec at a positional angle of 32.3~deg East of North, and an rms map noise of $91~\upmu$Jy~beam$^{-1}$.} \label{fig:cont_images} \end{figure*} | \label{section:discussion} Although the image flux ratios of gravitationally lensed quasars can be used to test models of galaxy formation and dark matter, the technique is currently limited by the small number of suitable objects that can be used in the analysis (e.g. \citealt{Xu:2015}). Here, we have shown the detection of a flux-ratio anomaly in the molecular gas from a lensed quasar for the first time. By targeting the high excitation CO (11--10) emission, we are able to probe a compact region that is close to the central super-massive black hole, which gives an independent measurement to the flux ratios observed in the mm-continuum and other wavebands. In addition, we have shown that there is no evidence of variability in the line emission from either intrinsic or extrinsic effects and, as the dust is optically thin to the line emission, the CO (11-10) flux-ratios are not affected by extinction. Thus, the study of molecular line emission appears to be a promising method to measure flux-ratio anomalies. As well as providing an independent measurement of the flux ratios, studies in the mm-regime have several advantages over other wavelengths. First, as shown here and in many other cases, the thermal dust emission in high redshift objects is sufficiently extended to produce an Einstein ring; such data can provide important constraints to the lensing macro-model, even at a low-angular resolution (e.g. \citealt{Rybak:2015a}). Therefore, lenses that produce only two-images could also now be used in the analysis, increasing the sample size. Second, interferometric arrays such as ALMA can provide sufficiently high angular resolution ($>10$~mas) such that any degeneracies between the inferred level of substructure and the source model (often assumed to be point-like) can be minimised. Finally, 70~percent of known gravitationally lensed quasars are FIR-bright \citep{Stacey:2018}, and so are expected to have detectable high excitation gas associated with the quasar. This, coupled with the large-scale lens surveys being carried out with DES, for example, will provide a large population of potential targets for this work in the near-term. A pilot study of a well-selected sample of lensed quasars to investigate this is planned. Our study of MG~J0414+0534 has not revealed the cause of the flux-ratio anomaly, although the similar flux ratios observed here and at other wavebands does suggest that it is due to some perturbation in the lensing potential. However, we have demonstrated that it is not due to a candidate dwarf satellite close to images A1 and A2 that was previously reported. As the magnitude of the anomaly is consistent with other studies, we have not tested the possible mass models that are consistent with the data, which we defer to a follow-up paper. This will include combining the continuum and line emission data (e.g. \citealt{Hezaveh:2013}) to determine whether the perturbation is due to a sub-halo or a more massive structure that is currently unaccounted for in the lensing macro-model. \begin{figure} \begin{subfigure}[b]{0.51\textwidth} \centering \includegraphics[width=\textwidth]{CO_mom1_zoom.pdf} \end{subfigure} \begin{subfigure}[b]{0.51\textwidth} \centering \includegraphics[width=\textwidth]{CO_mom2_zoom.pdf} \end{subfigure} \caption{The CO (11--10) velocity-field (moment one; upper) and velocity dispersion (moment two; lower), for the merging images A1 and A2. Pixels below the $5\sigma$-level are masked out. The synthesized beam FWHM is shown in the bottom left-hand corner.} \label{fig:moment_maps} \end{figure} | 18 | 8 | 1808.05571 |
1808 | 1808.09872_arXiv.txt | Constraining the planetary composition is essential for exoplanetary characterization. In this paper, we use a statistical analysis to determine the characteristic maximum (threshold) radii for various compositions for exoplanets with masses up to 25 Earth masses (M$_\oplus$). We confirm that most planets with radii larger than 1.6 Earth radius (R$_\oplus$) are not rocky, and must consist of lighter elements, as found by previous studies. We find that planets with radii above 2.6 R$_\oplus$ cannot be pure-water worlds, and must contain significant amounts of hydrogen and helium (H-He). We find that planets with radii larger than about 3 R$_\oplus$, 3.6 R$_\oplus$, and 4.3 R$_\oplus$ are expected to consist of 2\%, 5\% and 10\% of H-He, respectively. We investigate the sensitivity of the results to the assumed internal structure, the planetary temperature and albedo, and the accuracy of the mass and radius determination. We show that the envelope's metallicity, the percentage of H-He and the distribution of the elements play a significant role in the determination of the threshold radius. Finally, we conclude that despite the degenerate nature of the problem, it is possible to put limits on the possible range of compositions for planets with well-measured mass and radius. | \label{sec:intro} The ongoing efforts to detect and characterize exoplanets from Earth and space have led to the detection of thousands of exoplanets, and allows us to study planets as a class of astrophysical objects. Measured radii of planets from the {\it Kepler} mission combined with Radial Velocity (RV) and Transit Timing Variations (TTV) follow-ups provide information on the planetary radii and masses, and therefore, on their mean densities. The measured masses and radii can be compared to theoretical mass-radius (M-R) relations of planetary objects, which is used to infer the possible bulk composition \citep[e.g.][]{Weiss2014,Zeng2016,Wolfgang2015,Batygin2013}. \par Since the discovery of exoplanets with radii between that of Earth (1 R$_{\oplus}$) and Neptune ($\sim$4 R$_{\oplus}$), it was unclear whether they represent large-scale terrestrial planets (super-Earths) or small versions of Neptune (mini-Neptunes). Characterizing these planets is in particular challenging because we do not have similar objects in the Solar System, and they lie in a mass-regime where uncertainties in the Equation of State (EOS) and the material's distribution are the largest \citep[e.g.][]{Baraffe2008,Vazan2016}. \par Determining the exact planetary structure and composition is challenging due to the intrinsic degeneracy, i.e., exoplanets with very different interiors can have identical masses and radii \citep[e.g.][]{Rogers2010,Lopez2014,Dorn2015,Dorn2017}. Despite this inherent degeneracy, the least dense possible interiors for a given bulk composition can be derived. These represent end-member interiors that can be compared to observed exoplanets. For example, the lowest density among all rocky (silicate) interiors is the one of MgSiO$_3$. Based on the density of the idealized composition of MgSiO$_3$, previous studies suggest that most of the planets with radii larger than 1.6 R$_\oplus$ have too low densities to be consistent with purely rocky interiors \citep{Rogers2015, Weiss2014}, and therefore they are expected to contain volatiles. More specifically, \citet{Rogers2015} employed a hierarchical Bayesian statistical approach to determine threshold radii of various rocky compositions. The threshold radius of a given composition represents the radius above which a planet has very low probability to be of that specific composition. \citet{Rogers2015} used a sample of 22 short period (up to 50 days) \textit{Kepler} planets with Radial Velocity follow-ups. For purely rocky exoplanets, a threshold radius of 1.6 R$_\oplus$ was found. Interestingly, the distribution of observed radii of small exoplanets suggests a bimodal shape of planetary sizes \citep{Fulton2017, Fulton2018}. A gap found at radii 1.5 - 2.0 R$_\oplus$ splits the population of close-in planets (orbital period shorter than 100 days) into two regimes: planets with $R_p< 1.5$ R$_\oplus$ and planets with $R_p=2.0-3.0$ R$_\oplus$. This paucity in the distribution might be explained by photo-evaporation of their volatile atmospheres \citep{Owen2017,VanEylen2017, Lopez2014}. % Generally, in volatile-rich planets, the thickness of the gaseous envelope depends on the mass fraction of the light elements, the envelope's metallicity, and the temperature profile of the planet. % These parameters and the characteristics of the underlying deeper layers determine the planet's density. Similarly to pure-rocky planets, end-member interior models for volatile-rich compositions exist. For example, a planet with a mass fraction of 2\% of H-He is expected to have the lowest density when the envelope's metallicity is low, and the temperatures are high. % In this paper, we build on the statistical methodology of \citet{Rogers2015}, and determine different threshold radii for small and intermediate-size planets, accounting for the possibility of gaseous envelopes with different metallicities and internal structures. | \label{sec:Conclusions} We present a statistical analysis to determine the threshold radii of volatile-rich planets. We show that different assumed compositions and internal structures with fixed $\fHHe$ and $Z$ lead to a range threshold radii. As a result, in order to characterize individual planets information on their orbital properties and atmospheric compositions is required. However, despite the degenerate nature of the problem we suggest that there are characteristic threshold radii for different compositions. % \par First, we confirm that planets with radii larger than 1.6 R$_\oplus$ are not rocky, and must consist of lighter elements. This conclusion is consistent with the work of \citet{Rogers2015}, despite some differences in the statistical analysis and the used planetary sample. It is found that distinguishing a pure-water planet from a rocky planet with a thin H-He atmosphere is not possible. Therefore, planets that are classified as ocean planets might in reality be rocky core planets with a volatile atmosphere \citep{Adams2008}. \par Second, we show that most of the planets larger than $\sim 3$ R$_\oplus$ must contain at least 2\% of H-He, while most of the planets with radii larger than $\sim 3.6$ R$_\oplus$ and 4.3 R$_\oplus$ must contain at least 5\% and 10\% of H-He, respectively. While the exact value of R$_{th}$ depends on the model assumptions (i.e., composition, structure, thermal state, EOS), we find a range of threshold radii of $\sim 2.5-4.3 R_\oplus$ for planets with rocky cores and gaseous atmospheres. These radii are typically larger than the threshold radii for pure-water planets ($R_{th}$ $\sim 2.6 R_\oplus$). We find that although the planetary albedo and semi-major axis affect the planetary temperature, they have a relatively small impact on the inferred R$_{th}$. For albedos between zero and 0.9, R$_{th}$ varies from $\sim$ 3 $R_\oplus$ to $\sim$ 3.2 $R_\oplus$, suggesting that assumed albedo has a very small impact on R$_{th}$. We suggest that high planetary luminosity leads to somewhat larger R$_{th}$, in the range of sensible luminosities ($L \sim 10^{-1}-10^{2} L_N $, where $L_N$ is Neptune's luminosity) the change in R$_{th}$ is very small, suggesting that R$_{th}$ is relatively insensitive to the assumed planetary luminosity. \par The key conclusions of our study can be summarized as follows: \begin{itemize} \sitem We confirm that planets with radii larger than $\sim$1.6 R$_\oplus$ are not pure-rocky worlds and must consist of lighter elements. % \sitem Planets with radii larger than $\sim$2.6 R$_\oplus$ are not pure-water worlds and must consist of atmospheres (presumably of H-He). % \sitem By defining a mini-Neptune (or a Neptune-analog) as a planet with at least 2\% of H-He in mass, we find that the transition from super-Earths (planets consisting of less than 2\% of H-He) to mini-Neptunes occurs at $\sim$ 3 R$_{\oplus}$.% \sitem Planets with radii larger than $\sim$4 R$_\oplus$ are expected to consist of at least 10\% of H-He and are therefore gaseous-rich. % \end{itemize} Upcoming data from space missions such as CHEOPS, TESS and PLATO as well as ground-based facilities will further constrain the possible compositions of exoplanets. Measurements of planets with similar masses at larger radial distances will allow us to extend our scheme and characterize colder planets and reveal whether the threshold radii are expected to change with the distance to the host star. Finally, accurate measurements of both the masses and radii of small- and intermediate- mass exoplanets will allow us to determine whether threshold masses exist. This will significantly improve our understanding of the formation, and evolution, and internal structures of planets in the solar neighbourhood. % \newpage | 18 | 8 | 1808.09872 |
1808 | 1808.08874_arXiv.txt | {% The solar meridional flow is an essential ingredient in flux-transport dynamo models. However, no consensus on its subsurface structure has been reached. } {% We merge the data sets from SOHO/MDI and SDO/HMI with the aim of achieving a greater precision on helioseismic measurements of the subsurface meridional flow. } {% The south-north travel-time differences are measured by applying time-distance helioseismology to the MDI and HMI medium-degree Dopplergrams covering May 1996--April 2017. Our data analysis corrects for several sources of systematic effects: $P$-angle error, surface magnetic field effects, and center-to-limb variations. For HMI data, we used the $P$-angle correction provided by the HMI team based on the Venus and Mercury transits. For MDI data, we used a $P$-angle correction estimated from the correlation of MDI and HMI data during the period of overlap. The center-to-limb effect is estimated from the east-west travel-time differences and is different for MDI and HMI observations. An interpretation of the travel-time measurements is obtained using a forward-modeling approach in the ray approximation. } {% In the latitude range 20$\degr$--35$\degr$, the travel-time differences are similar in the southern hemisphere for cycles 23 and 24. However, they differ in the northern hemisphere between cycles 23 and 24. Except for cycle 24's northern hemisphere, the measurements favor a single-cell meridional circulation model where the poleward flows persist down to $\sim$0.8~$R_\odot$, accompanied by local inflows toward the activity belts in the near-surface layers. Cycle 24's northern hemisphere is anomalous: travel-time differences are significantly smaller when travel distances are greater than 20$\degr$. This asymmetry between northern and southern hemispheres during cycle 24 was not present in previous measurements, which assumed a different $P$-angle error correction where south-north travel-time differences are shifted to zero at the equator for all travel distances. In our measurements, the travel-time differences at the equator are zero for travel distances less than $\sim$30$\degr$, but they do not vanish for larger travel distances. This equatorial offset for large travel distances need not be interpreted as a deep cross-equator flow; it could be due to the presence of asymmetrical local flows at the surface near the end points of the acoustic ray paths. } {% The combined MDI and HMI helioseismic measurements presented here contain a wealth of information about the subsurface structure and the temporal evolution of the meridional circulation over 21 years. To infer the deep meridional flow, it will be necessary to model the contribution from the complex time-varying flows in the near-surface layers. } | We define solar meridional circulation as the axisymmetric component of the meridional flow in a spherical-polar coordinate system where the polar axis coincides with the Sun's rotation axis. We assume in this definition that all layers rotate about a single rotation axis. The surface meridional flow, first measured by \citet{Duvall1979}, is poleward with a peak speed of 10--20~m~s$^{-1}$ at low- and mid-latitudes, and is known to persist through the convection zone to some extent \citep{Giles1997}. It has been found by various methods that the magnitude and profile of the meridional flow varies with solar activity level \citep[e.g.,][]{Chou2001,Haber2002,Beck2002,Zhao2004,Hathaway2010,Ulrich2010,Liang2015b,Komm2015}. In parallel, a pattern of inflows toward the active regions develops \citep[e.g.,][]{Gizon2001,Loeptien2017} and moves equatorward in step with the activity belts as the cycle progresses. It is believed that these inflows, which in general have a component in the meridional plane, account for a significant part of the cyclic change of the meridional flow \citep{Gizon2004b,Svanda2008}. In addition, the meridional flow is capable of transporting magnetic flux and is considered a likely mechanism in flux-transport dynamo models for the conveyance of the field in the solar interior \citep[see, e.g., recent review by][]{Cameron2017}. Recently, a number of inconsistent helioseismic results were reported \citep{Zhao2013,Schad2013,Jackiewicz2015,Rajaguru2015,Boening2017,Chen2017,Lin2018}. The main reason is that helioseismic measurements suffer from a variety of systematic errors such as instrumental misalignment, the center-to-limb variation, and the influence of the surface magnetic field \citep[e.g.,][]{Beck2005,Duvall2009,Zhao2012,Liang2015a}. Furthermore, the meridional flow is more than one order of magnitude weaker than other major flows inside the Sun. Using the noise model by \citet{Gizon2004}, it has been suggested by \citet{Braun2008} and by \citet{Hanasoge2009} that an estimate of the meridional flow at the bottom of the convection zone at a level of precision of 1~m~s$^{-1}$ requires tens of years of data. Thus, helioseismic measurements of a deep meridional flow are in need of a very long observational time series. The Michelson Doppler Imager on board the Solar and Heliospheric Observatory \citep[SOHO/MDI:][]{Scherrer1995} and the Helioseismic and Magnetic Imager on board the Solar Dynamical Observatory \citep[SDO/HMI:][]{Scherrer2012,Schou2012} have accumulated nearly two solar cycles of full-disk Doppler observations since May 1996. Lately \citet{Liang2017}, taking care of most of the major systematics, performed a detailed comparison of the travel-time measurements of meridional circulation from the two data sets in a concurrent period from May 2010 to April 2011 and obtained a remarkable degree of consistency. Building upon the previous success, we merge the two data sets in this work with hopes of tying down the meridional flow profile below 0.9~$R_\odot$. In Sect.~\ref{sec:data}, we describe the data preparation and analysis. In Sect.~\ref{sec:obs}, we present the measured travel-time shifts. In Sect.~\ref{sec:model}, we model not only the global-scale meridional circulation that covers all latitudes, but also the inflows around the mean active latitudes in the upper convection zone as a separate component. The measured travel-time shifts are compared with the forward-modeled travel-time shifts in Sect.~\ref{sec:cmpr} We summarize in Sect.~\ref{sec:sum} with a discussion of the implications of the work presented here. | \label{sec:sum} We measured the travel-time shifts in the north-south and east-west directions from 14-yr of MDI data and 7-yr of HMI data covering 12-yr of cycle 23 and 9-yr of cycle 24. The measured travel-time shifts exhibit several interesting features. First, the temporal trends of the center-to-limb effect are different for the MDI and HMI data sets. Second, the solar cycle variations of the travel-time shifts induced by the subsurface meridional flows are not in phase for different distance ranges. Third, a significant reduction in the amplitude of travel-time shifts for large distances is seen in the northern hemisphere during the rising and maximum phases of cycle 24, which leads to an apparent cycle-to-cycle variation in the northern hemisphere and a strong north-south asymmetry for cycle 24. The forward-modeled travel-time shifts were computed in the ray approximation for some representative flow models and compared with the long-term averages of measured travel-time shifts. The 21-yr travel-time measurement in general favors the forward-modeled results for single-cell meridional circulation models in combination with the inflows toward the activity belts in the upper convection zone. However, in view of the contrasts in the two cycles and in the two hemispheres, the northern measurement of cycle 24 decreases rapidly for distances greater than 12$\degr$, implying a rapid decrease of the poleward flows with increasing depth. Due to limited flow models implemented in this work, we restrict ourselves from commenting further on the possible flow field that could produce such a rapid decline in the travel-time shifts. Moreover, the forward-modeled results for the local inflows might partly explain the solar cycle variations of travel-time shifts for different distance ranges. \paragraph{Systematic errors} One might worry that the unusual result of cycle 24 in the northern hemisphere is caused by some systematic errors, especially when the center-to-limb variations observed by MDI and HMI are so different in many aspects. We note that, after the removal of the center-to-limb effect, the results of \citet{Liang2017} measured from MDI and HMI observations in the period of one-year overlap (i.e., the rising phase of cycle 24) both showed a noticeable reduction in the northern hemisphere's measurements for $\Delta > 20\degr$. However, we cannot exclude the possibility that this unusual reduction is caused by unknown systematic error if the systematic error affects both data sets in a similar way (a good example is the systematic effect caused by the surface magnetic field). We also note that the strong north-south asymmetry of cycle 24, particularly the non-zero values at the equator, might be attributed to a $P$-angle error and thus be removed. Considering the problems posed by various systematic errors, it would be necessary in the future to carry out a comparison with helioseismic observations from the GONG++ data since 2001. \paragraph{Deep meridional flows} As for the flow profile below 0.8~$R_\odot$, we are hindered by the low signal-to-noise ratio from giving a conclusive result of the meridional circulation in the lower convection zone. While a possible flow in the lower convection zone may produce a travel-time shift of a few hundredths of a second, the noise level in this work, after averaging over 21-yr data and a large range of latitudes and travel distances, is on the order of 0.1~s in the regime of concern. Accordingly, a direct detection of deep meridional circulation with confidence would require an observation on the order of hundred years. This estimate is consistent with that made by \citet{Braun2008} and by \citet{Hanasoge2009}. A common way to boost the signal-to-noise ratio with limited observation in time-distance helioseismology is to apply a spatio-temporal filter in Fourier domain \citep[e.g.,][]{Kholikov2014}. One concern is how to incorporate the filter with the masking procedure for this type of filters may blend the Doppler signals in the quiet Sun with that in the active regions. Another approach is to average the travel-time shifts over a wider band in longitude at the expense of introducing systematic error. Although the resulting measurement might contain the center-to-limb variation, the relative change in them can still be used to study the temporal variation of deep meridional circulation as done by \citet{Liang2015b}, assuming that the temporal change in the center-to-limb variation is smooth. \paragraph{Local inflows} In addition to the global-scale meridional circulation that covers all latitudes, the local cellular flows around the activity belts have a crucial role to play in explaining the meridional-flow-induced time shifts. The presence of the near-surface inflows toward the active regions results in a time-shift pattern which alternates in latitude and in travel distance. As a consequence, the equatorward migration of the near-surface inflows causes an apparent solar cycle variation in the travel-time measurements of meridional circulation. These local flows in the upper convection zone, after averaging over decades of observations, may still produce a travel-time shift comparable to that by a deep meridional flow and complicates the interpretation of the measured travel-time shifts for large distances. We note that \citet{Lin2018} adopted a simplified model to separate the inflows from the global-scale meridional circulation and obtained a slower meridional flow at solar maximum than that at minimum. Apart from that, the anchoring depth of the inflow structure is of interest. The divergent-flow-like travel-time perturbation peaking at a depth of $\sim$0.9~$R_\odot$ \citep{Chou2001,Beck2002,Chou2005} might be linked with the near-surface inflow to form a cellular flow structure. However, it has been pointed out by \citet{Liang2015a} that these measurements were made without considering the influence of the surface magnetic field on the travel-time shifts (this issue is revisited in Appendix~\ref{app:nobmask}). The travel-time shifts measured in this work, with the surface magnetic field being taken care of though, are insensitive to the anchoring depth of the inflow structure for the distance range implemented in our measurements. To resolve a local cellular flow structure in the near-surface layers, higher spatial resolution Dopplergrams are required for the travel-time measurement in the distance range $\Delta < 6\degr$, which is absent from this work. \paragraph{Other uncertainties} Our data analysis did not take into account a potential error in the orientation of the solar rotation axis determined by the Carrington elements \citep{Beck2005,Hathaway2010}. In the frame determined from the observations by \citet{Carrington1863}, this uncertainty may introduce an apparent flow leaked from the solar rotation on the order of $\sim$3~m~s$^{-1}$ northward in summer and southward in winter when the $B_0$ angle is small. Thus, there would be a systematic effect if the northward and southward leaking flows did not cancel each other out in the periods over which the long-term averages were performed. We note that the MDI data we discarded since mid-2003 all have a small $B_0$ angle and the systematic effect due to the error in the Carrington elements is expected to be minimal. We also note that the ray approximation is expected to be inaccurate when applied to structures with length scale comparable to the first Fresnel zone \citep{Birch2001}. Furthermore, we compute the ray paths with merely one frequency even though the central frequency of the wavelet has a dependence on travel distance. On closer inspection, this may incur an uncertainty of 0.01--0.03~s, for short distances in particular (see Appendix~\ref{app:cf-freq}). In this regard, a sophisticated inversion with kernels that take into account finite-wavelength effects \citep[e.g.,][]{Boening2017,Gizon2017,Fournier2018,Mandal2018} is needed to correctly decipher the meaning of the measured travel-time shifts. It would be preferable to study the north-south asymmetry and other fine structures of meridional circulation by inversion instead of forward modeling. | 18 | 8 | 1808.08874 |
1808 | 1808.04788_arXiv.txt | {% \hi{Feedback by massive stars shapes the interstellar medium and is thought to influence subsequent star formation. Details of this process are under debate.} } {% \hi{We exploited observational constraints on stars, gas and nucleosynthesis ashes for the closest region of recent massive-star formation, Scorpius-Centaurus OB2, and combined them with 3D hydrodynamical simulations, in order to address physics and history for the case of the Scorpius-Centaurus superbubble.}} {\hi{We used published cold gas observations through \planck survey data processing, \her and \apexu, continuum and molecular line observations. We analysed the Galactic All Sky Survey (GASS) to investigate shell structures in atomic hydrogen, and used \hip and \gaia data in combination with interstellar absorption against stars to obtain new constraints for the distance to the \HI features. Hot gas is traced in soft X-rays via the \rosat all sky survey. Nucleosynthesis ejecta from massive stars were traced with new \itg spectrometer observations via $^{26}$Al radioactivity. We also performed 3D hydrodynamical simulations for the Sco-Cen superbubble. } } { \hi{Soft X-rays and a now more significant detection of $^{26}$Al confirm recent ($\approx 1$~Myr ago) input of mass, energy and nucleosynthesis ejecta, likely by a supernova in the Upper Scorpius (USco) subgroup. We confirm a large supershell around the entire OB association and perform a 3D hydrodynamics simulations with a conservative massive star population that reproduces the morphology of the superbubble. High resolution GASS observations of a nested supershell reveal that it is filamentary \rvb{possibly related to} the Vishniac clumping instability, but molecular gas (Lupus~I) is only present where the shell coincides with the connecting line between the subgroups of the OB association, suggesting a connection to the cloud, probably an elongated sheet, out of which the OB association formed. Stars have formed sequentially in the subgroups of the OB association and currently form in Lupus~I. To investigate the impact of massive star feedback on extended clouds, we simulate the interaction of a turbulent cloud with the hot, pressurised gas in a superbubble. The hot gas fills the tenuous regions of the cloud and compresses the denser parts. Stars formed in these dense clumps would have distinct spatial and kinematic distributions.} } {\hi{The combined results from observations and simulations are consistent with a scenario where dense gas was initially distributed in a band elongated in the direction now occupied by the OB association. Superbubbles powered by massive stars would then repeatedly break out of the elongated parent cloud, surround and squash the denser parts of the gas sheet and thus induce more star formation. The expected spatial and kinematic distribution of stars is consistent with observations of Sco-Cen. The scenario might apply to many similar regions in the Galaxy and also to AGN-related superbubbles.} } | \label{sec:i} % The interstellar medium is a multifaceted, dynamic place: massive stars inject energy, mass and freshly synthesised nuclei. This changes its metallicity and sustains multi-phase turbulence with volume-filling, hot and rarefied regions and dense layers compressed by converging flows which may make the transition to the molecular phase and form the next generation of stars \citep{MO77,dAB05,Heitschea08,Hennea08,Micicea13,GomVaz14,GongOst15a}. Key drivers of this process are massive stars with their ionising radiation, winds and core-collapse supernovae.% It is clear that in order to understand these complex phase transitions in the interstellar medium associated with multiple generations of stars, studies in any single wavelength cannot be sufficient. Molecular gas, often associated with ongoing star formation, is best studied at infrared and sub-mm wavelengths. Atomic gas is connected with shells pushed out by energy input of young massive stars and efficiently mapped in the 21~cm radio line and with optical absorption against stars. Tenuous gas shock-heated by the winds and explosions of massive stars to keV temperatures is only seen in X-ray observations, and the ejecta themselves can be traced via their radioactive decay lines at MeV energies. \begin{table*} \caption{Sco-Cen subgroups and their properties} \label{t:sc-overview} % \centering % \begin{tabular}{ccccccc} % \hline\hline % Sub-group & initial stellar mass & age & distance & diameter & B stars\tablefootmark{a} & past SN\tablefootmark{b}\\ name & $\sbr{M_\odot}$ & $\sbr{\rm Myr}$& $\sbr{\rm pc}$ & $\sbr{\rm pc}$ & $\sbr{\rm number}$ & $\sbr{\rm number}$\\ \hline USco & 2060 & $\approx$~5-10\tablefootmark{d} & 145 & 35 & 49 & 1\\ LCC & $\approx 2000$\tablefootmark{c} & $\approx$~15 & 118 & 50 & 42 & $\approx 4$\tablefootmark{e} \\ UCL & $\approx 3000$\tablefootmark{c} & $\approx~$17 & 140 & 100 & 66 & $\approx 7$ \\ \hline % \end{tabular} \tablefoot{ The data is taken from \citet{PreibMam08} unless indicated otherwise. \tablefoottext{a}{Current number of B stars according to \citet{deBruijne99}.} \tablefoottext{b}{Estimated number of past supernovae.} \tablefoottext{c}{Estimated from the number of B stars with Salpeter initial mass function and scaled to the well determined mass of USco. For the initial mass function used in \citep{PreibMam08} for USco, USco would have lost only one B-star from the main sequence while ageing from 5 to 16~Myr. } \tablefoottext{d}{Stars with $M< 1$~\msun have age determinations towards the lower end of the range; more massive ones are found to be older \citep{HH15,PecMam16}. The range is likely related to measurement and model uncertainties and not necessarily a real age spread \citep{Preibisch12,Donaldea17}. } \tablefoottext{e}{Scaled to the estimate in UCL using a Salpeter initial mass function. } } \end{table*} \begin{figure*} \centering \includegraphics[width=0.99\textwidth]{Sco_Cen_intro-sketch_v05.pdf} \caption{Summary sketch of observational information on the Scorpius-Centaurus region. The OB-association Sco-Cen OB2 has three subgroups of $\approx 2000$~\msun, each, formed over the last $\approx 15-20$~Myr. Stars are currently forming in the $\rho$~Ophiuchus (mov\hi{ing} away from USco at about 1 \kms) and Lupus~I (part of an expanding \HI loop around USco) molecular clouds. See Table~\ref{t:sc-overview} for more details on the stars. \HI shells are detected around the youngest OB subgroup, USco, and around the entire region. Diffuse soft X-ray emission is detected towards the superbubble at a level of $10^{35}$~erg~s$^{-1}$. One of the detected signatures of massive star winds and supernova explosions is \al, towards USco.} \label{fig:scocensketch}% \end{figure*} \object{Scorpius-Centaurus OB2} with its associated superbubble (Sco-Cen in the following) is the closest region with clear signs of recent massive-star activity \citep{PreibMam08}, and therefore an ideal object to study these effects in great detail. The OB association is at a distance of around 140~pc and consists of three subgroups, Upper Scorpius (USco), Upper Centaurus-Lupus (UCL), and Lower Centaurus-Crux (LCC) identified from Hipparcos parallax and proper motion measurements \citep{deZeeuwea99,MelDam09}, \hi{which has recently been confirmed with \gaia data \citep{WM18}}. The stellar populations are thus known in great detail \citep[e.g.,][]{deBruijne99,Preibea02,Mamea02}. The three sub-groups currently contain many B~stars and, given their ages between 5 and 17~Myr, probably had a number of more massive stars in the past that exploded already \citep[e.g.,][Table~\ref{t:sc-overview} for basic stellar parameters]{deGeus92}. With an angular diameter of roughly $90\deg$ on the sky, Sco-Cen is also well studied at all wavelengths that carry information about the diffuse interstellar medium (Fig.~\ref{fig:scocensketch} for a schematic overview). Large area and all-sky surveys, such as the \planck dust maps, or the ROSAT all-sky survey (details below) show an essentially post star formation region. Most of the molecular gas has been converted to stars, or dislocated into an \HI supershell \citep{Poepea10}. The region is a prominent source at keV X-rays \citep{Gaczkea15}. As X-ray bright superbubble, Sco-Cen likely hosted a supernova explosion within the past Myr \citep{KD14,Krausea14a}. This is consistent with the detection of radioactive \al towards one of the subgroups \citep{Diehlea10}. Causal connection in the observed sequence of star formation events has been suggested \citep[e.g.,][]{Preibea02}. \citet{PreibZinn07} detail a triggering scenario for the entire region: Star formation first starts in UCL and LCC. Feedback in UCL produces an expanding high pressure region headed by a shock wave. The shock reached the USco parent cloud about 10~Myr ago, pressurised the cloud and caused the denser parts of the cloud to collapse and form stars. The cloud was subsequently eroded by its internal feedback which caused star formation in nearby clouds, e.g. the Lupus~I cloud and the \object{$\rho$ Ophiuchus} cloud. Several authors have suggested a possible link between the onset of star formation in the \object{$\rho$ Ophiuchus} region and the influence of expanding shells in Sco-Cen: \citet{Wilkea79,Wilkea15} mention magnetic field structure and a higher velocity dispersion of the YSOs in the main core of \object{$\rho$ Ophiuchus}, which could be a result from the global collapse occurring in the region due to ta shock wave from USco \citep[compare also][]{Kwonea15}. \citet{Klose86} argues for an enhanced gamma ray flux $>100$~MeV, which he relates to an enhanced level of cosmic ray protons in the region, likely from an energetic explosion which might have played a role in triggering star formation in \object{$\rho$ Ophiuchus}. Proper motion studies show that USco has a projected velocity of $(\mu_l \cos b, \mu_b)= (-24.5,-8.1)$~mas/yr \citep{deZeeuwea99}. Subtracting this from the projected velocity \hi{of} the \object{$\rho$ Ophiuchus} star cluster, $(\mu_l \cos b, \mu_b)= (-23.8,-10.7)$~mas/yr \citep{Ducea17}, yields a relative motion of $(\mu_l \cos b, \mu_b)= (0.7,-2.6)$~mas/yr. This translates to a projected velocity of 1.7~\kms roughly radially away from USco, supporting the connection to feedback in USco. \begin{figure*} \centering \includegraphics[angle=0,width=0.99\textwidth]{scocen_850mu_labelled2c.jpg} \caption{\planck 850$\mu$m dust map of the Scorpius-Centaurus region in Galactic coordinates. The Milky Way plane can be seen as a pink band over the entire image. Scorpius-Centaurus~OB2 (green ellipses indicate the location of the three subgroups) is mainly above the plane and essentially dust-free apart from the $\rho$~Ophiuchus and Lupus~I clouds (labelled). Coordinates are Galactic and given in degree. The colour scale ranges from black (low) over green, pink, red, orange to yellow (high).} \label{fig:850mu}% \end{figure*} In a re-analysis of the {\sc Hipparcos} data, \citet{BouyAlves15} have recently expanded on this scenario towards lower Galactic longitudes: They find that the three Sco-Cen subgroups are part of a much larger stream of OB associations and open clusters which form a sequence of star formation over 60 Myr. Star formation would have started 65 Myr ago in the now open cluster \object{NGC 2451A} and propagated 350~pc along a filament with the last major episode in USco. Discrete star formation events took place every few to 30 Myr. While the basic case for sequential star formation in Sco-Cen is strong, details remain hard to understand. For example, if each star formation event sweeps up the dense interstellar medium in its surroundings before it triggers the formation of another OB association in its shell \citep[collect \& collapse, e.g.,][]{Whitwea94}, one would expect a monotonic age increase towards the oldest stars, which is not seen in Sco-Cen. This is similar for a distribution of clouds close to equilibrium between self-gravity and pressure that might be triggered to collapse by a passing shock wave. If such a distribution of clouds existed for a time similar to the age of the Sco-Cen stars, one would expect at least some of them to form stars independently of any trigger. Here we compile multiwavelength data on the interstellar medium in Sco-Cen, and interpret these with the help of dedicated superbubble simulation\hi{s}. We find that simple collect and collapse scenarios are disfavoured. Even the smallest, currently star-forming molecular cloud, shows evidence for a multi-stage formation process where cloud formation and the onset of star formation are independent from each other. Our simulations show that an initially homogeneous medium cannot fully explain the data. We suggest a multi-stage formation process where the high sound speed in a hot superbubble plays an important role in communicating pressure enhancements to different parts of an initially flattened linear cloud inside the superbubble. \begin{figure*} \centering \includegraphics[width=\textwidth]{Lupus_I_obs_summary_v02.pdf} \caption{Lupus I molecular cloud, combined image of the mosaics obtained with {\it SPITZER}/MIPS at 24~$\mu$m (blue), \heru/PACS at 160~$\mu$m (green) and the \heru/SPIRE at 500~$\mu$m (red), all in logarithmic scales. The right overplotted panels show the column density distribution functions of the northern (top) and southern (bottom) parts of LupusI and the central panel shows the histograms of the dust temperature in different parts of the cloud \citep[see details in][]{Gaczkea15}. The left panel shows the gas velocity profiles averaged along cuts A, B and C, which are highlighted by the black boxes. Ecliptic coordinates are used for this figure to help identifying northern and southern parts. Galactic coordinates of the centre of Lupus~I: $l=338\deg 50\arcmin$, $b=16\deg 40\arcmin$. } \label{fig:lupIapex}% \end{figure*} \begin{figure*} \centering \includegraphics[width=0.49\textwidth]{HI_bigbubble.png} \includegraphics[width=0.49\textwidth]{HI_small_bubble.png} \caption{Intensity of the 21cm line of neutral hydrogen towards Sco-Cen. The left (right) panel shows the emission integrated between -20~\kms and 0 (0 and 10~\kms). The numbers in the yellow boxes show upper distance limits for the \HI features derived from velocity-matched \NaI absorption lines against stars with distances known from parallax measurements by {\em Hipparcos} and {\em GAIA}. } \label{fig:HI-big}% \end{figure*} \begin{figure*} \centering \includegraphics[width=0.4\textwidth]{Usco33v74kms.pdf} \includegraphics[width=0.5\textwidth]{No_dist_3D_USco_loops_azim-30elev10.pdf} \caption{Intensity of the 21cm line of neutral hydrogen towards USco in the 7.4~km~s$^{-1}$ velocity channel (left) and by-eye characterisation of the \HI structures across multiple velocity channels (right). Three groups of rings are apparent: the USco loop (black), which corresponds to the dominant loop in the channel map on the left, and two smaller, tube-like structures (green and red) which are cospatial but separated in velocity space. The red and green rings might be a result of erosion of the \HI interface between the Sco-Cen superbubble and the local bubble, where the hot gas streaming through the hole ablates and accelerates the \HI. } \label{fig:HI-USco}% \end{figure*} \begin{figure} \centering \includegraphics[width=0.5\textwidth]{disthist.png} \includegraphics[width=0.5\textwidth]{disthist2a.png} \caption{Histograms of distances of HI features in Sco-Cen determined by association with \NaI absorption lines. Top: cumulative histogram for all available stars. Bottom histogram \hi{from} stars at distances between 100 and 300~pc. More lower (upper) limits are found up to (above) about 150~pc, Almost no lower limits are found above 220~pc. } \label{fig:HI-histos}% \end{figure} | \label{sec:conc} We have carried out a multiwavelength analysis of the interstellar medium in the Sco-Cen region. From cold and tenuous to hot X-ray gas, including freshly injected nucleosynthesis ejecta, we find consistent evidence for the gradual \hi{trans}formation of clouds, star \hi{formation} and the \hi{expansion of the} Sco-Cen superbubble. This superbubble has at times surrounded denser clouds of gas, and squashed them with the likely result of triggering more star formation. At the current time, this "surround \& squash" scenario predicts \al uniformly spread out through the cluster, which we detected with \itgu. The bubble gas should have X-ray temperatures which we found in the \rosat all sky survey. We have performed hydrodynamic simulation\hi{s} that reproduce key features of our observations. We suggest a refined scenario for the evolution of Sco-Cen: About 15-17~Myr ago star formation has started in an elongated, possibly flattened, gas cloud. Stars first formed in a particular part of the elongated cloud, now the UCL subgroup. A superbubble then cleared the immediate vicinity, but when it broke out, advancement of the supershell into the dense gas cloud almost stops, and a hot bubble forms around it. Hot gas also permeates the more tenuous parts of the dense cloud which are inside the superbubble, but leaves the denser parts \hi{in place and squashes them to higher densities}. \hi{This may have contributed to form the LCC subgroup.} Supernovae in UCL and LCC then suddenly increase the pressure within the superbubble after several Myr. This \hi{squashes} some gas in the elongated cloud, \hi{now} inside the superbubble, \hi{so that it} collapse\hi{s} gravitationally and form\hi{s} stars, thus producing the USco subgroup. Massive star winds in USco have then swept up the remaining parts of the cloud, leaving $\rho$ Ophiuchus and Lupus~I at opposite sides. We find kinematic and morphological evidence of shell instabilities, \rvb{as might be expected for} a cloud of swept-up gas. A recent supernova in USco could then have triggered star formation simultaneously in these two clouds. \hi{This scenario of hot gas surrounding and squashing the denser parts of clouds to induce the formation of stars probably makes the strongest prediction in the area of kinematics. The resulting star groups should be gravitationally unbound and subgroups should have coherent kinematics reflecting the local streaming velocity of the hot gas they were embedded in during the squashing. Since superbubbles can break out anisotropically from a gas distribution and have spatially and temporally varying pressure distributions (compare Fig.~\ref{fig:sim}), the kinematically coherent subgroups would be expected to move in different directions, not necessarily away from the next older part in the sequence of star formation. This is similar to the observed kinematics in Sco-Cen \citep{WM18}: The three subgroups are each unbound and have coherently moving sub-units. The velocity dispersion is so small that the groups cannot have formed on a much smaller spatial scale as bound clusters and expanded due to gas expulsion.} \hi{Sco-Cen OB2 is a typical OB association. Molecular cloud and star formation appear to occur frequently in dense sheets between superbubbles \citep[e.g.,][]{DawsonJea13,DawsonJea15,Inutsea15,Seifrea17}. A similar surround and squash mechanism might therefore operate frequently in the Milky Way and other star-forming galaxies. } The 'surround-and-squash' scenario \hi{might even apply to} AGN-related superbubbles affecting star formation \hi{on galaxy scale}. | 18 | 8 | 1808.04788 |
1808 | 1808.09725_arXiv.txt | We present an updated catalog of 4680 northern eclipsing binaries (EBs) with Algol-type light curve morphology (i.e., with well-defined beginning and end of primary and secondary eclipses), using data from the Catalina Sky Surveys. Our work includes revised period determinations, phenomenological parameters of the light curves, and system morphology classification based on machine learning techniques. While most of the new periods are in excellent agreement with those provided in the original Catalina catalogs, improved values are now available for $\sim 10\%$ of the stars. A total of 3456 EBs were classified as detached and 449 as semi-detached, while 145 cannot be classified unambiguously into either subtype. The majority of the SD systems seems to be comprised of short-period Algols. By applying color criteria, we searched for K- and M-type dwarfs in these data, and present a subsample of 609 EB candidates for further investigation. We report 119 EBs (2.5$\%$ of the total sample) that show maximum quadrature light variations over long timescales, with periods bracketing the range $4.5-18$~yrs and fractional luminosity variance of $0.04-0.13$. We discuss possible causes for this, making use of models of variable starspot activity in our interpretation of the results. | \label{sec:intro} Our thinking about eclipsing binary stars (EBs) has undergone a tremendous change in the last decade. EBs are one of nature's best laboratories for determining the fundamental physical properties of stars, and thus for testing the predictions of theoretical models (e.g., Torres et al. 2010; Catelan \& Smith 2015, and references therein). A large number of eclipsing Algol-type (EA) binaries, for which the beginning and end of eclipses are well defined, have been discovered recently as a by-product of several wide-field, ground-based photometric surveys, some of which are dedicated to the detection of variable stars. Among these surveys, one finds the Catalina Sky Survey (CSS, Larson et al. 2003), the Visible and Infrared Survey Telescope for Astronomy (VISTA) Variables in the Via Lactea (VVV, Minniti et al. 2010; Catelan et al. 2013), the asteroid survey LINEAR (Stokes et al. 2000; Palaversa et al. 2013), and the All Sky Automated Survey (ASAS, Pojmanski, 1997; Pojmanski et al. 2005), the Northern Sky Variability Survey (NSVS, Wo{\'z}niak et al. 2004), the Transatlantic Exoplanet Survey (TrES, Alonso et al. 2004; Alonso et al. 2007), the Optical Gravitational Lensing Experiment (OGLE, Udalski et al. 1992) survey, the Hungarian-made Automated Telescope Network exoplanet survey (HATNet, Bakos et al. 2004), and the Wide Angle Search for Planets (SuperWASP, Pollacco et al. 2006; Christian et al. 2006), among others \citep[see][for recent reviews and references]{gk17,is17}. On the basis of light curve (LC) morphology, EA-type eclipsing systems, with clearly defined eclipses on their LCs, include both (D) detached and semi-detached (SD) systems. As a rule, in order to establish the actual system configuration of any individual EB having such an Algol-type light curve morphology, a detailed physical modeling is required. The aforementioned projects are very useful to understand the photometric properties of the different types of binaries, affording for instance statistical studies of the properties of EA systems. In addition, large samples provide the opportunity for special cases of binaries that need dedicated follow-up observations to emerge, or even to reveal new classes (e.g., the Heartbeat stars, Welsh et al. 2011). In this work, we use the northern data from CSS (which continues collecting data to this day) in order to complete our search for detached EBs and to present an updated and more detailed catalog of their properties, in comparison with \citet{adea09,adea14a,adea14b,adea17}. The additional data allow not only to revise their periods and class but also to derive the phenomenological and physical parameters of selected detached systems. Furthermore, we were able to search for systems exhibiting long-term variation, or which may harbor low-mass components. This paper, the first of a series on the subject, is organized as follows. In Section~\ref{sec:Observations}, we briefly describe the CSS data that we use in our analysis. The construction of the sample and an outline of the analysis methods are explained in Section~\ref{sec:EA}. Estimates of the periods and morphological features, and a physical classification of the EA type, are given in Section~\ref{subsec:Classification}. In Sections~\ref{subsec:Long term variations} and \ref{sec:Low mass EBs}, we discuss all the results, followed by a brief summary of our work in Section~\ref{sec:conclusion}. | \label{sec:conclusion} Using CSS data covering a 12-yr timespan, we obtained an updated catalog of 4680 EA-type EBs, with revised period determination, phenomenological parameters of their light curves, and system morphology classification based on machine learning techniques. Our study includes many low-mass EB candidates, as well as systems that show additional variation in their maxima over long ($\sim 5-10$~yr) timescales. Most of the new periods are in excellent agreement with those provided in the original Catalina catalogs, but significantly revised values have been obtained for $\sim 10\%$ of the stars. A total of 3456 EBs were classified as D, 449 as SD, and 145 EBs had an uncertain classification. Our classification agrees with the findings of Lee (2015) for 83\% of the sources. The sample classified as SD contains $\sim 9\%$ systems with spectral types earlier than F0V, thus it seems that the majority of the systems in the sample are F-G spectral type EA systems with periods less than a day. These systems have been characterized as short period Algols \citep{2000MNRAS.315..587R} in the scenario of \cite{2006AcA....56..199S}. At the same time, they have also been described as being in near \citep{2001AJ....122.3436V, 2004PASP..116..931K} or broken \citep{2001AJ....122.3436V,2013AN....334..860A} contact. We again caution, as we did in the Introduction, that a detailed physical modeling of individual EBs is needed to reveal the true system configuration. Following our methodology of searching for K- and M-type dwarfs, we ended up with a sample of 609 low-mass EB candidates, increasing the total sample of stars at the low-mass end. Spectroscopic follow-up of these sources would be useful to help place constraints on models of low-mass stars. The majority of Lee's (2015) low-mass candidates are included in our sample, including four that have been verified as double-lined M dwarf EBs (Lee \& Lin 2017; Lee 2017). Moreover, we identified rare EA systems with periods close to the period cut-off at $P \sim 0.22$~d \citep{sr92,sr97}. In addition to these results, our analysis of the long-term trends in the CSS data revealed cyclic or quasi-cyclic modulation of the maximum brightness on long ($\sim 5-10$~yr) timescales for as many as 119 EA systems (2.5\% of the entire sample). The $\Delta L/L$ range is within $[0.04-0.13]$ with mean value $\langle\Delta L/L \rangle = 0.075\pm 0.017$, while the periods are in the range of $[4.5-18]$~yrs, with mean $P =12.1 \pm 3.3$~yrs. Recently, Marsh et al. (2017) reported similar behavior in 205 eclipsing W~UMa-type systems from CSS (2.2\% of the target sample), finding periods in the range $4-11$~yrs and fractional luminosity variance $\Delta L/L \approx 0.04-0.16$. Close binaries are known to be significantly more active than wide binaries and single stars (e.g., Shkolnik et al. 2010), most likely due to their being tidally locked and high rotational velocities, resulting in high levels of magnetic activity. This in late types is predicted to inflate their radii by inhibiting convective flow and increasing starspot coverage. The observed long-term variability can be explained by either the Applegate mechanism or by variable spot regions. Ol{\'a}h (2006) suggested that the magnetic field interaction has more effects on the starspot activities of the main-sequence stars than does the tidal force, because these stars have much higher surface gravities. As a consequence, the main-sequence stars often show active regions at quadrature phases. It should be noted that even though the vast majority of spotted stars cannot be easily imaged with special techniques (Doppler imaging or interferometry), our sample is useful to the future study of stellar activity cycles or other associated phenomena (e.g., flares). | 18 | 8 | 1808.09725 |
1808 | 1808.05217_arXiv.txt | The {\it Planck} Galactic Cold Clumps (PGCCs) are the possible representations of the initial conditions and the very early stages of star formation. With an objective to understand better the star and star cluster formation, we probe the molecular cloud associated with PGCC G108.37-01.06 (hereafter, PG108.3), which can be traced in a velocity range $-$57 to $-$51 km s$^{-1}$. The IPHAS images reveal H$\alpha$ emission at various locations around PG108.3, and optical spectroscopy of the bright sources in those zones of H$\alpha$ emission disclose two massive ionizing sources with spectral type O8-O9V and B1V. Using the radio continuum, we estimate ionizing gas parameters and find the dynamical ages of \hii regions associated with the massive stars in the range 0.5$-$0.75 Myr. Based on the stellar surface density map constructed from the deep near-infrared CHFT observations, we find two prominent star clusters in PG108.3; of which, the cluster associated with \hii region S148 is moderately massive ($\sim$ 240 M$\sun$). A careful inspection of JCMT $^{13}$CO(3$-$2) molecular data exhibits that the massive cluster is associated with a number of filamentary structures. Several embedded young stellar objects (YSOs) are also identified in the PG108.3 along the length and junction of filaments. We find the evidence of velocity gradient along the length of the filaments. Along with kinematics of the filaments and the distribution of ionized, molecular gas and YSOs, we suggest that the cluster formation is most likely due to the longitudinal collapse of the most massive filament in PG108.3. | \label{sec:intro} The formation of the star cluster is a topic of considerable interest since most stars in our Galaxy form in groups within clustered environments \citep[e.g.,][]{2003ARA&A..41...57L}. Several environmental conditions can breed young clusters such as: i) fragmentation of the swept up matter in the shells of the expanding \hii regions \citep[e.g.][]{1977ApJ...214..725E,2008ApJ...688.1142K,2014A&A...566A.122S,2016ApJ...818...95L}, ii) external compression of pre-existing clumps by nearby massive stars \citep[e.g.][]{1994A&A...289..559L,2011MNRAS.415.1202C,2013MNRAS.432.3445J,2015ApJ...799...64D}, iii) matter sandwiched between bubbles \citep[e.g.][]{2001ApJ...553L.185Y,2011ApJ...738..156O}, and iv) at the collision point of molecular clouds \citep[e.g.][]{2014ApJ...780...36F,2018PASJ..tmp....9W,2018PASJ..tmp...10H}, v) at the junction of converging filaments or hub of filamentary systems \citep[e.g.][]{2012A&A...540L..11S,2013ApJ...766..115K}. As molecular clouds are often comprised of \hii regions, bubbles, and dense filamentary structures \citep[][]{2009AJ....138..975B, 2013ApJ...779..113M, 2015A&A...581A...5S, 2016ApJS..226...13B, 2017ApJ...836...98J}, therefore, understanding what shapes the molecular clouds to dense and massive enough to form a young cluster is of great interest. Additionally, the initial cloud configuration decides the future location of cluster formation \citep[e.g.][]{2004ApJ...616..288B}. Therefore, the exact role of environment on the star and star cluster formation of a cloud can only be thoroughly understood by tracing various components of the interstellar medium (ISM) through multiwavelength observations. {\it Planck} is the third generation mission to measure the anisotropy of the cosmic microwave background (CMB) at nine frequency bands from 30 to 857 GHz, with beam sizes ranging from 33$\arcmin$ to 5$\arcmin$ \citep[][]{2011A&A...536A..23P}. As the high-frequency channels of {\it Planck} cover the peak thermal emission of dust colder than 14 K, therefore, {\it Planck} images probe the coldest parts of the ISM. In fact, using (sub)millimeter and millimeter wavelengths {\it Planck} have revealed an extremely cold population of dense molecular clumps, namely the {\it Planck} Galactic Cold Clumps \citep[PGCCs,][]{2016A&A...594A..28P}. The {\it Planck} Early Release Compact Source Catalogue \citep[ERCSC,][]{2011A&A...536A..13P} provides lists of positions and flux densities of compact sources at each of the nine {\it Planck} frequencies. The properties of PGCCs, however, are still not well known due to the lack of observations at high spatial resolution. Analysis of a part of the ERSSC suggests that depending upon distances, these cold sources trace a broad range of objects, from low-mass dense cores to giant molecular clouds \citep[][]{2018arXiv180503883Z}. High angular resolution {\it Herschel} and CO isotopologues follow-up observations have revealed that substantial fraction of such cold sources are filamentary \citep[][]{2012A&A...541A..12J,2017arXiv171109425J} and correspond to the very initial evolutionary stages of star formation \citep[][]{2012ApJ...756...76W,2013ApJS..209...37M,2016ApJS..224...43Z,2018ApJS..234...28L}. In particular, \citet[][]{2016A&A...591A.105Z} pointed out based on {\it Herschel} data, about 25\% of PGCCs near the Galactic mid-plane may be massive enough to form high-mass stars and star clusters. In this work, we examine the molecular cloud and stellar content associated with PGCC G108.37-01.06 (hereafter PG108.3, for details see section \ref{sec:overview}) located at {\it l} = 108\degr.3710, {\it b} = $-$1\degr.0649 ($\alpha_{2000}$ = 22$^{\rm h}$ 56$^{\rm m}$ 21$^{\rm s}$, $\delta_{2000}$ = +58\degr 30\arcmin 55\arcsec) using multiwavelength datasets. The large-scale CO morphology of the cloud is filamentary, it hosts a number of \hii regions \citep[][]{1985PASJ...37..345T}. Moreover, in the shallow near-infrared (NIR) 2MASS images, it appears to be a possible site of cluster formation. Therefore, PG108.3 is a good target to investigate the environmental effect on the formation of star and star clusters. Our aim is to use multiwavelength high-resolution observations to understand what dominates the star formation process in this planck cold clump. In this work, we made use of optical spectroscopic observations to identify the ionizing stars of the \hii regions. We studied ionized gas content and dynamical status of the \hii regions using radio continuum data. Using deep $H$, $K$ band data sets along with {\it Spitzer}-IRAC data, we identify and characterize the various young stellar objects (YSOs) and young clusters of the region. We examine the cold gas distributions and their kinematics using JCMT $^{13}$CO($3-2$) observations and probe their correlation with ionized gas, YSOs, and clusters. In the final step, we attempt to comprehend the star formation scenario of the complex. We organize our work as follows. In Section \ref{sec:overview} we present a brief overview of the PG108.3. Section \ref{sec:observations} describes our observational details. Section \ref{sec:results_of_ionized_gas} deals with various results on the ionized, stellar and young stellar, and gas component of the cloud. Section \ref{sec:understanding_of_star_and_cluster_formation} is devoted to our studies of star and cluster formation in the cloud based on the various results obtained in this analysis. We summarize our main findings in Section \ref{sec:conclusions}. \begin{figure*} \centering \includegraphics[height=11.0cm, width=13.0cm]{fig01.eps} \caption{{\it Planck} 857 GHz view of the PG108.3. The simbad positions of three \hii regions are marked. The contour levels of background image are at 60.0, 80.0, 100.0, 130.0, 160.0 MJy sr$^{-1}$. Our FOV ($\sim$ 23$\arcmin$ $\times$ 19$\arcmin$) in CFHT WIRCam is shown in black box, whereas green box is the region covered by {\it Spitzer}-IRAC. See text for details.} \label{fig:planck} \end{figure*} | \label{sec:conclusions} In this paper, we carried out an extensive study using multi-wavelength datasets to explore the physical conditions of the molecular cloud and star formation activity around PG108.3 cloud complex. Our analyses focused on the massive ionizing sources and ionized gas, associated young clusters, molecular gas, filamentary structure, and embedded young stellar population. We summarize the important findings of the present analyses as follows: \begin{itemize} \item From our optical spectroscopic analysis, we identified that \hii regions associated with the PG108.3 cloud harbor at least two massive ionizing sources. The S148 \hii region is powered by the star S01 (O9 V), whereas S149 is from the star S02 (B1 V). \item Two compact radio continuum sources (i.e., S01 and S02) are traced with the NVSS 1.4 GHz data, and thus our analyses reveal the \hii regions are excited by corresponding massive sources. The dynamical age of S148 was estimated to be 0.5$-$0.75 Myr. \item The stellar surface density map reveals that at least two prominent subclusters (`A' and `B') in the vicinity of the PG108.3 cloud complex. The $K$-band luminosity function of S148 resembles that of Trapezium cluster. The total mass of S148 cluster is estimated to be $\sim$ 240 M$\sun$. The cluster follows the size-mass linear relation like other nearby clusters given in \citet[][]{2003ARA&A..41...57L}. \item Using the {\it Spitzer}-IRAC and deep NIR observations, we identified 111 candidate YSOs, which include 16 Class I, 39 Class II. Our YSO estimation is sensitive to $\sim$ 0.2 M$\sun$. \item The molecular cloud is depicted using JCMT $^{13}$CO(3$-$2) observations in the velocity range $-$57 to $-$51 km s$^{-1}$. A careful inspection of the molecular line data shows that the peak velocity of molecular gas around cluster A and cluster B (around S148) are at $-$55.2 km s$^{-1}$ and -52.7 km s$^{-1}$. The molecular data reveals the filament F1, where we found a gas motion from its southern end to S148 \hii regions via junction head at subcluster `A'. By comparing the distribution of ionized, molecular, and young stellar content, our findings do not point any triggering star formation at the \hii region boundaries, we rather suggest that the cluster formation is most likely due to the longitudinal collapse of the filament to the center of the potential well. \end{itemize} | 18 | 8 | 1808.05217 |
1808 | 1808.09995.txt | We investigate the N{\sevensize V} absorption signal along the line of sight of background quasars, in order to test the robustness of the use of this ion as criterion to select intrinsic (i.e. physically related to the quasar host galaxy) narrow absorption lines (NALs). We build composite spectra from a sample of $\sim$ 1000 C{\sevensize IV} absorbers, covering the redshift range 2.55 < z < 4.73, identified in 100 individual sight lines from the XQ-100 Legacy Survey. We detect a statistical significant N{\sevensize V} absorption signal only within 5000 km s$^{-1}$ of the systemic redshift, z$\rm_{em}$. This absorption trough is $\sim$ 15$\sigma$ when only C{\sevensize IV} systems with N(C{\sevensize IV}) > 10$^{14}$ cm$^{-2}$ are included in the composite spectrum. This result confirms that N{\sevensize V} offers an excellent statistical tool to identify intrinsic systems. We exploit the stacks of 11 different ions to show that the gas in proximity to a quasar exhibits a considerably different ionization state with respect to gas in the transverse direction and intervening gas at large velocity separations from the continuum source. Indeed, we find a dearth of cool gas, as traced by low-ionization species and in particular by Mg{\sevensize II}, in the proximity of the quasar. We compare our findings with the predictions given by a range of Cloudy ionization models and find that they can be naturally explained by ionization effects of the quasar. | Quasar outflows have been increasingly invoked by popular evolution models to regulate both star formation in host galaxies and the accretion of material on to central supermassive black holes (SMBH; e.g. \citealp{Granato2004, DiMatteo2005, Hopkins2016, Weinberger2017}). A SMBH at the centre of a galaxy can produce a large amount of energy ($\sim$ 10$^{62}$ erg). Even if just a few per cent of the quasar bolometric luminosity were injected into the interstellar medium (ISM) of the host galaxy, it could have a significant impact on the host galaxy evolution \citep{Scannapieco2004, Prochaska2009, Hopkins2010}. Such feedback offers a natural explanation for the observed mass correlation between SMBHs and their host galaxy spheroids (e.g., \citealp{King2003, McConnell2013}). However, the actual mechanisms of feedback remain highly uncertain. Outflows are often studied in the rest-frame ultraviolet (UV) via blueshifted narrow absorption lines (NALs) defined by full width at half maximum (FWHM) $\lesssim$ 300 km s$^{-1}$. These absorbers are ubiquitously found and detected in all active galactic nuclei (AGN) subclasses (e.g., \citealp{Crenshaw1999, Ganguly2001, Vestergaard2003, Misawa2007}). The limitation of NALs is that they arise from a wide range of environments, from high-speed outflows to halo gas to physically unrelated gas or galaxies at large distances from the quasar. Those forming in the proximity of quasars provide valuable tools to study the gaseous environment of quasar host galaxies (e.g. \citealp{Dodorico2004, Wu2010, Berg2018}). With the aim of investigating quasar winds, we must select NALs that truly trace outflowing gas cautiously. The outflow/intrinsic origin of individual NALs can be inferred from: i) time variability of line profiles, ii) absorption profiles significantly broader and smoother compared with the thermal line widths, iii) high space densities measured directly from excited-state fine-structure lines, iv) partial coverage of the background emission source measured via resolved, optically thick lines with too-shallow absorption troughs, v) higher ionization states than intervening absorbers. Nonetheless, NALs can still be connected to the quasar host galaxy without exhibiting such properties \citep{Hamann1997}. Previous works have shown that, among the highly-ionized absorption species commonly used to identify outflows (e.g. C{\sevensize IV}$\lambda \lambda$1548,1550; Si{\sevensize IV}$\lambda \lambda$1393,1402; N{\sevensize V}$\lambda \lambda$1238,1242; O{\sevensize VI}$\lambda \lambda$1031,1037), N{\sevensize V} is detected with the lowest frequency, but with the highest intrinsic fraction. \citet{Misawa2007} derived an intrinsic fraction of 75 per cent for N{\sevensize V} NALs, through partial coverage and line locking methods. \citet{Ganguly2013}, using the same techniques, found a value of 29-56 per cent and suggested using this ion in building large catalogs of intrinsic NALs with lower resolution and/or lower signal-to-noise ratio (S/N) data. In \citeauthor{Perrotta2016} (\citeyear{Perrotta2016}, hereafter P16) we used the spectra of 100 quasars at emission redshift z$\rm _{em}$ = 3.5 - 4.72 to build a large, relatively unbiased, sample of NALs and study their physical properties statistically. The spectra have been obtained with the echelle spectrograph X-shooter \citep{Vernet2011} on the European Southern Observatory (ESO) Very Large Telescope (VLT) in the context of the XQ-100 Legacy Survey \citep{Lopez2016}. P16 showed that N{\sevensize V} is a key line to study the effects of the quasar ionization field, offering an excellent statistical tool for identifying outflow/intrinsic candidate NALs. Indeed, most of the N{\sevensize V} systems in our sample exhibit distinctive signatures of their intrinsic nature with respect to intervening NALs (described above), and N{\sevensize V}/C{\sevensize IV} column density ratios larger than 1 (see \citealp{FR09}). The large number of Ly$\alpha$ lines characterizing the forest in the spectra of quasars at z$\rm_{em}$ = 3.5 - 4.72 prevents us in P16 from searching for individual N{\sevensize V} lines at large velocity offsets (we could reliably identify N{\sevensize V} only within 5000 km s$^{-1}$ of z$\rm_{em}$). Most of the Ly$\alpha$ forest is associated with moderate overdensities and traces filamentary structure on large scales, but some strong forest absorbers along with Lyman-limit systems (LLSs, N(H{\sevensize I}) $\geq$ 10$^{17.2}$ cm$^{-2}$) and damped Lyman alpha absorptions (DLAs, N(H{\sevensize I}) $\geq$ 10$^{20.3}$ cm$^{-2}$), are thought to be associated with galaxies and the circumgalactic medium (CGM; e.g. \citealp{Faucher2011, Fumagalli2011}). Therefore, intervening metals associated with Ly$\alpha$ absorbers can probe different environments: from the ISM to the outer regions of galaxies (the CGM) far from the quasar to the more diffuse intergalactic medium (IGM). In this work, we apply the stacking technique to the XQ-100 spectra to look for N{\sevensize V} at large velocity separations from z$\rm_{em}$. Indeed, when an ensemble of independent sight lines is stacked to produce a composite spectrum, the numerous stochastic H{\sevensize I} absorptions average together and the resulting spectrum is flat, revealing any strong metal signal buried within the Ly$\alpha$ forest. These measurements will complement our previous study on N{\sevensize V} and test the robustness of our claim on the use of N{\sevensize V} as a criterion to select intrinsic NALs. The work is organized as follows: Section~\ref{civ_sample} describes our methodology for identifying NALs; Section~\ref{stacking} collects the details of the procedure followed to build the stacking spectra. Our results are presented in Section~\ref{results} and Section~\ref{discussion} discusses the original findings of our study. Our conclusions are summarized in Section~\ref{conclusions}. We adopt a $\Lambda$CDM cosmology throughout this manuscript, with $\rm \Omega_M$ = 0.315, $\Omega_{\Lambda}$ = 0.685, and H$_0$ = 67.3 km s$^{-1}$ Mpc$^{-1}$ \citep{planck14}. | \label{conclusions} In this work, we present new measurements of metal NALs made using composite quasar spectra. Our sample includes 100 individual lines of sight from the XQ-100 Legacy survey, at emission redshift z$\rm_{em}$ = 3.5-4.72. To build the stacking spectra, we start from a large number ($\sim$ 1000) of C{\sevensize IV} absorption systems identified in P16, covering the redshift range 2.55 < z$\rm_{abs}$ < 4.73. The main goal of our analysis is to investigate the N{\sevensize V} absorption signal at large velocity separations from z$\rm_{em}$. This study complement our previous work on N{\sevensize V} and test the robustness of the use of N{\sevensize V} as a criterion to select intrinsic NALs. We also characterize the ionization state of the gas, both near and at great distance from the quasar. Our primary results are as follows. (1) We show the absence of a statistically significant intervening N{\sevensize V} absorption signal along the line of sight of background quasars. Indeed, the N{\sevensize V} exhibits a strong absorption trough only within 5000 km s$^{-1}$ of z$\rm_{em}$. This feature is much stronger ($\sim$ 15$\sigma$ confidence level) if C{\sevensize IV} systems with N(C{\sevensize IV}) > 10$^{14}$ cm$^{-2}$ are considered to build the composite spectrum. This result supports our previous claim (see P16) that N{\sevensize V} is an excellent statistical tool for identifying intrinsic systems. (2) We use photoionization models to show that, in a scenario where associated systems are photoionized by a quasar, the ionization parameter is expected to drop dramatically with distance from the continuum source. This is the main reason for intervening N{\sevensize V} systems being weaker than associated ones. Another possible contribution to this trend is a lower metallicity or N/O abundance. ( 3) The gas close to the continuum source (v$\rm_{abs}$ < 5000 km s$^{-1}$) exhibits a different ionization state with respect to the intervening one. Moving farther from the quasar, we can appreciate the appearance of the low-ions associated with the strongest systems in the sample (N(C{\sevensize IV}) > 10$^{14}$ cm$^{-2}$) and the drop in the N{\sevensize V} absorption signal. In contrast, the O{\sevensize VI} NAL is detected even at large velocity shifts from z$\rm_{em}$. We also note that O{\sevensize VI} and C{\sevensize IV} do not exhibit a change in their absorption signal in line with the decrease of the N{\sevensize V} signal. We run photoionization models that describe these trends well (see Fig.\ref{fig:cloudy_R}). We use these models to illustrate that a drop in the U parameter cannot be the only element to explain the data and that an additional thermal component (T $\sim$ 3 $\times$ 10$^5$ K) can produce the O{\sevensize VI} absorption we detect at large distance from the quasar. We show how it is possible that N{\sevensize V} is photoionized in environments where O{\sevensize VI} is mostly collisionally ionized, but, being weak, this can be challenging to detect. (4) We verify the deficiency of cool gas, as traced by low ions and in particular by Mg{\sevensize II}, in the proximity of the quasar. This finding is in agreement with the dearth of CII NALs (as shown in P16) and confirms that the gas in the proximity of the quasar along the line of sight has a different ionization state with respect to the gas in the transverse direction (e.g. \citealp{Prochaska2014, Farina2014}). | 18 | 8 | 1808.09995 |
1808 | 1808.02437_arXiv.txt | {The increase in discovered close binary central stars of planetary nebulae is leading to a sufficiently large sample to begin to make broader conclusions about the effect of close binary stars on common envelope evolution and planetary nebula formation. Herein I review some of the recent results and conclusions specifically relating close binary central stars to nebular shaping, common envelope evolution off the red giant branch, and the total binary fraction and double degenerate fraction of central stars. Finally, I use parameters of known binary central stars to explore the relationship between the proto-planetary nebula and planetary nebula stages, demonstrating that the known proto-planetary nebulae are not the precursors of planetary nebulae with close binary central~stars.} \keyword{\textls[-15]{planetary nebulae; stars: binaries; central stars of planetary nebulae; proto-planetary nebulae}} \begin{document} \setcounter{section}{0} % | The study of binary stars in planetary nebulae (PNe) has the potential to provide information about the formation of PNe and the evolution of the central stars (CSs). A number of different surveys, using different methods, have identified binary central stars of planetary nebulae (CSPNe). Photometric surveys looking for variability due to a companion tend to find close binary systems with orbital periods of about a week or less (e.g., \cite{bon90,mis09,dem15}). Such photometric surveys are sensitive to both cool companions and double degenerate systems \cite{hil11,san11}. Infrared surveys are designed to be sensitive to cool companions at all orbital periods \cite{dou15}, and radial velocity surveys are sensitive to stellar mass companions in orbital periods of up to several years \cite{dem04,jon17}. Once binary CSPNe are identified, follow-up work can determine system parameters. A number of such systems have recently been studied, with various sets of orbital and stellar parameters published (see the updated list of known close binary CSPNe maintained by David Jones at \url{http://drdjones.net/?q=bCSPN}). Because it is more difficult to determine parameters for binaries with long periods, the majority of binary CSPNe with known physical parameters are close binaries. Of the close binary CSPNe, the large majority have been discovered using photometry. In addition to providing discovery data for binaries, the light curves of those binaries can also tell us a great deal about the nature of the binary system. For example, close binary CSPNe with a main sequence companion have light curves dominated by an irradiation effect in which the inner hemisphere of the cool star is irradiated and heated by the hot CS. This behavior results in a nearly sinusoidal light curve with one maximum and one minimum per orbit. However, if the companion to the hot CS is a compact object (e.g., a white dwarf, WD), then any detected variability will likely be through eclipses or, more likely, ellipsoidal variability that is due to one of the stars nearly filling its Roche lobe and thus being elongated by the mutual gravity of the two stars. Typically, it will be the CS that nearly fills its Roche lobe, since these objects can still be large, not having contracted yet to the WD cooling track. As the CS contracts, the ellipsoidal variability decreases in amplitude, eventually becoming unobservable. A light curve dominated by ellipsoidal variability has two maxima and two minima per orbit due to the different projected surface areas (brighter when seen edge-on, fainter when the two stars are aligned along the line of sight). To date, all well-studied close binary CSPNe with light curves dominated by ellipsoidal variability have been shown to be double degenerate (DD) systems. Technically, the CS is typically not yet fully degenerate, but we use this terminology here to provide a better comparison with true DD systems, which these will become. They are also related to core degenerate (CD) systems \cite{kas11}, though in this case the common envelope (CE) has detached and left a close binary system. With the growing number of discovered binary CSPNe, we are beginning to reach a point at which statistically relevant statements can be made about how binary companions may influence both the ejection of the nebula and the evolution of the central star. Below I review some conclusions that have already been discussed in the literature, along with several connections that are currently being explored but still need confirmation. I then discuss what our current knowledge of close binary CSPNe can tell us about the evolution of PNe with close binary nuclei, especially in the context of proto-planetary nebulae (PPNe). | I have summarized some of the recent conclusions about the relationship between close binary CSs and their PNe. These conclusions are possible due to the increase in the number of discovered close binary CSPNe and by the increase in the number of systems with full binary modeling. I also demonstrated that the PPN-to-PN connection is not as clear as it may seem. The observed sample of PPNe are \emph{not} progenitors of PNe with close binary stars. In addition, close binary CSPNe seem to alter the post-AGB evolution of the core of the evolving star, with possible implications for CE evolution in addition to PN shaping and close binary evolution. \vspace{6pt} \funding{This material is based upon work supported by the National Science Foundation under Grant No. AST-1109683. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.} \conflictsofinterest{The author declares no conflict of interest.} \reftitle{References} | 18 | 8 | 1808.02437 |
1808 | 1808.00742_arXiv.txt | Molecules with an amide functional group resemble peptide bonds, the molecular bridges that connect amino acids, and may thus be relevant in processes that lead to the formation of life. In this study, the solid state formation of some of the smallest amides is investigated in the laboratory. To this end, CH$_{4}$:HNCO ice mixtures at 20~K are irradiated with far-UV photons, where the radiation is used as a tool to produce the radicals required for the formation of the amides. Products are identified and investigated with infrared spectroscopy and temperature programmed desorption mass spectrometry. The laboratory data show that NH$_{2}$CHO, CH$_{3}$NCO, NH$_{2}$C(O)NH$_{2}$, CH$_{3}$C(O)NH$_{2}$ and CH$_{3}$NH$_{2}$ can simultaneously be formed. The NH$_{2}$CO radical is found to be key in the formation of larger amides. In parallel, ALMA observations towards the low-mass protostar IRAS~16293--2422B are analysed in search of CH$_{3}$NHCHO (N-methylformamide) and CH$_{3}$C(O)NH$_{2}$ (acetamide). CH$_{3}$C(O)NH$_{2}$ is tentatively detected towards IRAS~16293--2422B at an abundance comparable with those found towards high-mass sources. The combined laboratory and observational data indicates that NH$_{2}$CHO and CH$_{3}$C(O)NH$_{2}$ are chemically linked and form in the ice mantles of interstellar dust grains. A solid-state reaction network for the formation of these amides is proposed. | \label{sec.int} \begin{figure*} \begin{center} \includegraphics[width=\hsize]{peptides.pdf} \caption{The reaction between the acid and base groups of two glycine molecules (1) forms a peptide bonded molecular chain (2). The peptide bond shows similarities to the smallest amide formamide (3), but the larger peptide chain incorporates structures that are similar to acetamide (4), N-methylformamide (5) and methyl isocyanate (6). Note that carbon atoms are not indicated, except for terminal groups.} \label{fig.pep_bond} \end{center} \end{figure*} Prebiotic molecules are species that resemble functional groups of biogenic molecules and are thought to be involved in the formation of molecules that are relevant to life, such as amino acids, nucleobases and sugars \citep{herbstdishoeck2009,caselliceccarelli2012}. The interstellar presence of prebiotic molecules supports the idea that the building blocks of life may have an extraterrestrial origin. A number of these molecules have been detected in the InterStellar Medium (ISM), such as the simplest ``sugar'' glycolaldehyde \citep[CH(O)CH$_{2}$OH,][]{hollis2004,jorgensen2012,jorgensen2016} and precursor molecules to the amino acid glycine, such as methylamine \citep[CH$_{3}$NH$_{2}$,][]{kaifu1974} and aminoacetonitril \citep[NH$_{2}$CH$_{2}$CN,][]{belloche2008}. Among prebiotics, molecules with an amide (--NH--C(O)--) or amide-like structure, such as isocyanic acid (HNCO), hereafter generally called amides, are of particular interest because they resemble a peptide bond, see Fig \ref{fig.pep_bond}. In terrestrial biochemistry amino acids are connected by peptide bonds resulting in long chains which eventually form proteins, the engines of life. Reactions involving molecules with an amide functional group offer alternative pathways to form peptide chains. Amides are widespread throughout the ISM. HNCO and formamide (NH$_{2}$CHO) are the most abundant ones and have been detected in a large variety of interstellar sources \citep[e.g.][]{bisschop2007,kahane2013,adande2013,corby2015,bergner2017} and comets, including 67P/Churyumov-Gerasimenko \citep[67P/C-G,][]{bockelee-morvan2000,goesmann2015,altwegg2017}. Observational evidence exists for a chemical relationship between HNCO and NH$_{2}$CHO, which is thought to originate in interstellar ice \citep{bisschop2007,lopez-sepulcre2015,coutens2016}. In the form of the OCN$^{-}$ anion, HNCO has been directly detected in interstellar ices at abundances as high as 2\% with respect to water \citep{lacy1984,gibb2004,vanbroekhuizen2005}. Tentatively, the presence of formamide in interstellar ice has been claimed towards NGC 7538 IRS9 \citep{raunier2004}. The more complex molecule acetamide (CH$_{3}$C(O)NH$_{2}$) has been detected towards Sagittarius B2 (Sgr B2) and Orion KL \citep{hollis2006,cernicharo2016,belloche2017} and on 67P/C-G \citep{goesmann2015,altwegg2017}. Formation of this molecule has been linked to that of formamide \citep{halfen2011}, although it is inconclusive whether gas-phase or solid-state chemistry is involved \citep[see also][]{quan2007}. Methyl isocyanate (CH$_{3}$NCO) has been detected towards Sgr B2 and Orion KL \citep{halfen2015,cernicharo2016} and recently towards the sun-like protostar IRAS 16293--2422 \citep{ligterink2017a,martin-domenech2017}. Its formational origin is likely found in interstellar ices, although some non-negligible gas-phase production routes are available \citep{quenard2018}. Hydrogenation of CH$_{3}$NCO is hypothesised to lead to N-methylformamide (CH$_{3}$NHCHO), a molecule that has tentatively been detected towards Sgr B2 \citep{belloche2017}. Carbamide, also known as urea (NH$_{2}$C(O)NH$_{2}$), has tentatively been identified towards Sgr B2 as well \citep{remijan2014}. Finally, cyanamide (NH$_{2}$CN) has been observed towards various galactic and extragalactic sources \citep[e.g.][]{turner1975,martin2006,coutens2017}. The high interstellar abundances of HNCO and NH$_{2}$CHO have resulted in many solid-state laboratory studies with the aim to understand their formation \citep{hagen1979,gerakines2004,raunier2004,jones2011,islam2014,munozcaro2014,fedoseev2015,noble2015,fedoseev2016,kanuchova2016,fedoseev2018}. In these studies, ice mixtures containing a source of carbon, like CH$_{3}$OH or CO, and a source of nitrogen, such as HCN, HNCO, NH$_{3}$, N$_{2}$ or NO are hydrogenated and/or energetically processed. A number of mechanisms have been shown to produce these species, such as the NH + CO reaction to produce HNCO \citep{fedoseev2015} and the NH$_{2}$ + CHO radical combination to produce NH$_{2}$CHO \citep{jones2011}. \citet{raunier2004} proposed that HNCO can be hydrogenated to NH$_{2}$CHO by hot H-atom addition (i.e. hydrogen atoms produced by energetic dissociation processes that carry enough excess energy to overcome reaction barriers). On the other hand, \citet{noble2015} showed that hydrogenation of HNCO with ``cold'' ($\sim$300~K) hydrogen atoms produced in a beam line does not result in the formation of NH$_{2}$CHO. \\ The larger amides NH$_{2}$C(O)NH$_{2}$ and CH$_{3}$C(O)NH$_{2}$ have been produced in various ice experiments \citep[e.g.][]{berger1961,agarwal1985,bernstein1995,raunier2004,henderson2015,forstel2016}, but formation mechanisms have not been extensively investigated. Some reactions have been proposed, such as the NH$_{2}$ + NH$_{2}$CO radical addition to form NH$_{2}$C(O)NH$_{2}$ \citep{agarwal1985,raunier2004}. Modelling investigations have predicted the formation of CH$_{3}$C(O)NH$_{2}$ through the CH$_{3}$ + HNCO reaction followed by hydrogenation \citep{garrod2008} or the hydrogen abstraction of NH$_{2}$CHO followed by CH$_{3}$ addition \citep{belloche2017}. The formation of CH$_{3}$NHCHO has been claimed in far-UV (also known as vacuum-UV or V-UV) irradiated CH$_{3}$NH$_{2}$:CO ice mixtures through the reaction CH$_{3}$NH + CHO \citep{bossa2012}, while modelling investigations have shown that hydrogenation of CH$_{3}$NCO is one of the main channels of CH$_{3}$NHCHO formation \citep{belloche2017}. Recently, the solid-state reaction CH$_{3}$ + (H)NCO was proposed as the most likely candidate to explain the formation of CH$_{3}$NCO \citep{ligterink2017a}. Other solid-state pathways, such as formation via a HCN$^{...}$CO van der Waals complex, have also been proposed as relevant pathways in modelling studies \citep{majumdar2018}. The aim of this work is to elucidate the chemical network that links various small amides that have been detected in the ISM and explain their formation. This work is complementary to that of \citet{ligterink2017a} on CH$_{3}$NCO and investigates reactions that can occur simultaneously with the formation of this molecule. Ice mixtures of CH$_{4}$:HNCO, two astronomically relevant precursor species, are irradiated with far-UV radiation. The far-UV radiation is used as a tool to form radicals, which engage in recombination reaction to form amides, amines and other molecules. On interstellar dust grains these radicals could be formed by far-UV photodissociation, but also non-energetically by hydrogenation of atomic carbon, oxygen and nitrogen. This paper is organised in the following way. Section \ref{sec.lab} discusses the laboratory set-up and measurement protocol. The results of the experiments are presented in Sec. \ref{sec.res}. Observations and the comparison between laboratory and observational results are presented in Sec. \ref{sec.obs}, followed by the discussion in Sec. \ref{sec.dis}. The conclusions of this work are presented in Sec. \ref{sec.con}. \begin{table} \caption[]{Overview of performed far-UV irradiation experiments on ice mixtures.} $$ \begin{tabular}{l l l l l l} \hline \hline \noalign{\smallskip} Exp. & $N$(HNCO) & $N$(CH$_{4}$) & $N$(CO)$^{a}$ & Lyman-$\alpha$ \\ & \multicolumn{3}{c}{ML (10$^{15}$ molecules cm$^{-2}$)} & High/Low\\ \noalign{\smallskip} \hline \noalign{\smallskip} 1 & 14.3 & 17.0 & -- & H \\ 2 & 17.4 & - & -- & H \\ 3 & 15.1 & 15.7$^{b}$ & -- & H \\ \noalign{\smallskip} \hline \noalign{\smallskip} 4 & 29.9 & 5.5 & -- & H \\ 5 & 11.4 & 24.6 & -- & L \\ \hline 6 & 2.9 & 8.5 & 95.6 & L & \\ 7 & 4.8 & 13.9 & 164.2 & L & \\ \noalign{\smallskip} \hline \end{tabular} $$ \emph{\rm Notes. $^{a}$Total $^{12+13}$CO column density calculated from the $^{13}$CO band multiplied by 91. $^{b}$Experiment using $^{13}$CH$_{4}$, the bandstrength value of 1.1$\times$10$^{-17}$ cm molecule$^{-1}$ is assumed to apply to the $^{13}$CH$_{4}$ degenerate stretching mode as well. Other bandstrength values are found in Table~\ref{tab.IRfeat}.} \label{tab.exper} \end{table} \begin{table*} \caption[]{Peak positions and transmission bandstrengths of precursor and product species} \label{tab.IR_param} \begin{tabular}{l l l l l l} \hline \hline \noalign{\smallskip} Species & Name & band & \multicolumn{2}{c}{Peak position} & Bandstrength \\ & & & \multicolumn{2}{c}{(cm$^{-1}$)} & cm molecule$^{-1}$ \\ & & & Literature & Experiment* & Transmission \\ \noalign{\smallskip} \hline \noalign{\smallskip} HNCO (water poor) & Isocyanic acid & OCN str.$^{a}$ & 2260 & 2266 & 7.8$\times$10$^{-17}$ \\ CH$_{4}$ & Methane & d-str.$^{b}$ & 1301 & 1302 & 7.3$\times$10$^{-18}$ \\ CH$_{4}$ & Methane & d-str.$^{b}$ & 3010 & 3010 & 1.1$\times$10$^{-17}$ \\ CO & Carbon monoxide & CO str.$^{c}$ & 2138 & 2142 & 1.1$\times$10$^{-17}$ \\ \noalign{\smallskip} \hline \noalign{\smallskip} OCN$^{-}$ (water poor) & Cyanate anion & OCN str.$^{a}$ & 2160 & 2170 & 1.3$\times$10$^{-16}$ \\ CO$_{2}$ & Carbon dioxide & CO a-str.$^{c}$ & 2342 & 2341 & 7.6$\times$10$^{-17}$ \\ HCN & Hydrogen cyanide & CN str.$^{d}$** & 2099 & 2108 & -- \\ CH$_{3}$NCO & Methyl isocyanate & NCO a-str.$^{e}$** & 2322 & 2322 & -- \\ CH$_{3}$CH$_{3}$ & Ethane & CH$_{3}$ d-str.$^{f}$ & 2975 & 2976 & 6.5$\times$10$^{-18}$ \\ NH$_{4}^{+}$ & Ammonium cation & deform.$^{a}$ & 1485 & 1466 & 4.6$\times$10$^{-17}$ \\ NH$_{2}$CHO & Formamide & CO str.$^{g,h}$ & 1700 & $\sim$1687 & 3.3$\times$10$^{-17}$ \\ NH$_{2}$CONH$_{2}$ & Carbamide & CO str.$^{h}$ & 1590 & -- & -- \\ NH$_{2}$CONH$_{2}$ & Carbamide & NH sym. bend$^{h}$ & 1675 & $\sim$1687 & -- \\ NH$_{2}$CONH$_{2}$ & Carbamide & NH asym. bend$^{h}$ & 1630 & $\sim$1687 & -- \\ \hline \end{tabular} \emph{\rm Notes. *Peak positions found for experiment 1 (see Table~\ref{tab.exper}); **Indicates IR data obtained from reflection experiments. $^{a}$\citet{vanbroekhuizen2004}; $^{b}$\citet{hudgins1993,boogert1997}; $^{c}$\citet{bouilloud2015}; $^{d}$\citet{gerakines2004}; $^{e}$\citet{ligterink2017a}; $^{f}$\citet{gerakines1996}; $^{g}$\citet{wexler1967}; $^{h}$\citet{raunier2004}} \label{tab.IRfeat} \end{table*} \begin{figure} \begin{center} \includegraphics[width=\hsize]{UV_comp.pdf} \caption{Far-UV spectrum of the MDHL emission between 115 and 170 nm. The top spectrum (red) shows lamp emission poor in Lyman-$\alpha$, while the bottom spectrum (black) shows lamp emission rich in Lyman-$\alpha$.} \label{fig.VUV_comp} \end{center} \end{figure} \begin{figure} \begin{center} \includegraphics[width=\hsize]{Tdes_plot.pdf} \caption{TPD traces of $m/z$ 59 of pure CH$_{3}$NHCHO (black) and pure CH$_{3}$C(O)NH$_{2}$ (red). Note that CH$_{3}$C(O)NH$_{2}$ has a double desorption peak with the first peak potentially caused by a phase change in the ice.} \label{fig.Tdes} \end{center} \end{figure} | \label{sec.con} This work has investigated the solid-state formation of amides and amines in far-UV irradiated CH$_{4}$:HNCO ice mixtures. Observations towards the low-mass sun-like protostar IRAS~16293--2422B were analysed in search of CH$_{3}$NHCHO and CH$_{3}$C(O)NH$_{2}$. The conclusions of this work are summarised as follows: \begin{itemize} \item Acetamide (CH$_{3}$C(O)NH$_{2}$), formamide (NH$_{2}$CHO), carbamide (NH$_{2}$CONH$_{2}$), methyl isocyanate (CH$_{3}$NCO) and methylamine (CH$_{3}$NH$_{2}$) are shown to simultaneously form in the solid-state, under conditions mimicking those of ice mantels on interstellar dust grains. \item The carbamoyl radical (NH$_{2}$CO) is identified as an important reaction intermediate in the formation of amides, specifically CH$_{3}$C(O)NH$_{2}$ and NH$_{2}$C(O)NH$_{2}$. \item CH$_{3}$C(O)NH$_{2}$ is tentatively identified in ALMA observations towards the low-mass sun-like protostar IRAS~16293--2422B at column densities of 9$\times$10$^{14}$ cm$^{-2}$ and 25$\times$10$^{14}$ cm$^{-2}$ for $T_{\rm ex}$ = 100 and 300~K, respectively. The resulting ratio of [CH$_{3}$C(O)NH$_{2}$] / [NH$_{2}$CHO] = 0.09--0.25 matches with abundances found towards high-mass YSOs. CH$_{3}$NHCHO is not detected down to an upper limit column density of $\leq$1$\times$10$^{15}$ cm$^{-2}$. \item A comparison between experimental TPD data and observations of CH$_{3}$C(O)NH$_{2}$ and NH$_{2}$CHO, indicates that solid-state chemistry on interstellar dust grains likely is responsible for the formation of these two species. \item A solid-state reaction network for the smallest generations of amides and amide-like molecules is proposed, focussing on reactions between N, NH and NH$_{2}$ radicals with CO and CHO, followed by CH$_{3}$ addition. \end{itemize} | 18 | 8 | 1808.00742 |
1808 | 1808.02571_arXiv.txt | The integral field spectrograph configuration of the LMIRCam science camera within the Large Binocular Telescope Interferometer (LBTI) facilitates 2 to 5~$\mu$m spectroscopy of directly imaged gas-giant exoplanets. The mode, dubbed ALES, comprises magnification optics, a lenslet array, and direct-vision prisms, all of which are included within filter wheels in LMIRCam. Our observing approach includes manual adjustments to filter wheel positions to optimize alignment, on/off nodding to track sky-background variations, and wavelength calibration using narrow band filters in series with ALES optics. For planets with separations outside our 1''x1'' field of view, we use a three-point nod pattern to visit the primary, secondary and sky. To minimize overheads we select the longest exposure times and nod periods given observing conditions, especially sky brightness and variability. Using this strategy we collected several datasets of low-mass companions to nearby stars. | \label{sec:intro} % At low temperatures gas-giant atmospheres transition from relatively warm, cloudy, and CO-rich (L-types) to relatively cool, and cloud free with CH$_4$ not CO as the dominant carrier of atmospheric carbon (T-types). This transition is not completely understood especially because it is so abrupt and dramatic. For example, the field brown dwarf population transition beginning at $T_{\mathrm{eff}}$$\sim1300$~K and ending by $T_{\mathrm{eff}}\sim1100$~K (e.g., Ref. \citenum{Kirkpatrick2005}). The temperature of the L-to-T transition is a function of gravity, with low-gravity objects observed to loft photospheric clouds and maintain atmospheric CO down to much lower temperatures than their higher-gravity counterparts (e.g., Ref. \citenum{Stephens2009}). Thus, degeneracies between surface gravity, effective temperature, cloudiness, and chemistry make interpreting spectra of substellar objects challenging. Thermal-infrared (3--5$\mu$m) measurements are essential for breaking these degeneracies because they probe the peak of the emission spectra for cool ($T_{\mathrm{eff}}\lesssim1000$~K) atmospheres and because they probe the fundamental transitions of the CH$_{4}$ and CO molecules. Thus, 3--5~$\mu$m spectral energy distributions are sensitive indicators for understanding carbon chemistry in substellar objects\cite{Noll2000}. While essential for a complete understanding of planetary atmospheres, thermal-infrared characterization of exoplanets has received relatively little attention, partly because of the challenging nature of ground-based observations at these wavelengths. The LBTI instrument at the LBT was specifically designed to provide low thermal background for the study of exoplanetary systems~\cite{Hinz2016}. The instrument takes advantage of the observatory's twin deformable mirror adaptive optics systems to observe at very-high spatial resolution using the minimum number of warm optics. To facilitate thermal-infrared spectroscopic observations of exoplanets, our team has built the world's only thermal infrared integral field spectrograph~\cite{Skemer2015}. Integral field spectrographs (IFS) facilitate exoplanet spectroscopy because the techniques of high-contrast imaging can be used to separate planet light from starlight at each wavelength. The instrument, named the Arizona Lenslets for Exoplanet Spectroscopy (ALES), has recently been commissioned and is now producing high-spatial resolution L-band spectra in high-contrast environments. The ALES IFS was built into the existing LMIRCam module~\cite{Skrutskie2010} of LBTI. To accomplish this, all the ALES optics, including magnifiers, a lenslet array, and direct-vision prisms, are located within LMIRCam filter wheels. This architecture requires a careful set-up and calibration approach that accommodates the number of moving parts. In this manuscript, we document our ALES observing strategy which we developed over three semesters while using ALES to observe multiple systems including exoplanets, brown dwarfs and solar system objects. This summer, ALES will undergo a significant upgrade that will improve our usable field of view, spectral resolution, and overall optical quality. New Magnification optics will also make imaging interferometry possible with ALES. (See Hinz et al., and Skemer et al., in these proceedings for more specifics.) After the upgrade, some of the details of our observing strategy may change, but the overall process and general steps of our approach will remain unchanged. | 18 | 8 | 1808.02571 |
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1808 | 1808.07481_arXiv.txt | We demonstrate that the UHECRs produced in the nuclear cascade in the jet of Low-Luminosity Gamma-Ray Bursts (LL-GRBs) can describe the UHECR spectrum and composition and, at the same time, the diffuse neutrino flux at the highest energies. The radiation density in the source simultaneously controls the neutrino production and the development of the nuclear cascade, leading to a flux of nucleons and light nuclei describing even the cosmic-ray ankle at $5 \cdot 10^{18} \, \mathrm{eV}$. The derived source parameters are consistent with population studies, indicating a baryonic loading factor of about ten. Our results motivate the continued experimental search of LL-GRBs as a unique GRB population. | \label{sec:intro} Gamma-Ray Bursts (GRBs) are extreme electromagnetic outbursts, see, for example, \citet{Piran:2004ba}. Here we consider the possibility that low-luminosity GRBs (LL-GRBs, with isotropic luminosity $\lesssim 10^{49} \mathrm{erg\,s^{-1}}$) and high-luminosity GRBs (HL-GRBs, with isotropic luminosity $\gtrsim 10^{49} \mathrm{erg\,s^{-1}}$) are two distinct populations, based on the different local rate of the two samples \citep{Guetta:2006gq,Liang:2006ci,Virgili:2008gp,Sun:2015bda}. Being locally much more abundant than HL-GRBs ($\approx 1 \, \mathrm{Gpc^{-3}\,yr^{-1}}$), LL-GRBs ($\approx 300 \, \mathrm{Gpc^{-3}\,yr^{-1}}$, as predicted in \citet{Liang:2006ci}) have been proposed as sources of cosmic rays and neutrinos \citep{Murase:2006mm,Murase:2008mr,Liu:2011cua,Senno:2015tsn}. More recently, LL-GRBs as sources of ultra-high energy cosmic-ray (UHECR) nuclei have been studied in \citet{Zhang:2017moz} including possible injection compositions. Due to the low radiation density, it has been proposed that the nuclei can escape intact from the sources, leading to compatibility with the UHE chemical composition measured by the Pierre Auger Observatory \citep{Aab:2014kda} after propagation. However, the low radiation density required for nuclei to escape implies at the same time low neutrino production efficiencies -- possibly too low to simultaneously describe the diffuse neutrino flux in a one zone model. HL-GRBs have been tested as the possible origin of UHECRs for both protons in \citet{Baerwald:2014zga} and nuclei in \citet{Biehl:2017zlw} describing cosmic-ray and neutrino data explicitly. It has been shown that for nuclei and for high enough radiation densities, a nuclear cascade due to the photo-disintegration of nuclei develops -- while at the same time neutrinos are efficiently produced by photo-hadronic interactions. Very tight constraints on neutrinos from HL-GRBs have been obtained by using direction, timing and energy information from GRB catalogues for stacking limits \citep{Abbasi:2012zw,Aartsen:2017wea}. These constraints limit the parameter space to low radiation densities, such as high collision radii and low luminosities in the internal shock model -- parameters which may not be favorable for HL-GRBs, and point already towards LL-GRBs \citep{Biehl:2017zlw}. A possible caveat are multi-zone collision models in which the different messengers originate from different regions of the same GRB, predicting somewhat lower neutrino fluxes~\citep{Bustamante:2014oka,Globus:2014fka,Bustamante:2016wpu} -- which however cannot explain the diffuse neutrino flux. The stacking bounds do not apply to LL-GRBs due to their much longer duration (making the background suppression less efficient) and their low luminosity (limiting the detection of resolved sources). Note that the luminosity mentioned here represents the X-ray luminosity, which may differ from the intrinsic kinetic luminosity of the jet. The latter can be higher by a factor $\sim 100$ taking into account the energy conversion efficiency \citep{Aloy:2018czj}. In this work, we study if LL-GRBs with a nuclear cascade in the jet can power the diffuse neutrino and cosmic-ray fluxes at the highest energies at the same time, using methods similar to \citet{Biehl:2017zlw,Biehl:2017hnb}. We inject a nuclear composition which is found to be reasonable in the jet of GRB progenitors \citep{Woosley:2005gy,Zhang:2017moz}, and we include the transition to the next population (at lower energies). As an important ingredient, it was noted in \citet{Unger:2015laa} in a generic model and in \citet{Globus:2015xga,Biehl:2017zlw} for GRBs that the nuclear cascade also controls the production of nucleons below the change of the slope in the measured CR energy spectrum, called the ``ankle'' at $\sim 5 \cdot 10^{18} \, \mathrm{eV}$ \citep{Fenu:2017}, \ie, spectrum and composition may be described in a much larger energy range across the ankle. Our analysis is based on a combined source-propagation model, which means that we include the interactions of the injected nuclei in the source in addition to the UHECR propagation, whereas a propagation-only model starts off at the interface between source and extragalactic space. Compared to earlier studies, we perform extensive parameter space scans focusing on a combined description of UHECR and neutrino data and including the description of the cosmic-ray ankle. Similar to previous studies, we use the internal shock scenario as a baseline scenario and comment on alternatives where applicable. We also motivate future searches in next-generation telescopes such as CTA. | \label{sec:conclusions} We have demonstrated that a global description of the cosmic-ray and neutrino data at the highest energies can be obtained by considering LL-GRBs as the sites of acceleration and interaction of the cosmic rays. We have shown that if the diffuse neutrino flux is to be powered by LL-GRBs, high photon densities in the source are required for efficient neutrino production. As a consequence, nuclei will disintegrate in the source, and the nuclear cascade developing within the source has to be taken into account. Our results are therefore based on a source-propagation model including the nuclear cascade in the source and cosmic-ray propagation. Interestingly, the light nuclei and nucleons (protons and neutrons) produced in the nuclear cascade can be used to describe the cosmic-ray spectrum and composition below the ankle at $5 \cdot 10^{18} \, \mathrm{eV}$. For a detailed analysis, we have included the next population dominating the cosmic-ray flux at energies $\lesssim 7 \cdot 10^{17} \, \mathrm{eV}$ as an unconstrained additional model component -- which may be of Galactic origin. As a consequence, we have obtained a near-perfect description of cosmic-ray spectrum and composition across the ankle, while at the same time powering the neutrino flux at the highest energies. In conclusion, the efficient modeling of the processes in the jet together with the extragalactic propagation allows a direct connection between data and the characteristics of the source. The investigation of alternative source classes to HL-GRBs and AGN blazars is motivated by constraints on the diffuse contribution from recent IceCube stacking analyses. Therefore, alternative scenarios, including LL-GRBs, are potentially needed to describe the diffuse IceCube neutrinos. Especially if the connection between the neutrinos and the UHECRs exists, it is likely that strong enough magnetic field effects on the secondary pions, muons, and kaons break the correlation between neutrino peak energy and maximal cosmic ray energy, as we have in LL-GRBs. For the same reason, it is difficult to postulate the UHECR connection in AGN blazars \citep{Murase:2014foa,Rodrigues:2017fmu,Gao:2018mnu}. Thanks to our estimate of the gamma-ray cascades from escaping EeV photons, we strongly encourage future progress in experimental studies of candidate source classes such as LL-GRBs from CTA. | 18 | 8 | 1808.07481 |
1808 | 1808.00574_arXiv.txt | The SuperMALT survey is observing 76 MALT90 clumps at different evolutionary stages (from pre-stellar or quiescent to HII) in high excitation molecular lines and key isotopomers using the Apex 12m telescope with an angular resolution of $\sim$20'' and a velocity resolution of $\sim$0.1 km/s. The aim of this survey is to determine the physical, chemical, and kinematical properties of the gas within clumps as they evolve. Here we report some preliminary results based on observations of the $J$=3-2 \& 4-3 lines of HNC, HCN, HCO$^+$, N$_2$H$^+$ and of the $J$=3-2 line of the isotopologue H$^{13}$CO$^+$. We find that the morphologies and line profiles vary with the evolutionary stage of the clumps. The average line width increases from quiescent to HII clumps while line ratios show hint of chemical differences among the various evolutionary stages. | Several observations have shown that high-mass stars form in massive dense clumps which have typically masses of $\sim$ 10$^{3}$ M$_{\odot}$, densities of $\sim$10$^{5}$ cm$^{-3}$ and dust temperatures $\le$ 20 K. (eg., \cite[Faundez et al. 2004]{Faundez2004}, \cite[Garay et al. 2004]{Garay2004}). Despite of the great role that high-mass stars play as energy input in the Galaxy, their formation mechanism is not cleary understood. Determining the properties of massive dense clumps and their fragmentation process, giving rise to cores, is crucial to understand the formation process of high-mass stars. Several molecular and continuum surveys have been made to investigate the characteristics and evolutionary stages of massive dense clumps (eg., ATLASGAL: \cite[Schuller et al. 2009]{Schuller2009}, Hi-GAL: \cite[Molinari et al. 2010]{Molinari2010}, GLIMPSE/MIPSGAL: \cite[Benjamin et al. 2003]{Benjamin2003}; \cite[Carey et al. 2009]{Caray2009}, MALT90: \cite[Foster et al. 2011]{Foster2011}; \cite[Jackson et al. 2013]{Jackson2013}). In particular, the Millimeter Astronomy Legacy Team 90 GHz (MALT90) Survey observed $\sim$3200 dense molecular clumps in different evolutionary states -- from pre-stellar, to proto-stellar, and H II regions -- in 16 low excitation molecular lines (mostly $J$=1$\rightarrow$0), near 90 GHz. This survey, made using the 22m Mopra Telescope with an angular resolution $\sim$40'' and a velocity resolution of $\sim$ 0.1 km/s, has provided important information concerning the morphology and kinematics of the massive dense clumps showing that clumps in different evolutionary stages indeed show different physical and kinematical properties (eg., \cite[Guzman et al. 2015]{guzman2015}, \cite[Rathborne et al. 2016]{jill2016}, \cite[Contreras et al. 2017]{contreras2017}). However, since for a given molecular species only a single rotational transition (mostly $J$=1$\rightarrow$0) was observed, the MALT90 data are insufficient to perform a robust derivation of the gas properties. Only with multi-level excitation analysis a robust determination of clump temperatures, densities, and column densities can be obtained, which are important to understand the differences in physical and chemical properties of clumps at different evolutionary stages. We conducted the SuperMALT survey to observe a subsample of MALT90 clumps in high excitation molecular lines and key isotopomers using the Apex 12m telescope with the aim of providing an important new legacy database to characterise the evolution of high-mass star forming clumps (\cite[Contreras et al. in preparation]{Yanett2017}). In this work, we present some preliminary results from the survey. | We report preliminary results from the SuperMALT survey. We find that the morphology of the emission observed in the $J$=3-2 lines of HNC, HCN, HCO$^+$, N$_2$H$^+$ are similar, although there are some differences. The HCO$^{+}$ emission is more extended than that seen in the N$_{2}$H$^{+}$ and HNC. The morphology of the molecular emissions are also similar to that of the dust emission at 870 micron (ATLASGAL). The line profiles appear different from quiescent clumps to HII clumps. Most quiescent clumps show nearly Gaussian line profiles while evolved clumps show features such as line wings and self absorption. Clumps in the late stages (HII) exhibit brighter lines than those of quiescent clumps and an increase in line widths or turbulence suggesting active star formation within the former clumps. \\ {\textit Acknowledgements:} S.N. and G.G. greatfully acknowledge support from CONICYT project PFB-06. | 18 | 8 | 1808.00574 |
1808 | 1808.09455_arXiv.txt | We present a new precision measurement of gas-phase abundances of S, O, N, Si, Fe, P, Al, Ca as well as molecular hydrogen (H$_2$) in the Leading Arm (region II, LA\,II) of the Magellanic Stream (MS) towards the Seyfert galaxy NGC\,3783. The results are based on high-quality archival ultraviolet/optical/radio data from various different instruments (HST/STIS, FUSE, AAT, GBT, GB140\,ft, ATCA). Our study updates previous results from lower-resolution data and provides for the first time a self-consistent component model of the complex multi-phase absorber, delivering important constraints on the nature and origin of LA\,II. We derive a uniform, moderate $\alpha$ abundance in the two main absorber groups at $+245$ and $+190$ km\,s$^{-1}$ of $\alpha$/H$\,=0.30\pm 0.05$ solar, a low nitrogen abundance of N/H$\,=0.05\pm 0.01$ solar, and a high dust content with substantial dust depletion values for Si, Fe, Al, and Ca. These $\alpha$, N, and dust abundances in LA\,II are similar to those observed in the Small Magellanic Cloud (SMC). From the analysis of the H$_2$ absorption, we determine a high thermal pressure of $P/k\approx 1680$ K\,cm$^{-3}$ in LA\,II, in line with the idea that LA\,II is located in the inner Milky Way halo at a $z$-height of $<20$ kpc where it hydrodynamically interacts with the ambient hot coronal gas. Our study supports a scenario, in which LA\,II stems from the break-up of a metal- and dust-enriched progenitor cloud that was recently ($200-500$ Myr ago) stripped from the SMC. | The gravitational and hydrodynamical interaction between the Milky Way (MW) and the Magellanic Clouds (MCs) and the subsequent star-formation activity in the Clouds have transported more than one billion solar masses of gas from the MCs into the circumgalactic medium (CGM) of the Milky Way. These processes have produced extended gaseous streams and clouds at distances of $d\approx 20-100$ kpc that cover more than $40$ percent of the sky (Wannier et al.\,1972; Mathewson et al.\,1974; Putman et al.\,1998; Br\"uns et al.\,2005; Fox et al.\,2014; Richter et al.\,2017). This gigantic circumgalactic gas reservoir dominates the Milky Way's current and future gas accretion rate (see recent reviews by D'Onghia \& Fox 2016; Putman et al.\,2012; Richter 2017) and thus has a major impact on the Galaxy's evolution. The neutral gas bodies of these CGM clouds and streams can be observed in H\,{\sc i} 21\,cm emission, while the more diffuse and more extended ionized gaseous envelopes can be traced in ultraviolet (UV) metal absorption against distant, extragalactic point sources (such as quasars and other type of AGN, for simplicity hereafter referred to as QSOs) as well as in H$\alpha$ emission (see, e.g., Richter 2017; Barger et al.\,2013, 2017). Absorption-line studies of the Milky Way gas environment, in particular, can be readily compared to similar studies of the CGM around other, more distant galaxies at low and high redshift (e.g., Werk et al.\,2013; Stocke et al.\,2014; Liang \& Chen 2014; Richter et al.\,2016). Detailed studies of the spatial distribution and chemical composition of the Milky Way's CGM provide important clues on the on-going formation and evolution of the Galaxy in its Local Group environment through gas infall and satellite interaction. They also deliver detailed constraints on the filling factor and physical conditions of multi-phase circumgalactic gas around star-forming disk galaxies and thus are of high relevance to evaluate the importance of the CGM for galaxy evolution, in general. The most prominent of the circumgalactic gas features generated by the interaction between the MCs and the MW is the Magellanic Stream (MS), an enormous stream of neutral and ionized gas in the southern hemisphere extending over more than 200 degrees (Nidever et al.\,2010), and possibly over several hundred kpc in linear size (D'Onghia \& Fox 2016). The main body of the MS is believed to be at a distance of $50-100$ kpc, thus located in the outer Milky Way halo. The spatial extension of the MS into the northern sky at $l>300$ is the so-called Leading Arm (LA), a conglomerate of scattered clouds seen in 21\,cm data and in UV absorption whose ionized enveleope possibly extends far into the northern sky (Fox et al.\,2018; Richter et al.\,2017). It was previously argued that the LA must be of purely tidal origin, as it leads the orbital motion of the Magellanic System (which includes the MS, the LA, and the Magellanic Bridge) around the MW (e.g., Putman et al.\,1998). However, more recent hydrodynamical simulations indicate that only the combination of multiple tidal stripping events, outflows from the Magellanic Cloud, and ram-pressure stripping can explain the complex spatial distribution of the MS and LA gas components and the observed abundance variations therein (Besla et al.\,2010, 2012; Fox et al.\,2018; Pardy et al.\,2018). The LA, which is sub-divided into four main substructures, LA\,I-IV (Putman et al.\,1998; Br\"uns et al.\,2005; Venzmer et al.\,2012; For et al.\,2012) is much closer than the MS ($d<20$ kpc), as evident from the analysis of young stars that recently have been found in the LA (Casetti-Dinescu et al.\,2014; Zhang et al.\,2017). In our previous studies, we have used UV absorption-line data for many dozens of QSO sightlines together with 21\,cm emission-line data from various instruments to characterize the overall chemical composition of the MS and the LA, to estimate their total mass, and to pinpoint their contribution to the Milky Way's gas-accretion rate (Fox et al.\,2010, 2013, 2014, 2018; Richter et al.\,2013, 2017). In addition to these large surveys, detailed analyses for individual sightlines are likewise highly desired. Such studies provide a tremendous amount of accurate information on the chemical enrichment pattern and the local physical conditions in the gas, which is very important to discriminate between different scenarios for the origin of the MS and LA (e.g., LMC vs.\,SMC). For instance, the substantially higher metallicity and enhanced dust abundance found in the MS along the Fairall\,9 sightline (Richter et al.\,2013; hereafter referred to as R13) compared to other MS sightlines (Fox et al.\,2013) demonstrates that the trailing arm of the MS has a dual origin with spatially and kinematically distinct filaments stemming from both LMC {\it and} SMC (see also Nidever et al.\,2008, 2010). This result sets important constraints for dynamical models of the LMC/SMC/MW gravitational interactions and provides independent evidence for an enhanced star-formation activity in the LMC that has lifted $\alpha$-enriched gas into the Milky Way halo (see discussions in R13; Bustard et al.\,2018; Pardy et al.\,2018). The stellar activity of the LMC is currently driving over $10^7$ solar masses out of this galaxy in a large-scale galactic wind (Barger 2016) and similar winds were likely generated during episodic periods of elevated star formation in the past. Following our long-term strategy to explore the origin and fate of the Magellanic System and its role for the past and future evolution of the Milky Way and the Local Group (LG), we here present a detailed study of the chemical abundances and physical conditions in the LA\,II along the line of sight towards NGC\,3783. Because of the brightness of the background Seyfert galaxy NGC\,3783 and the high neutral gas column density, the NGC\,3783 direction is the best-studied QSO sightline for absorption-line measurements in the LA with a number of detailed studies (Lu et al.\,1994, 1998; Sembach et al.\,2001; Wakker et al.\,2002, hereafter WOP02; Wakker 2006) using medium-resolution UV data from the early-generation HST/UV spectrograph GHRS (Goddard High Resolution Spectrograph), data from the {\it Far Ultraviolet Spectroscopic Explorer} (FUSE), and 21\,cm inferometer data from the Australian Compact Telescope Array (ATCA). The best available UV data set (in terms of spectral resolution and S/N) for NGC\,3783 was obtained using the high-resolution echelle E140M grating of the Space Telescope Imaging Spectrograph (STIS) in 2000-2001 to monitor the UV variability of NGC\,3783 and to study its intrinsic absorption (Gabel et al.\,2003a, 2003b), but these spectacular STIS spectra never were used to confirm and extend the earlier abundance results in the LA or to use the full diagnostic power of the combined STIS/FUSE/ATCA data set to constrain the physical conditions in the gas. An update of the earlier LA abudance measurements for the NGC\,3783 sightline is also highly desired to better characterize the dust-depletion properties of the gas in the LA and to account for the change in the solar reference abundances for the elements sulfur and oxygen since 2002 (Asplund et al.\,2009). With this paper, we are closing this gap and provide the most precise determination of gas and dust abundances in two individual clouds with the LA based on the archival E140M STIS data of NGC\,3783. Combining the STIS, FUSE, and ATCA data we further determine important physical quantities such as density, temperature, thermal pressure, and absorber size in the LA. By combining our precision results for the NGC\,3783 sightline with our recent measurements of the large-scale metal distribution in the LA based on lower-resolution UV data from the Cosmic Origins Spectrograph (COS; Fox et al.\,2018), our multi-instrument spectral survey provides important new constraints on the origin of the LA, its physical properties, and its location in the Milky Way halo. | 18 | 8 | 1808.09455 |
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1808 | 1808.05956_arXiv.txt | Projected quasar galaxy pairs provide powerful means to study the circumgalactic medium (CGM) that maintains the relics of galactic feedback and the accreted gas from the intergalactic medium. Here, we study the nature of a Lyman Limit system (LLS) with N(\HI)=10$^{19.1\pm0.3}$\,cm$^{-2}$ and a dust-uncorrected metallicity of [Fe/H]$=-1.1\pm0.3$ at $z=0.78$ towards Q0152$-020$. The \MgII\ absorption profiles are composed of a main saturated and a few weaker optically thin components. Using MUSE observations we detect one galaxy close to the absorption redshift at an impact parameter of $54$\,kpc. This galaxy exhibits nebular emission lines from which we measure a dust-corrected star formation rate of $10^{+8}_{-5}$\,M$_\odot$\,yr$^{-1}$ and an emission metallicity of [O/H]$=-0.1\pm0.2$. By combining the absorption line kinematics with the host galaxy morphokinematics we find that while the main absorption component can originate from a galactic wind at $V_{\rm w}=110\pm4$\,\kms\ the weaker components cannot. We estimate a mass ejection rate of $\dot M\gtrsim0.8$\,M$_\odot$\,yr$^{-1}$ that translates to a loading factor of $\eta\gtrsim0.1$. Since the local escape velocity of the halo, $V_{\rm esc}\simeq430$\,\kms, is a few times larger than $V_{\rm w}$, we expect this gas will remain bound to the host galaxy. These observations provide additional constraints on the physical properties of winds predicted by galaxy formation models. We also present the VLT/X-Shooter data analysis of 4 other absorbing systems at $1.1<z<1.5$ in this sightline with their host galaxies identified in the MUSE data. | The circumgalactic medium (CGM) is the region around a galaxy through which the baryon exchange between the intergalactic medium (IGM) and the galaxy occurs. On the one hand it hosts the fresh pristine gas entering the halo from the IGM but on the other hand it contains relics of metal enriched galactic winds. While the extent of the CGM varies based on the definition, usually it is considered as the region with a radius of $\sim300$\,kpc surrounding galaxies \citep[e.g.,][]{Steidel10,Shull14_a}. To understand the full cycle of gas from IGM into galaxies and from galaxies towards IGM it is crucial to understand the distribution, kinematics and metal content of the gas in the CGM. Due to the very low density of the gas it is not yet possible to probe the CGM in emission \citep[but see,][]{Frank12}. Absorption lines towards background quasars provide the most sensitive tools to carry out CGM studies. In particular, quasar absorbers with large \HI\ column density, N(\HI)$\gtrsim10^{19}$\,${\rm cm^{-2}}$, afford robust measurements of the metal content of the CGM. There are several studies that also show that the majority of the neutral gas reservoirs in the Universe are associated with such absorbers \citep{Peroux03,Prochaska05,Noterdaeme09dla,Noterdaeme12dla,Zafar13}. Therefore, by studying such strong \HI\ absorbers one not only probes the CGM but also traces the possible fuel for forming stars in galaxies. \citet{Quiret16} reported a bimodal distribution of metallicity for a subsample of strong \HI\ absorbers ($19.0<\log [N(\HI)/{\rm cm}^{-2}] <20.3$) at $z<1$. The high metallicity side of the distribution could be interpreted as a possible tracer for galactic winds and the low metallicity side as a possible tracer for the pristine IGM gas accretion \citep[see also][for similar results but for Lyman limit systems (LLS) with N(\HI)$>10^{16}$\,cm$^{-2}$]{Lehner13,Wotta16}. \citet{Hafen16} studied a sample of LLS at similar redshifts extracted from hydrodynamical simulations where they demonstrated that such absorbers are associated with gaseous complexes in the CGM. While their mean LLS metallicity ([X/H]=$-0.9$) is consistent with that of observations, they did not find a bimodal metallicity distribution. It is worth also noting that in their simulation, \citet{Hafen16} did not find as many low metallicity systems as those detected in observations. However, to disentangle the role of the CGM gas one needs to study together the LLS and its host galaxy. The CGM can also be studied from the metal absorption lines in the spectra of the host galaxies themselves, the so called down-the-barrel technique \citep[e.g.,][]{Pettini02_dtb,Steidel10,Rubin12,Martin12,Kacprzak14,Erb15,Bordoloi16,Finley17}. Blueshifted and redshifted absorption lines are produced in the spectra of galaxies in the cases of, respectively, galactic winds and infalling gas. Large sample studies have been successful in finding predominantly outflows; infalling gas has been unambiguously identified in only a few cases \citep{Rubin12,Martin12}. \citet{Rubin12} postulated that the strong and broad absorption lines from outflowing gas can smear out the absorption signature from infalling gas. In any case, the low resolution spectra of galaxies and the unknown distance between absorbing gas and the host galaxy limit the ultimate accuracy of the derived physical parameters from down-the-barrel studies. The nature of the gas in the CGM can be studied in more detail when the host galaxies of quasar absorbers are also detected in emission \citep{Moller98a,Rahmani10,Peroux11a,Peroux11b,Noterdaeme12,Bouche13,Christensen14,Rahmani16,Rahmani18,Moller17}. As an example \citet{Noterdaeme12} studied multiple emission lines from the host galaxy of a strong \HI\ absorber at $z=2.2$ and demonstrated that the absorption is most likely associated with an outflow \citep[see also,][]{Kulkarni12}. \citet{Krogager12} observed a tight anti-correlation between the impact parameter and the N(\HI) which is likely driven by feedback mechanisms \citep[see also][for similar results]{Monier09,Rao11,Rahmani16}. Much better understanding on the nature of the CGM can be achieved if one combines the absorption line kinematics with those of the host galaxy absorber extracted from integral field unit (IFU) observation. As part of the SIMPLE survey \citep{Bouche07_simple,Bouche12a}, in a study of the host galaxies of strong \MgII\ absorbers, \citet{Schroetter15} demonstrated that such absorbers are often associated with galactic winds. \citet{Peroux11a} used VLT/SINFONI IFU observations of the field of a handful of Damped Lyman-$\alpha$ systems (that are absorbers having $\log$\,[$N(\HI)$/cm$^{-2}$]$>20.3$) at $z\sim1$ to study the kinematics of their host galaxies. They also showed that the detection rate is higher for the subsample of absorbers with higher metallicity. \citet{Rahmani18} used VLT/MUSE observation of an LLS host galaxy along with high resolution Keck/HIRES data of the quasar absorption line to show that the LLS originates from a warped disk. Such warped disks are probably associated with cold gas accreting onto galaxies \citep[see also][for similar studies]{Bouche13,Burchett13,Diamond-Stanic16}. Here we present the detailed study of an LLS at $z=0.78$ towards Q0152$-020$\footnote{RA(J2000)=28.113464, Dec(J2000)=$-20.018435$.} \citep{Sargent88_MgII,Rao06}. We combine new VLT/MUSE observation of the quasar field with high spectral resolution Keck/HIRES, low spectral resolution HST/FOS spectroscopy of the quasar and HST/WFPC2 $F702W$ high spatial resolution image to understand the nature of the CGM gas absorber. The paper is organized as follows. In section \ref{section_observation} we provide a summary of the available observations. In section \ref{section_absline} we study the absorption line and in section \ref{section_host_gal} we present the properties of the host galaxy absorber. In section \ref{section_conclude} we study the nature of the gas we see in absorption and finally we summarize the paper in section \ref{section_summary}. We also mention other absorbing systems in this sightline having redshifts $1.0<z<1.5$ in appendices \ref{appendix_absline} and \ref{appendix_hostgal}. Throughout this paper, we assume a flat $\Lambda$CDM cosmology with $H_0=69.3$\,\kms\,Mpc$^{-1}$ and $\Omega_m=0.286$ \citep{Hinshaw13}. | In this work we have studied an LLS with $\log$[N(\HI)/cm$^{-2}$]=$19.1\pm0.3$ at $z=0.78$ towards Q0152$-$020. The metal absorption profiles associated with this LLS consist of a strong main component (seen in \MgII\ and \FeII) but also another two well separated weaker components at velocity differences of $\sim200$\,\kms\ (that are only detected in \MgII). We have measured a dust-uncorrected absorption metallicity of [Fe/H]$=-1.1\pm0.3$\,dex.% At the absorption redshift of \zabs=0.78 our VLT/MUSE data is sensitive to a dust-uncorrected SFR=0.5\,M$_\odot$\,yr$^{-1}$. Inspecting our VLT/MUSE data we find one galaxy at a velocity separation of $v=78$\,\kms\ with respect to the main absorption component. This galaxy is located at an impact parameter of $b=54$\,kpc. We confirm the redshift of this galaxy by detecting [\OII], [\OIII]5007 and H$\beta$ emission lines. % By modeling the HST/WFPC2 $F702$ broad-band image of this galaxy using a S\'ersic profile we obtained a fit with $n=1.45\pm0.07$. The morphokinematic modeling, using the MUSE data cube, shows this galaxy has a rotation curve with a $V_{\rm max}=(2.0\pm0.1)\times10^2$\,\kms. Such analyses further lead us to conclude that this disky galaxy has not suffered from recent strong interactions with other galaxies. We infer the stellar mass using the Tully-Fisher relation to be $M_\star=(3.6\pm1.5)\times10^{10}$\,M$_\odot$. We further measure a dust-corrected SFR for this galaxy to be $10^{+8}_{-5}$\,M$_\odot$\,yr$^{-1}$ and oxygen abundance of [O/H]=$-0.1\pm0.2$. We also estimate a metallicity gradient of $\sim0.03$\,dex\,kpc$^{-1}$. At the position of the quasar, the predicted velocity of gas corotating with the disk of the galaxy is +50\,\kms. Since the velocities of different absorbing components are $v\sim-80$\,\kms\ and $v\sim-300$\,\kms\ we reject a scenario in which the LLS originates from the stellar disk or a warped disk associated with this galaxy. HVCs, associated with the galaxies in the Local group, have similar metallicities and velocities to this LLS and can be found at comparable impact parameters to their host galaxies. Hence, we can not refute the possibility that one or more absorption components can be associated with HVC like clouds in the CGM of the host galaxy. % We also explore the possibility that the LLS arises from a galactic wind phenomenon. This model is supported by the azimuthal angle of $\phi=70^\circ$ which shows the quasar sightline passes close to the galactic pole. We try to reproduce the absorption profile using a wind model where gas clouds are confined within a cone and have reached their terminal velocity. A good match to the absorption profile of one of the absorption components is achieved with a wind velocity of $V_{\rm w}=110\pm4$\,\kms\ and cone opening angle of $\theta_{\rm max}=15\pm2^\circ$. We estimate a total mass ejection rate of $\dot{M}\gtrsim0.8$\,M$_{\odot}$\,yr$^{-1}$ and a loading factor of $\eta\gtrsim0.1$. {This parameter plays a crucial role in reproducing the observed population of galaxies in simulations \citep[e.g.][]{Dutton12,Dave13,Muratov15,Hayward17}. However, our estimated value of $\eta\gtrsim0.1$ is in agreement with those from FIRE simulations \citep[][]{Muratov15}.} We further find the escape velocity from the halo of this galaxy is almost four times larger than the wind speed. Therefore, this outflowing material will not be able to leave the halo of this galaxy. \citet{Guillemin97} had also searched for the host galaxy of this absorbing system where they associated it with a galaxy at $z=0.603$ ($\Delta v\gtrsim3\times10^{5}$\,\kms). Thanks to the large field of view and the high sensitivity of MUSE we can now reliably identify and study the host galaxy absorber. Further combining MUSE data with the high spectral resolution quasar absorption line data and HST image of the field we further studied the nature of the absorbing gas. Altogether our results show the power of IFU observations of quasar galaxy pairs in CGM studies. | 18 | 8 | 1808.05956 |
1808 | 1808.06955_arXiv.txt | We present the first X-ray and UV/optical observations of a very bright and fast nova in the disc of M31, M31N 2013-01b. The nova reached a peak magnitude $R\sim$15\,mag and decayed by 2 magnitudes in only 3\,days, making it one of the brightest and fastest novae ever detected in Andromeda. From archival multi-band data we have been able to trace its fast evolution down to $U>21$\,mag in less than two weeks and to uncover for the first time the Super-Soft X-ray phase, whose onset occurred 10-30\,days from the optical maximum. The X-ray spectrum is consistent with a blackbody with a temperature of $\sim$50 eV and emitting radius of $\sim$4$\times 10^{9}$ cm, larger than a white dwarf radius, indicating an expanded region. Its peak X-ray luminosity, 3.5$\times 10^{37}$\,erg\,s$^{-1}$, locates M31N 2013-01b among the most luminous novae in M31.\\ We also unambiguously detect a short 1.28$\pm$0.02\,h X-ray periodicity that we ascribe to the binary orbital period, possibly due to partial eclipses. This makes M31N 2013-01b the first nova in M31 with an orbital period determined. The short period also makes this nova one of the few known below the 2-3\,h orbital period gap.\\ All the observed characteristics strongly indicate that M31N 2013-01b harbours a massive white dwarf and a very low-mass companion, consistent with being a nova belonging to the disc population of the Andromeda Galaxy. | \label{sec:intro} Classical Novae (CN) are close binary systems ($\rm P_{orb} \sim$ 1.5 - 10\,h) consisting of a late type main sequence or red giant secondary and an accreting white dwarf (WD) primary that experience an outburst triggered by a thermonuclear runaway in the hydrogen-rich accreted material \citep{bod08}. CN thus belong to the cataclysmic variable (CV) class. After a maximum (brightening up to 7-16\,mag), the optical emission declines due to the receding photosphere at a rate which defines the Nova speed class \citep{pay64}. Part of the material is expelled at high velocities \citep{sho11}. The decline time, typically defined as the time needed to decline by 2 magnitudes (t2) was found to be related to the peak maximum \citep{del95} and to the expansion velocity of the ejected envelope \citep{del02,sha11,sch11}. Fast novae tend to be brighter and to display high expansion velocities. After one to a few weeks, the receding atmosphere towards the hotter inner regions is such that the emission moves to X-rays, making the Nova a Super-Soft X-ray Source (SSS) with temperature of the order of $\sim$20-80\,eV and X-ray luminosities $> 10^{36}$ erg s$^{-1}$. This phase is powered by stable hydrogen burning within the part of the accreted envelope and is observable when the ejected matter becomes optically thin to soft X-rays \citep{ori01,kra02}. Few novae during the SSS phase were found to display short period X-ray oscillations or quasi-oscillations on time-scales of several tens of sec with relatively short duty cycles \citep{bea08,nes15}, ascribed to $g-mode$ pulsations of the burning envelope, which, if confirmed, have the potential to estimate the WD mass \citep[see][and references within]{nes15,wol18}. Oscillations on time-scales of thousands of seconds detected during the SSS phase have been interpreted as non-radial pulsations \citep{dra03,dob10} while those found to be coherent signify the rotation period of a magnetic WD \citep{pie11,dob10}. On the other hand, periodic variations on longer (hours) timescales are compatible with the binary orbit, showing either eclipses \citep{sal08} or X-ray modulations \citep{hen10,hen14,pag10,bea12}. Most Milky Way novae are found either in the Galactic Bulge or lying in the Galactic plane, thus suffering from strong absorption. Novae in Local Group galaxies provide instead the best targets to study outburst evolution and especially the SSS phase. Thanks to its close distance (776\,pc, \citet{dal12}; we will adopt this distance throughout this article) and low Galactic extinction \citep{dal12} M31 is an ideal target to study nova populations. \citet{hen10,hen14} published $\sim$25 novae (out of the $\sim$80 known) in M31 that show the SSS phase, covering the period until 2012. Only two of them, M31N 2006-04a and M31N 2011-11e, possess a candidate X-ray orbital period (1.6\,h and 1.3\,h respectively), not confirmed due to the low statistics \citep{hen14,pie11}. The Exploring the X-ray Transient and variable Sky (EXTraS) project \citep{del16} developed new techniques and tools to extract and describe the timing behavior of archival X-ray sources detected by {\it XMM-Newton/EPIC} \citep{str01,tur01}. Thanks to the great improvement of EXTraS with respect to the 3XMM source catalog, we performed a systematic search for eclipsing and/or dipping sources in the {\it XMM-Newton} data set pointing the M31 Galaxy, finding significant and periodic dips in the light curve of 3XMM J004401.9+412544. The X-ray source is located at R.A.$=00^{{\rm h}}~44^{{\rm m}}~01.^{{\rm s}}97$, Dec.$=+41^{\circ}~25'~44.5''$ (J2000)\footnote{3XMM-DR8: http://xmmssc.irap.omp.eu/Catalogue/3XMM-DR8/3XMM\_DR8.html} (1$\sigma$ statistical plus systematic error of 2''), positionally consistent with the optical nova PNV J00440207+4125442=M31N 2013-01b (henceforth M31N2013), {located at R.A.$=00^{{\rm h}}~44^{{\rm m}}~02.^{{\rm s}}09$, Dec.$=+41^{\circ}~25'~44.4''$ (J2000) (5'' statistical plus systematic error) \citep{hor13}.} It was detected only once by {\it XMM-Newton} (out of 12 observations) on Feb.8, 2013, $\sim$14 days after the optical maximum reported by \citet{hor13} and during a follow-up observation of the nova, thus confirming the association of the two sources.\\ M31N2013 was found as a bright optical transient \citep{hor13}, with a peak R-band magnitude on 2013, Jan. 25.728\,UT (this will be considered as T0 for the rest of the paper) of 15.05(7). Follow-up observations\footnote{IAU CBAT: http://www.cbat..eps.harvard.edu/unconf/followups/J00440207+4125442.html} showed a rapid fading in a few days. M31N2013 was spectroscopically confirmed as a He/N or hybrid (Fe\,IIb) nova by \citet{sha13}, displaying broad (FWHM $\rm \sim 5500\,km\,s^{-1}$ Balmer emission lines. The magnitude and X-ray luminosity (see Table 1) are consistent with a nova in M31, while excluding a foreground nova or a nova in our Galaxy (that would be much more or much less luminous than the other observed novae, respectively). It is located in the M31 disc \citep{hor13}, further supporting the M31 membership. During the optical decline, apart from {\it XMM-Newton}, M31N2013 was also observed with the {\it Neil Gehrels Swift} satellite (henceforth {\it Swift}) in the X-rays and UV and optical, the {\it Chandra} X-ray Observatory, using either HRC \citep{mur00} or ACIS instruments \citep{nou98}. Here we present a complete analysis of optical/UV and X-ray SSS phase of this nova exploiting all the available data. Section \ref{data} describes the data reduction, Section \ref{anal} the analysis, and in Section \ref{discussion} we discuss the results. | \label{discussion} \subsection{The X-ray periodicity} Our analysis of the {\it XMM-Newton} X-ray data resulted in a periodic modulation with a period of 1.28$\pm$0.02 h, obtained in an observation lasting about 5\,cycles. The amplitude of this variability is large $\sim$40$\%$. The modulation appears to be also structured with hints of a double-peaked maximum and a flat minimum. The timescale and large amplitude of the modulation exclude that the SSS luminosity variation is due to pulsations (see Section \ref{sec:intro}). We ascribe the periodicity to the orbital period of the system. It is the first nova in M31 with an orbital period unambiguously detected in the X-rays. Only candidate periods of two novae in M31 were found by \citet{hen10,hen14} during the SSS phase of M31N 2006-04a and M31N 2011-11e, 1.6\,h and 1.3\,h, respectively. The orbital period of CVs is an observational proxy of their evolutionary state, where the secular mass transfer rate decreases as the systems evolve towards short orbital periods \citep{how01,bar03}. Hence, systems below the 2-3\,h orbital period gap \citep{war95} are old systems and expected to accrete at a low-rates $\lesssim 5\times 10^{-11}\, M_{\odot}\,yr^{-1}$ \citep{kni11}. Assuming a 1.3\,h orbital period, the donor in M31N2013 is expected to be a very low-mass ($\rm M_2 \leq 0.1 M_{\odot}$) star and of late M or even later spectral type \citep{kni11}. Observed spectral types of CV donors at these short periods have been found to be late M dwarfs, M6-M9 as it is the case for the short period (1.4\,h) old novae CP\,Pup or GQ Mus ($>$M6) \citep{szk88}. For L-type donors the systems could also be period-bouncers and a few have been found so far with only one confirmed X-ray emitting magnetic system \citep{ste17} accreting at very low rate ($\sim 10^{-14}\,M_{\odot}\,yr^{-1}$). The very short 1.28\,h period locates M31N 2013-01b close to the expected theoretical orbital minimum of CVs ( 1.1\,h) \citep{how01}. Comparing the orbital period distribution of novae from \citet{rit03} this nova is among the shortest orbital period systems and one of the few detected to display an orbital modulation in the SSS phase. Orbital X-ray variability could be due to structures in the accretion disc although absorption effects should be present. Additionally, a partial eclipse from fixed regions such as the disc-rim or by the donor star could be viable solutions. Similar partial eclipses were claimed for two Galactic novae HV Cet \citep{bea12} and V5116\,Sgr \citep{sal08}. Both the models require an high-inclination system (i$\gtrsim60^{\circ}$) Another possibility is that M31N2013 harbours a magnetic WD of the polar type \citep[see ][ for a review]{cro90}, where no disc is formed due to the high magnetic field of the WD primary ($\rm B \geq 10\,MG$) that locks its rotation at the orbital period. The polars show strong X-ray variations (up to 100$\%$) due to localised accretion spots at the magnetic poles. However, the burning in the very early phases of a nova would rapidly reach a spherical symmetry, heating the whole WD surface \citep{cas10}. The radius of the SSS emitting region is found to be in the range $\sim 3-7\times 10^{9}$\,cm that is indeed much larger than those of WDs, indicating an expanded emitting envelope. The lack of observed spectral variability along the phase may favour an eclipse, possibly partial, of the X-ray emitting region. \subsection{M31N\,2013-Ib: a very fast nova in M31} M31N2013 is a very fast nova, with a $t_2$ decay in the R band of $\sim$3\,d. The rate of decay is then $d\,m/d\,t \sim 0.7$ and consistent with the universal law by \citet{hac06} where the flux decays as $\rm F \propto t^{-\alpha}$ with $\alpha$=1.7-1.75. We find $\alpha$=1.7. \citet{lee12} suggests that such very fast novae are more rarely encountered in M31 than in our Galaxy, even if selection effects should be taken into account. In particular, the online catalogue of optical apparent novae in M31 by Pietsch \& Haberl\footnote{http://www.mpe.mpg.de/~m31novae/opt/m31/index.php} reports a handful (11) of very fast novae with similar $t_2\sim$2-4\,d. Fast novae are found to harbour massive WD ($\rm M_{WD} \geq 1.2\,M_{\odot}$) \citep{del95,del02} and the light curves are predicted to be almost independent of the chemical composition once iron content is fixed \citep{hac06}. The SSS X-ray turn-on and turn-off times cannot be fully constrained due to the lack of a deep monitoring shortly after the optical maximum. While the {\it Swift/XRT} upper limit on Feb. 1 is too weak to constrain the turn-on time, the detections by {\it Swift}/XRT and {\it XMM-Newton} could hint to a fast turn-on t$_{on}<$ 10\,d (taking into account the T0 reported in Section \ref{sec:intro}). The turn-off time could have occurred about one month after the optical maximum given the non-detection in the {\it Chandra} data, or before. Such short times are consistent with the correlation found between the turn-on and turn-off times found for M31 novae by \citet{hen14}. The hot blackbody temperature as derived from the X-ray spectral fit is also in agreement with a short turn-off time on similar timescale \citep[see e.g. ][]{hen14}. Additionally, results from the M31N2013 datasets -- including the short t2, the extremely high expension velocity detected in the optical spectrum of the early phases of the outburst \citep{sha13}, turn-on and turn-off time -- appear to be consistent with the relations found for the M31 novae \citep{hen14}. Althought consistent, in all these graphs M31N2013 is always at the edge of the relationships, owing to its short t2, turn-on time and turn-off time, thus allowing for a better constraint of the link among these quantities. The X-ray luminosity of the SSS of M31N2013 is consistent with other M31 novae found so far (L$_{50}<8.7\times10^{37}$ erg s$^{-1}$, \cite{hen14}), although it is in the bulk of the most luminous ones (only 4 out of the 24 M31 novae reported in \citet{hen14} show a higher L$_{50}$). Using the dust maps by \citet{mon09} that include the contribution of the Galactic foreground extinction (E$_{gal}$(B-V)=0.10), we derive a total extinction $\rm 0.10\lesssim E(B-V)\lesssim 0.21$, since the location of M31N2013 within M31 is not known. This translates into a range of extinction in the R band of $\rm 0.26 \leq A_R \leq 0.55$. Assuming a distance of 776$\pm$18\,kpc and this range of extinction, we estimate an absolute magnitude at the peak of the optical light curve in the range $\rm -9.60 \leq M_R \leq -10.0$. This range is fully consistent with the absolute peak magnitude observed in fast novae with similar $t_2$ and in particular with the super-bright Galactic nova V1500\,Cyg \citep{kat13}. Unfortunately, due to the loosly constrained optical and X-ray positions, a search for the nova progenitor resulted inconclusive. Although not studied until now, M31N2013 is one of the brightest and fastest nova ever detected in M31, belonging to the disc population and likely harbouring a massive WD and one of the few known at very short orbital periods. \begin{figure}[ht!] \plotone{longterm_v5.eps} \caption{The long term light curves of M31N2013. On the x axis, we report time since the first detection of Nova. {\it Upper Panel:} the UV/optical magnitudes clearly show an exponential decrease of the optical nova (dashed line) after the maximum (vertical dotted line). {\it Lower Panel:} the equivalent X-ray luminosity L$_{50}$, as obtained from all X-ray instruments, shows instead a less clear decay of the X-ray source. Errors are at 1$\sigma$ and upper limits at 3$\sigma$. \label{fig:lc-long}} \label{longterm} \end{figure} | 18 | 8 | 1808.06955 |
1808 | 1808.06678_arXiv.txt | We present the first fully general relativistic dynamical simulations of Abelian Higgs cosmic strings using 3+1D numerical relativity. Focusing on cosmic string loops, we show that they collapse due to their tension and can either (i) unwind and disperse or (ii) form a black hole, depending on their tension $G\mu$ and initial radius. We show that these results can be predicted using an approximate formula derived using the hoop conjecture, and argue that it is independent of field interactions. We extract the gravitational waveform produced in the black hole formation case and show that it is dominated by the $l=2$ and $m=0$ mode. We also compute the total gravitational wave energy emitted during such a collapse, being $0.5\pm 0.2~ \%$ of the initial total cosmic string loop mass, for a string tension of $G\mu=1.6\times 10^{-2}$ and radius $R=100~\mpl^{-1}$. We use our results to put a bound on the production rate of planar cosmic strings loops as $N\lsim 10^{-2}~\mathrm{Gpc}^{-3}~\mathrm{yr}^{-1}$. | \label{sect:intro} The recent detection of Gravitational Waves (GW) from black hole (BH) \cite{Abbott:2016blz} binaries by the LIGO/VIRGO collaboration marked the start of a new era of observations. Beyond astrophysical objects such as BH and neutron stars, this paved the way for the use of GW to search directly for signatures of new physics. One of the key targets for this search are cosmic strings \cite{Kibble:1976sj,Abbott:2017mem,Copeland:2009ga}. Cosmologically, cosmic strings networks naturally arise after a phase transition in the early universe, possibly during GUT symmetry breaking. More speculatively, string theory also suggests the presence of cosmological fundamental superstrings, especially through the mechanism of brane inflation \cite{Pogosian:2003mz,Jones:2003da}. These networks may manifest themselves through several channels, such as imprints via lensing on the Cosmic Microwave Background \cite{Ade:2013xla} and possibly through the presence of a stochastic gravitational wave background. The latter in particular is recently searched for by the LIGO/VIRGO collaboration \cite{Abbott:2017mem}. More intriguingly, one can search for localized coherent events of these strings, such as when the strings self-interact through the formation of sharp cusps or through the collisions of traveling kinks that are formed during the intercommutation (i.e. collisions) of cosmic strings. Before this work, the two primary methods of modeling cosmic strings has been through solving the field theory equations in flat or expanding spacetime, or through an effective Nambu-Goto prescription with weak coupling to gravity (see e.g. \cite{Vilenkin:2000jqa}). In either case, by considering the stress-energy of a network of strings, one can then compute in the weak gravity limit a stochastic GW background \cite{Vilenkin:1981bx,Damour:2004kw}. Local events such as the collisions of traveling kinks and cusps along the strings are expected to produce bursts of GW -- these bursts events have been computed using the Nambu-Goto approximation, again in the weak field limit \cite{Damour:2004kw}. These two methods do not coincide in general, mainly due to their disagreement on the primary energy loss mechanism of the cosmic strings (see \cite{Hindmarsh:2017qff,Vincent:1997cx,Moore:1998gp,Olum:1998ag, Moore:2001px,BlancoPillado:2011dq}). \begin{figure}[tb] \begin{center} {\includegraphics[width=0.9\columnwidth]{GWsignal.png}} \caption{\textbf{GW for a BH formed from circular cosmic string loop collapse:} We plot the real part of the dominant $l = 2$ $m = 0$ mode of $r\Psi_4$ over time. The loop has tension $G\mu = 1.6\times 10^{-2}$ and an initial radius $R = 100~\mpl^{-1}$. The grey shaded area of the plot are mixed with stray GWs that arise as artifacts of the initial data. The x-axis $t_{\mathrm{ret}}=t-r_\mathrm{ext}$ is the retarded time where $r_\mathrm{ext}$ is the extraction radius.} \label{fig:WaveForm_sub} \end{center} \end{figure} Going beyond the weak field limit requires the finding of the solutions to the full field theory coupled to general relativity -- and in this work we present the \emph{first numerical relativity simulation of Abelian Higgs cosmic strings in full general relativity}. In this first paper of a series, we numerically explore the collapse of a circular cosmic string loop in extreme regimes ($4\times 10^{-3}<G\mu<4\times 10^{-2}$). We show that whether the loop collapses into a BH or unwinds itself depends on a simple analytic relation derived using the hoop conjecture. In the former case, we computed both the gravitational waveform (fig. \ref{fig:WaveForm_sub}) and its integrated GW energy emitted from such a collapse. For the latter, we found that the total energy emitted in gravitational waves is $0.5\pm 0.2~\%$ of the initial mass, which is in agreement with the bound of $<29\%$ \cite{HAWKING199036}.We will discuss direct detection prospects of such individual collapse events with GW detectors in section \ref{sect:discussion}. | \label{sect:discussion} We have extracted the gravitational wave signal for the case $G\mu = 1.6\times 10^{-2}$, and $R = 100~ \mpl^{-1}$ and found that the efficiency $\epsilon=0.5\pm 0.2 ~\%$ of the initial mass is radiated into gravitational waves. The QNM frequency of our GW waveform (fig. \ref{fig:WaveForm_sub}) is in the UV range and out of any current or future detectors. On the other hand, if we assume that our numerical results scale, we can ask whether we can detect suitably massive cosmic strings loops with current or future detectors. The two key parameters are $(i)$ the frequency and $(ii)$ the luminosity of the event, both which depend on the masses. The former constraints our loop parameter space to $2\pi\mu R \approx M_{detector}$. We choose $M_{detector}$ such that its frequency lies at peak sensitivity of LIGO/VIRGO ($f\sim~100Hz$). For the latter, the strain $h$ observed at a distance $d$ from a source of GWs is \begin{equation} \left(\frac{h}{10^{-21}}\right)\sim \sqrt{\frac{E_{\mathrm{GW}}}{3\times 10^{-3} M_\odot}}\left(\frac{10~\mathrm{Mpc}}{d}\right)~. \label{eqn:distance_d} \end{equation} Cosmic strings loops are generated during the evolution of the string network when strings intercommute, although there is presently no consensus on the probability distribution of loops and their classification (see e.g. \cite{Blanco-Pillado:2017oxo,Ringeval:2017eww}). Furthermore, it is not clear that all loops will collapse due to the presence of non-intersecting loop configurations and the uncertainty in their angular momentum loss mechanisms. Hence, we will take the agnostic view that only planar loops will collapse -- assuming that planar loops will circularize as argued by \cite{Hawking1989}. Suppose then $N(R,z)$ is the co-moving production density rate of planar loops of radius $R$ at redshift $z$ (i.e. it has dimensions $[N(R,z)] = L^{-3}T^{-1}$), then the detection rate is given by \begin{equation} \Gamma = \int_{0}^{z_d}4\pi \left[\int_{0}^{z}\frac{dz'}{H(z')}\right]^2\frac{N(r,z)dz}{H(z)}~,~d= \int_{0}^{z_d}\frac{dz}{(1+z)H}~. \end{equation} such that $z_d$ is the maximum range in redshift of the detector, which itself depends on the energy of the GW $E_{\mathrm{GW}}$ emitted. Our numerical results \eqn{eqn:epsilon} suggest that $0.5\%$ of the total string loop mass is emitted, which is an order of magnitude smaller than that of the typical BH-BH mergers, translating to about a factor of 3 shorter in detectable distance $d$. For LIGO/VIRGO and ET, the maximum redshift range is then $z_d\sim 0.005$ and $z_d\sim 0.05$ respectively. In this limit, $\Gamma$ can be approximated as \begin{equation} \Gamma \approx \epsilon^{3/2}\left(\frac{R}{GM_{\odot}}\right)^{3/2}(G\mu)^{3/2}\left(\frac{10^{-19}}{h}\right)^{3}\left(\frac{N(R,z)}{\mathrm{Mpc}^{-3}}\right)~. \end{equation} Clearly, $\Gamma$ depends linearly on $N(R,z)$, which itself depends on the cosmic string model and its network evolution, which at present is still being debated vigorously as mentioned above. For example, in \cite{Hawking1989}, it was estimated that $N(R,z)\propto (G\mu)^{2R/s-4}$ where $s$ is the correlation length of the loop. Other estimates are given in \cite{Polnarev1991, Caldwell1996}. On the other hand, we can use the non-detection of such collapse events in the present LIGO/VIRGO to put a constraint on $N(R,z)$. For $G\mu\sim 10^{-10}$ which leads to solar system sized loops of $R\sim {\cal O}(100)$ a.u., this is $N(R,z)< 10^{-2}~ \mathrm{Gpc}^{-3}~\mathrm{yr}^{-1}$, which is a lower detection rate than what is expected from BH mergers of ${\cal O}(10)~ \mathrm{Gpc}^{-3}~\mathrm{yr}^{-1}$ \cite{LIGOScientific:2018mvr}. Finally, we note that this is a conservative estimate since these solar system sized loops satisfy $R_{\mathrm{BH}}\sim{\cal O}(10^{40}) \times \delta$ and hence are thin loops. In this limit, $\epsilon$ might be closer to $29~\%$, with a corresponding increase in $d$. We will numerically investigate the collapse of these thin loops in a future work.\\ | 18 | 8 | 1808.06678 |
1808 | 1808.04027_arXiv.txt | Models of spontaneous baryogenesis have an interaction term $\partial_\mu\theta j^\mu_B$ in the Lagrangian, where $j^\mu_B$ is the baryonic current and $\theta$ can be a pseudo-Nambu-Goldstone boson. Since the time component of this term, $\thetadot j^0_B$, equals $\thetadot n_B$ for a spatially homogeneous current, it is usually argued that this term implies a splitting in the energy of baryons and antibaryons thereby providing an effective chemical potential for baryon number. In thermal equilibrium, one {then obtains} $n_B \sim \thetadot T^2$. We however argue that a term of this form in the Lagrangian does not contribute to the single particle energies of baryons and antibaryons. We show this for both fermionic and scalar baryons. But, similar to some recent work, we find that despite the above result the baryon number density obtained from a Boltzmann equation analysis can be proportional to $\thetadot T^2$. Our arguments are very different from that in the standard literature on spontaneous baryogenesis. | Observations indicate that our Universe possesses a baryon asymmetry. The conventional approach to baryogenesis in cosmology is based on the three well known (and necessary) Sakharov's conditions \cite{Sakharov:1967dj}: (i) violation of baryon number (ii) violation of C- and CP-symmetries and (iii) being out of thermal equilibrium. However, there exist some interesting scenarios wherein one or more of these conditions are not satisfied. The spontaneous baryogenesis scenario is one such novel scenario in which the CP-symmetry is not violated and the baryon asymmetry is generated in thermal equilibrium. Models of spontaneous baryogenesis \cite{Cohen:1987vi,Cohen:1988kt} have an interaction of the form $\partial_\mu\theta j^\mu$ in the Lagrangian density, where $j^\mu$ is related to the baryonic current and $\theta$ may be a pseudo-Nambu-Goldstone boson. Now, $\int d^3x j^0=Q$, where $Q$ is the charge associated with $j^\mu$, and ignoring spatial variations in $j^0$, $j^0=Q/V= n$, where $n$ is the net number density of the quanta associated with scalars or fermions $\phi$ or $\psi$. The coefficient of $n$, i.e. $\thetadot$, has been interpreted to be equivalent to an energy splitting in particle and antiparticle energies and thus an effective chemical potential for $\phi$ or $\psi$, provided the rate of change of $\dot\theta$ is sufficiently slow. This can then give rise to a particle-antiparticle asymmetry in thermal equilibrium. This interpretation has been invoked in spontaneous baryogenesis, including at the electroweak phase transition, and in flat direction baryogenesis, radion baryogenesis, quintessential baryogenesis, etc. In this article we question the arguments underlying the above interpretation. We argue that a $\thetadot n$ term in the Lagrangian density does not necessarily imply a split in the energies of particles and antiparticles and hence does not automatically lead to an interpretation of $\thetadot$ being an effective chemical potential. We also argue that dispersion relations $k^0({\bf k})$ do not necessarily give particle and antiparticle energies. For the latter one must obtain the Hamiltonian and take its expectation value in single particle and antiparticle states. The energies one obtains do not always agree with the expressions for $k^0$. In particular, while $k^0$ may contain $\thetadot$ the single particle and antiparticle energies may not. For the models under discussion we include a baryon number violating interaction and further study the Boltzmann equation, similar to the approach of Ref. \cite{Arbuzova:2016qfh}. For scenarios with a $\thetadot j^0$ term in the Lagrangian density, the dispersion relations are modified, but, interestingly, even for cases where single particle and antiparticle energies are the same one does get a net baryon asymmetry due to the modified dispersion relations. Depending on the baryon number violating term, one gets different expressions for the asymmetry. This mechanism of generation of asymmetry from a $\thetadot j^0$ term is very different from that originally proposed in spontaneous baryogenesis and similar scenarios. Spontaneous baryogenesis models \cite{Cohen:1987vi,Cohen:1988kt} also consider the generation of baryon asymmetry in the oscillating phase of the $\theta$ field. This has been further commented upon in Refs. \cite{Dolgov:1994zq,Dolgov:1996qq} and we do not consider this here. The outline of our article is as follows. In Sections \ref{sec:fermions} and \ref{mynote} we discuss the case of a fermion current coupled to the derivative of a field $\theta$. We obtain the dispersion relation and the single particle and antiparticle energies. We then perform an analysis using the Boltzmann equation. In Sec. \ref{sec:scalars1} we consider the case of a scalar field with an interaction similar to that in Sec. II, i.e., a coupling of the scalar field current with $\partial_\mu\theta$. In Sec. \ref{sec:scalars2} we consider the case of a scalar field with a self interaction of the form $g_2 n$, where $g_2$ is a constant. In both cases, as in the fermionic case, we obtain the dispersion relations and single particle and antiparticle energies and then perform an analysis using the Boltzmann equation. Finally, we summarize our conclusions in Sec. \ref{sec.conclusion}. | \label{sec.conclusion} Spontaneous baryogenesis presumes that a term of the form $\thetadot j^0 \sim \thetadot n$ in the Lagrangian density, where $j^0$ is the zeroth component of the particle current and $n$ is the net particle number density, translates into a splitting of energies of particles and antiparticles, and therefore acts as an effective chemical potential which then gives rise to a matter-antimatter asymmetry or baryon asymmetry in a system in thermal equilibrium. Our analysis above implies that there are two separate issues here. One is whether or not the term in the Lagrangian density gives rise to an energy splitting, and the second is whether or not one obtains a matter-antimatter asymmetry. For both fermions and scalars we find in Secs. \ref{sec:fermions} and \ref{sec:scalars1} that $\thetadot j^0$ modifies the mode functions of the corresponding quantum fields, but it does not lead to a splitting of single particle and antiparticle energies. However, because of the modified mode functions, if baryon number violating interactions are in the thermal equilibrium then equating the collision integral on the r.h.s. of the Boltzmann equation to 0 gives a non-zero chemical potential for particle number. This then gives rise to a matter-antimatter asymmetry, or baryon asymmetry if the particles carry baryon number. It may be noted that obtaining the Hamiltonian density in terms of the field and its time derivative and trying to relate the presence or absence of a $\thetadot j^0$ in the Hamiltonian density with particle-antiparticle energy splitting is inappropriate and misleading. For the fermionic case there is no $\thetadot j^0$ term in the Hamiltonian density while it does appear for the scalar case. But in both cases the single particle energies for particles and antiparticles are the same. The presence or absence of $\thetadot j^0$ in the Hamiltonian density does not depend whether or not $\theta$ is a dynamical field or an external field (like a classical background field). In the latter case, the equation of motion of the field is not entirely determined by the Lagrangian under study but one still includes $\Pi_\theta \thetadot$ in the Hamiltonian density. Therefore our conclusions above are the same irrespective of whether $\theta$ is a dynamical or an external field. The presumption that a term of the form $\thetadot j^0 \sim \thetadot n$ in the Lagrangian density gives rise to a splitting in energies of particles and antiparticles has been widely used in the literature in models of spontaneous baryogenesis, including at the electroweak phase transition, flat direction baryogenesis, radion baryogenesis, quintessential baryogenesis, etc. Our analysis above indicates that this presumption may not hold even though such a term may ultimately give rise to a matter-antimatter asymmetry. We have also studied the case where the field derivative $\thetadot$ is replaced by a constant, in Sec. \ref{sec:scalars2}. This modifies the mode functions and gives a splitting in energies of particles and antiparticles. Then both the energy splitting and any chemical potential, depending on the number violating interactions in thermal equilibrium, will contribute to the asymmetry in number densities of particles and antiparticles. These conclusions would also hold for a term of the form $h(\theta) j^0$. We conclude by comparing our results with that of Ref. \cite{Arbuzova:2016qfh} for the case of a fermionic current coupled with the derivative of the $\theta$ field. In Eq. (4.3) of Ref. \cite{Arbuzova:2016qfh} the energies of particles and antiparticles are obtained from the dispersion relation. We argue that the dispersion relation gives expressions for $k^0$ which enter in the mode functions of the fermion field $\psi$ (see our Eqs. (\ref{f}) and (\ref{g})) but the single particle and antiparticle energies are given by the expectation value of the Hamiltonian in Eq. (\ref{eq:finalfermionhamiltonian}) in single particle and antiparticle states. We too find that the $k^0$ associated with $u$ and $v$ spinors depend on $\dot\theta$ and are different, but our Hamiltonian implies that particles and antiparticles have the same energy which is independent of $\dot\theta$. Our analysis of the collision integral in the Boltzmann equation in Sec. \ref{mynote} was done for the case where the quark field is rotated to absorb a phase factor of $\exp(i\theta)$ after spontaneous symmetry breaking which gives rise to the $(\partial_\mu \theta) J^\mu$ in the Lagrangian density. In Ref. \cite{Arbuzova:2016qfh} it was done for the unrotated case and our result for the baryon asymmetry generated in thermal equilibrium for the rotated quark fields agrees with their result. Ref. \cite{Arbuzova:2016qfh} also obtains a result for the baryon asymmetry for the rotated case using other arguments. This has an error but after correction it is in agreement with that for the unrotated case and our result (see the footnote below our Eq. (\ref{eq:fermionbaryonnumber})). | 18 | 8 | 1808.04027 |
1808 | 1808.06807_arXiv.txt | {We have examined dust emission in galaxy clusters at millimeter wavelengths using the \Planck\ $857 \, {\rm GHz}$ map to constrain the model based on {\em Herschel} observations that was used in studies for the Cosmic ORigins Explorer (CORE) mission concept. By stacking the emission from \Planck-detected clusters, we estimated the normalization of the infrared luminosity versus mass relation and constrained the spatial profile of the dust emission. We used this newly constrained model to simulate clusters that we inject into \Planck\ frequency maps. The comparison between clusters extracted using these gas+dust simulations and the basic gas-only simulations allows us to assess the impact of cluster dust emission on \Planck\ results. In particular, we determined the impact on cluster parameter recovery (size, flux) and on \Planck\ cluster cosmology results (survey completeness, determination of cosmological parameters). We show that dust emission has a negligible effect on the recovery of individual cluster parameters for the \Planck\ mission, but that it impacts the cluster catalog completeness, reducing the number of detections in the redshift range [0.3-0.8] by up to $\sim 9\%$. Correcting for this incompleteness in the cosmological analysis has a negligible effect on cosmological parameter measurements: in particular, it does not ease the tension between \Planck\ cluster and primary cosmic microwave background cosmologies.} | Quantifying dust emission from galaxy clusters is interesting for both astrophysical and cosmological studies. Dust emission from member galaxies is a tracer of the star formation rate (SFR) in dense environments \citep{alberts2014, alberts2016}, and the question of intracluster dust embedded in the hot intracluster medium (ICM) concerns stellar feedback and the physical state of the ICM \citep{montier2004}. Cluster dust emission also has potentially important ramifications for cosmology from Sunyaev-Zeldovich (SZ) cluster counts because it can contaminate the SZ signal and modify survey selection functions. One of the first appearances of contaminating dust emission was found by \cite{planck2013-xi} when stacking the SZ signal from central halo galaxies. Contamination by dust emission came to dominate the SZ signal when approaching the low-mass group scale. \citet{hurier2016} and \citet{comis2016} examine dust emission by stacking signal in the high frequency \Planck\ maps around massive clusters from the \Planck\ catalogs. The former work separated the dust and SZ signals to conclude that the dust emission evolved with redshift and was more spatially extended than the SZ signal. The authors of the latter work combined IRAS data with \Planck\ observations to measure dust temperature and determine dust masses in cluster systems. These studies extend the work of \citet{montier2005} and \citet{giard2008}, who detected dust emission by stacking IRAS maps around clusters. Because the \Planck\ beam has an angular extent similar to or larger than that of the studied clusters, these observations integrate their total emission. \citet{hurier2016} find that the dust emission could be fully accounted for by cluster member galaxies, in agreement with previous work \citep{roncarelli2010}, a conclusion further supported by the temperatures of $T\sim 20$\,K determined by \citet{comis2016} that are typical of late-type galaxies. Little attention has yet been given to studying the impact of dust emission on SZ cluster surveys, largely because the level of the emission relative to the SZ signal is poorly known. The large surveys by the Atacama Cosmology Telescope \citep[ACT,][]{hasselfield2013}, the South Pole Telescope \citep[SPT,][]{bleem2015} and the \Planck\ mission \citep{planckcatalogue2016} do not model the effect of dust emission on their selection functions and photometry. Any effect will depend in detail on the observation bands and how they are used in cluster detection. In this paper, we examine dust emission from massive, intermediate redshift clusters using \Planck\ observations and evaluate its impact on the \Planck\ SZ cluster selection function and photometry. The data are presented in Sect.~\ref{sec:data}. We proceed by first establishing a baseline model (Sect. \ref{sec:dust_model}) for cluster dust emission and fit key model parameters with our \Planck\ measurements (Sect.~\ref{sec:constraints}). These parameters are the normalization of the infrared (IR) luminosity-cluster mass relation and the spatial extent of the dust emission. We then simulate \Planck\ observations of clusters with both SZ signal and dust emission to quantify the effect of the dust emission on the \Planck\ SZ selection function and photometry (size and SZ flux) in Sect.~\ref{sec:impact}. We conclude in Sect.~\ref{sec:ccl}. Throughout the paper, we adopt the \Planck\ $\Lambda$CDM cosmology \citep[TT,TE,EE+lowP+lensing+ext in Table 4 of][]{planckcosmo2016}: $h = H_0 / (100 \, \rm{km s^{-1} Mpc^{-1})} = 0.6774$, $\Omega_{\rm m}=1-\Omega_{\Lambda}=0.3089$, $\Omega_b=0.0485976$, $n_{\rm s}=0.9667$. \begin{figure} \centering \adjincludegraphics[width=\hsize,trim={0 0 0 {.25\width}},clip]{Figs/fluxdens_vs_z_857GHz_planck_lowz_withcounts_v4-eps-converted-to.pdf} \caption{{\it Top:} Redshift distribution of our PSZ2 sample (1091 clusters). {\it Bottom:} Predicted cluster dust flux density versus redshift from the~\cite{dezotti2016} model. The dust emission is integrated within a sphere of radius $R_{500}$ for a cluster of mass $M_{500}=10^{14.5} M_\sun$ in the 857~GHz \Planck\ band. We fixed $r_L=1$ (see Eq.~\ref{eq:lirlowz}) in this figure.} \label{fig:cai_flux_vs_nu} \end{figure} | \label{sec:ccl} We have modeled dust emission in galaxy clusters at millimeter wavelengths using the model by~\cite{dezotti2016}, which we augmented by stacking PSZ2 clusters. The model now gives the shape of the dust profile and a normalization for the dust emission based on the \Planck\ 857~GHz channel. We used this model to simulate clusters that we injected into the \Planck\ maps. We then assessed the impact of dust emission on \Planck\ cluster results, finding that: \begin{itemize} \item Dust emission is not responsible for the cluster size over-estimation seen in the real data. \item The size over-estimation is plausibly caused by a mismatch, in the external regions, between the true cluster pressure profiles and the UPP adopted in the cluster extraction tool. \item When fixing cluster size and position, dust emission biases \Planck\ cluster flux measurements low at only the 1 to 2\% level. \item Dust emission impacts the completeness of the cluster cosmology catalog over the redshift range [0.3-0.8], with a maximum loss of $\sim 9\%$ of clusters between $z=0.5$ and $z=0.8$. \item This cluster loss has a negligible effect on cosmological parameter estimation. Taking dust contamination into account in the \Planck\ cluster cosmology analysis does not help to ease the tension with the primary CMB. \end{itemize} Our constraints on the cluster dust emission model are general, and the calibrated model can be used to evaluate the impact of dust emission on other SZ surveys. This is of particular interest for the highly sensitive next generation SZ cluster surveys such as the Advanced ACT \citep{advact}, SPT-3G \citep{spt-3g}, the Simons Observatory\footnote{\tt http://simonsobservatory.org}, CMB-S4 \citep{cmb-s4}, CORE~\citep{delabrouille2017,melin2017}, and PICO~\citep{hanany2018}. | 18 | 8 | 1808.06807 |
1808 | 1808.04802_arXiv.txt | We use numerical simulations of molecular cloud formation in the colliding flow scenario to investigate the reliability of observational estimates of the angular momenta of early-state, low-mass protostellar cores. We show that, with suitable corrections for projection factors, molecular line observations of velocity gradients in NH$_3$ can be used to provide reasonable estimates of core angular momenta within a factor of two to three, with a few large underestimates due to unfavorable viewing angles. Our results differ from previous investigations which suggested that observations might overestimate true angular momenta by as much as an order of magnitude; the difference is probably due to the much smoother velocity field on small scales in our simulations, which result from allowing turbulence to decay and gravitational infall to dominate. The results emphasize the importance of understanding the nature of ``turbulent'' velocities, with implications for the formation of protostellar disks during core collapse. | \label{sec:intro} The angular momenta of collapsing protostellar clouds establish the properties of binary and multiple stars and the initial conditions of their protoplanetary disks. Recent observations have demonstrated that disks of appreciable size can form early, during the Class I and even the Class 0 protostellar phases (\citealt{tobin12b,ohashi14,aso15,yen15,oya16}; see \citealt{takakuwa17} for a review). However, simulations of collapsing magnetized protostellar cores over the last few years have indicated that magnetic fields can in some circumstances remove so much angular momentum that large disks do not form \citep[see, e.g.][for reviews]{seifried13,li14}. Ambipolar diffusion can play a crucial role in reducing the magnetic flux responsible for angular momentum transport outward \citep{masson16,vaytet18}, although it is unclear whether it is sufficient to allow the formation of disks during the Class 0 phase \citep{dapp12}. Alternatively, substantial misalignment of the magnetic field relative to the net rotation axis, and/or diffusion of the field in a turbulent environment, can allow early disk formation \citep{joos12,joos13,santos-lima12,santos-lima13,seifried12,seifried13,krumholz13,lewis15,wurster16,wurster17}. In view of the theoretical uncertainties, observational estimates of infalling protostellar core angular momenta (and their directions) can be useful. Following the classic work by \cite{goodman93} on the rotation of protostellar cores, additional observational estimates of angular momenta have been made \citep{caselli02} \citep[see the review by][]{belloche13}, with added emphasis on the angular momenta of inner envelopes \citep{yen15} to help constrain disk formation. However, numerical simulations by \cite{offner08} and \cite{dib10} suggest that observations can overestimate true angular momenta by as much as a factor of 10. These large bias factors presumably result from the assumed complexity of the turbulent velocity field, which cancels out some of the angular momentum on small scales that is smoothed out on large scales with the usual observational procedures. This interpretation is supported by the results of \citeauthor{offner08} indicating that the bias is smaller if the initial seeded turbulent velocity field is allowed to decay rather than be continuously driven. The approach to developing initial conditions that we favor is one in which the turbulence is the result of cloud formation, followed by gravitational collapse \citep[ e.g.,][]{Heitsch+06,bp2011}. The supersonic velocity fields at the time of star (sink particle) formation are then the result (mostly) of overdensities in the cloud which drive motion gravitationally. This arguably results in a smoother velocity field than in the case of continued driving of turbulence by some unspecified mechanism; in turn, this could mean that observational estimates of angular momenta could be more representative of true values. To investigate this further, in this paper we employ the results of numerical simulations for dense cores, post-process them with radiative transfer, and use the same methods of analysis for closer comparison with observational results. | \label{sec:conclusion} We have analyzed the results of numerical simulations of star formation with and without magnetic fields with decaying turbulence, with and without radiative transfer post-processing. Using the same methods as typically used to obsevationally estimate specific angular momenta $j$ for protostellar cores, we find that even though the assumption in these methods of uniform rotation and simple power-law density distributions are not correct in detail, the methods can still serve to produce estimates of $j$ that are within a factor of two to three of the real values. These results are in contrast with other findings in cases where the turbulence is continually driven by an unspecified mechanism. Our simulations do not include stellar feedback. Outflows from low-mass protostars pose a particular challenge to distinguish expansion from undisturbed core velocity fields \citep[e.g.,][]{tobin11}, which can dominate uncertainties in estimating core angular momenta. Simulations including schematic bipolar outflows could help constrain these uncertainties. In any case, our results suggest that it is worthwhile and important to continue observational efforts to estimate angular momenta of protostellar cores, especially given the significance of such studies for understanding the formation of protoplanetary disks. | 18 | 8 | 1808.04802 |
1808 | 1808.03878_arXiv.txt | Vela~X is the prototypical example of a pulsar wind nebula whose morphology and detailed structure have been affected by the interaction with the reverse shock of its host supernova remnant. The resulting complex of filamentary structure and mixed-in ejecta embedded in a nebula that is offset from the pulsar provides the best example we have of this middle-age state that characterizes a significant fraction of composite SNRs, and perhaps all of the large-diameter PWNe seen as TeV sources. Here we report on an {\it XMM-Newton} (hereafter {\it XMM}) Large Project study of Vela~X, supplemented by additional \chandra\ observations. Through broad spectral modeling as well as detailed spectral investigations of discrete emission regions, we confirm previous studies that report evidence for ejecta material within Vela~X, and show that equivalent width variations of O VII and O VIII are consistent with temperature maps within the PWN that show low-temperature regions where the projected SNR emission appears to dominate emission from the ejecta. We identify spectral variations in the nonthermal emission, with hard emission being concentrated near the pulsar. We carry out investigations of the Vela~X ``cocoon'' structure and, with hydrodynamical simulations, show that its overall properties are consistent with structures formed in the late-phase evolution of a composite SNR expanding into a surrounding medium with a density gradient, with ejecta material being swept beyond the pulsar and compressed into an elongated structure in the direction opposite the high external density. | Located at a distance of only $\sim 290$~pc \citep{Dodson_etal03}, the Vela supernova remnant (SNR; Figure 1) houses a young pulsar that powers the extended pulsar wind nebula (PWN) Vela X. The PWN extends to the south of the pulsar, apparently the result of an asymmetric reverse shock (RS) interaction associated with a large-scale density anisotropy surrounding the SNR \citep[e.g.,][]{Blondin_etal01}. Observations across the electromagnetic spectrum have been used to characterize the emission properties and overall structure of the PWN, as described below. Radio observations of the Vela SNR \citep[G263.9$-$3.3;][]{Milne68} show a large ($\sim 6^\circ$ diameter) shell-type remnant with a central flat-spectrum PWN (Vela X). Studies of neutral hydrogen in the Vela direction reveal a thin shell surrounding the SNR, with a density $n_0 \approx 1 {\rm\ cm}^{-3}$ \citep{Dubner_etal98}, while X-ray observations of the SNR with the {\it ROSAT} observatory \citep{Aschenbach_etal95} show a limb-brightened shell of thermal X-rays whose brightest emission is in the northeastern hemisphere, toward the Galactic plane. An overall asymmetry of Vela is evident, with the emission in the south/southwest extending to larger radii (Figure 1). Distinct bowshock-like structures are observed along, or just outside, the SNR shell, with subsequent studies confirming that these appear to be high-velocity ejecta fragments that have exited the SNR \citep[e.g.,][]{Tsunemi_etal99}. \citet{Bocchino_etal99} show that the emission in the northeast (NE) region is characterized by two distinct temperature components ($kT_1 \approx 0.1$~keV, $kT_2 \approx 0.5$~keV), possibly indicating expansion into a surrounding medium with dense clouds and a low-density inter-cloud medium (ICM; $n_{\rm ICM} \approx 0.01 - 0.1 {\rm\ cm}^{-3}$). Using \xmm\ observations, \citet{Miceli_etal05} carried out observations of shock-cloud interactions along the northern rim of Vela and concluded that the two distinct temperature components reported in earlier studies are both associated with the clouds, leading to an upper limit $n_{\rm ICM} < 0.06 {\rm\ cm}^{-3}$. Optical and far-UV studies of Vela ``fragment D,'' located outside the eastern limb, indicate interaction of this ejecta knot with an external cloud with $n_{cl} \approx 4 - 11 {\rm\ cm}^{-3}$, providing additional evidence for a higher overall density in the northern and western regions of the SNR \citep{Sankrit_etal03}. \begin{figure*}[t] \includegraphics[width=7in]{fig1.pdf} \caption{ Left: { \it ROSAT} image of the Vela SNR (G263.9$-$3.3). Soft emission is shown in red and hard emission in blue/green. The contour is the outer radio boundary of Vela X. The individual circles identify \xmm\ pointings discussed in the text. The dashed circular region was taken in small-window mode and was used for imaging, but not for spectral analysis. The numbered circles in the lower right illustrate regions used for spectral fitting of each pointing (see Figure 6). Upper right: Zoomed-in region of {\it ROSAT} image showing pulsar and cocoon. The outermost radio contour is shown to illustrate the extent of Vela~X. Lower right: \chandra\ image of the Vela pulsar and its surrounding compact nebula. The jet axis is in the SE-NW direction, roughly aligned with the pulsar proper motion. The direction to the Vela~X cocoon is indicated. } \label{fig1} \end{figure*} A $^{12}{\rm CO}$ survey of molecular clouds (MCs) in the vicinity of the Vela SNR \citep{Moriguchi_etal01} establishes the presence of a high MC concentration outside the northern limb of the remnant, but little CO in the west/southwest where the X-ray emission is more extended. This suggests an inhomogeneous density distribution in the pre-explosion environment with an inter-cloud density $n_{\rm ICM} \approx 0.01{\rm\ cm}^{-3}$, similar to that inferred from X-ray studies, although the presence of an HI shell indicates a cooling shock that requires a considerably higher density given the observed SNR radius, illustrating that the medium surrounding the SNR is complex. The Vela pulsar, located in the central regions of the SNR, has a spin period of 89.3 ms, a characteristic age $\tau_c = 11.3$~kyr, and a spin-down power $\dot{E} = 7 \times 10^{36} {\rm\ erg\ s}^{-1}$. \chandra\ observations show that the pulsar is surrounded by a compact nebula (Figure 1, lower right) with distinct features corresponding to an inclined jet-torus structure \citep{Helfand_etal01}. VLBI parallax measurements \citep{Dodson_etal03} establish a distance $d = 287^{+19}_{-17}$~pc and a proper motion that, when combined with the torus inclination, establishes a pulsar velocity of $\sim 80 {\rm\ km\ s}^{-1}$. The proper motion is along the direction of the pulsar jet axis, providing evidence for alignment of the kick velocity with the pulsar spin axis. Radio observations of Vela~X reveal a morphology concentrated to the south of the pulsar itself, suggesting that the SNR RS has propagated more rapidly from the northern direction due to a higher ambient density, thus leading to disruption of the northern part of the PWN. Higher resolution radio images \citep{Bock_etal98} show a network of filamentary structure in the PWN, possibly formed by Rayleigh-Taylor (R-T) instabilities in this interaction with the RS (Figure 2). ROSAT observations of the Vela~X region \citep{MO95} reveal a large emission structure -- the so-called ``cocoon'' -- extending $\sim 45$~arcmin to the south of the pulsar (seen as a distinct blue structure in Figure 1, upper right). The region is characterized by a hard spectrum and appears to lie along a bright elongated radio structure \citep{Frail_etal97}. ASCA observations established a two-component X-ray spectrum with the hard component adequately described by either a power law or a hot thermal plasma \citep{MO97}. Radio polarization maps show that the magnetic field in Vela~X is roughly aligned with the cocoon in the central region \citep{Bock_etal02}. \begin{figure*}[t] \centerline{\includegraphics[width=6.5in]{fig2.pdf}} \caption{ Left: MOST radio image of Vela X showing filamentary structure within the extended nebula. The single contour is the outermost contour from the diffuse radio emission in Vela X. White regions in the image correspond to negative fluxes associated with missing coverage due to a lack of small baselines. Right: Exposure-corrected mosaic of \xmm\ pointing in Vela~X, shown in blue, along with the MOST radio image, shown in red. The bright, elongated radio structure in the central region lies adjacent to the X-ray cocoon. } \label{fig2} \end{figure*} Using the {\it CANGAROO} telescope, \citet{Yoshikoshi_etal97} detected $\gamma$-ray emission at energies above 2.5~TeV, located somewhat southeast of the pulsar. Assuming inverse-Compton scattering off of the same electron population responsible for the hard X-ray spectrum, they estimated a magnetic field strength $B \lsim 4 \mu$G. Subsequent observations with {\it H.E.S.S.} identified a TeV nebula larger than the X-ray cocoon, with a spectrum consistent with either a broken power law or a single power law with an exponential cutoff \citep{Aharonian_etal06}. The brightest region of TeV emission is concentrated along the X-ray cocoon. Using \xmm\ observations along the central region of the cocoon, \cite{LaMassa_etal08} detected two distinct emission components -- a power law with a spectral index of $\sim 2.2$ and a thermal plasma with enhanced abundances of O, Ne, and Mg, presumably associated with ejecta that have been mixed into the PWN upon interaction with the RS. Broadband spectral modeling showed that the radio, nonthermal X-ray, and TeV $\gamma$-ray emission can be understood as the result of synchrotron and IC emission, but with a broken power law or a distinct population of radio-emitting electrons. A magnetic field strength of $\sim 5 {\mu}$G was estimated, consistent with the value estimated from TeV studies. Modeling by \cite{deJager_etal08} predicted observable GeV emission from Vela X based on a leptonic model for the TeV emission. The PWN is observed at energies up to $\sim 200$~keV with {\it BeppoSAX} \citep{Mangano_etal05}, and observations with \fermi\ reveal \gamray\ emission extended over the entire radio-emitting region, but with a centroid that is distinctly offset toward the west from the peak of the TeV emission \citep{Abdo_etal10}. X-ray observations with \suzaku\ \citep{Katsuda_etal11} identify nonthermal emission in the northeast that appears to extend beyond the radio emission, leading to the suggestion that perhaps the X-ray and radio-emitting components are associated with different electron populations. Here we describe X-ray observations carried out in an \xmm\ Large Project to study Vela X, along with \chandra\ observations coincident with the early region studied by LaMassa et al. (2008). We apply a hydrodynamical model, constrained by the observed properties of Vela~X, to investigate the formation of the cocoon structure. In Section 2, we describe our observations and data reduction. The results of our spectral analysis are presented in Section 3, and our hydrodynamical simulations of the Vela SNR and Vela~X are described in Section 4. We present a discussion of our results in the context of the multiwavelength data on Vela~X in Section 5, and summarize our conclusions in Section 6. | We have carried out X-ray observations and hydrodynamical simulations of Vela~X in an effort to understand its overall structure in the context of the evolution of a PWN within the confines of an SNR. Our results show that the properties of Vela~X are well described by a model in which the PWN has undergone asymmetric disruption from the SNR RS, resulting from the Vela SNR evolving in a non-uniform ISM with a higher density in the NE, as originally suggested by Blondin et al. (2001). Our large-scale mapping of key regions within Vela~X provides important details of the overall structure that confirm earlier measurements of both thermal and nonthermal emission \citep{MO97,LaMassa_etal08}. We produce temperature, abundance, spectral index, and equivalent width maps that reveal significant variations in the properties of the thermal and nonthermal gas within Vela~X. We find the following: 1. At an age equal to the characteristic age of the pulsar, the overall size of the SNR and PWN can be approximately reproduced assuming evolution into a density gradient whose values are constrained by previous density measurements for the Vela SNR. 2. The interior of Vela~X contains both relativistic gas, producing synchrotron radiation, and ejecta material whose thermal emission establishes enhanced abundances of Ne, O, and Mg. This is consistent with HD simulations that show significant mixing of ejecta into the PWN during the RS-crushing phase of evolution. 3. Equivalent-width maps show a complex variation in the relative contributions of O~VII and O~VIII to the emission. Both broad fitting of the ejecta temperature and modeling of spectra from discrete regions show that these variations are associated with temperature differences that are primarily associated with regions where the SNR shell component dominates emission from the ejecta. 4. The Vela~X ``cocoon'' consists of an extended region whose emission is from a combination of ejecta and PWN material. High-resolution X-ray images provide some indication of fine-scale filamentation, but show that the structure is largely diffuse on small scales. Simulations show that such a structure can naturally be formed as part of the hydrodynamical development of the RS-PWN interaction. 5. The spectrum of the nonthermal emission is harder near the pulsar than at regions farther along the cocoon, and beyond. The HD simulations indicate that this is consistent with both the cocoon and the relic PWN being dominated by particles injected by the pulsar at an earlier age, now subject to synchrotron losses, while the region nearer the pulsar contains particles more recently injected by the pulsar. 6. The nonthermal X-ray spectrum at the peak position of the GeV emission observed with the \fermi-LAT appears harder than emission at similar offset distances from the pulsar and the cocoon. The overall picture from our observations and simulations are consistent with the picture that the overall morphology of Vela~X is the result of an asymmetric RS interaction associated with expansion of the SNR into medium whose density is higher in the NE regions. The cocoon appears to result from the RS approaching and displacing the PWN from the NE, then wrapping around the highest pressure regions of the nebula close to the pulsar, finally coming together on the opposite side of the pulsar and over-pressuring the entrained plasma there. There remain important considerations for which extension of our modeling to MHD will be necessary. In particular, since our HD simulations do not treat the evolution of the PWN magnetic field, our knowledge of the synchrotron losses suffered by particles in different regions of the PWN is quite incomplete. This, along with magnetic effects that might dominate the dynamics on some spatial scales, will be important to consider in future works in order to address the overall distribution of the emission observed in the GeV and TeV bands. | 18 | 8 | 1808.03878 |
1808 | 1808.03563_arXiv.txt | There is significant scientific value to be gained from combining {\em AKARI} fluxes with data at other far-IR wavelengths from the {\em IRAS} and {\em Herschel} missions. To be able to do this we must ensure that there are no systematic differences between the datasets that need to be corrected before the fluxes are compatible with each other. One such systematic effect identified in the BSCv1 data is the issue of beam corrections. We determine these for the BSCv2 data by correlating ratios of appropriate {\em IRAS} and {\em AKARI} bands with the difference in 2MASS $J$ band extended and point source magnitudes for sources cross matched between the {\em IRAS} FSC, {\em AKARI} BSCv2 and 2MASS catalogs. We find significant correlations ($p<< 10^{-13}$ correlations indicating that beam corrections are necessary in the 65 and 90$\mu$m bands. We then use these corrected fluxes to supplement existing data in spectral energy distribution (SED) fits for ultraluminous infrared galaxies in the HERUS survey. The addition of {\em AKARI} fluxes makes little difference to the results of simple $(T,\beta)$ fits to the SEDs of these sources, though there is a general decrease in reduced $\chi^2$ values. The utility of the extra {\em AKARI} data, however, is in allowing physically more realistic SED models with more parameters to be fit to the data. We also extend our analysis of beam correction issues in the {\em AKARI} data by examining the Herschel Reference Sample, which have {\em Herschel} photometry from 100 to 500$\mu$m and which are more spatially extended than the HERUS ULIRGs. Thirty-four of the HRS sources have good {\em Herschel} SEDs and matching data from {\em AKARI}. This investigation finds that our simple 2MASS-based beam correction scheme is inadequate for these larger and more complex sources. There are also indications that additional beam corrections at 140 and 160$\mu$m are needed for these sources, extended on scales $>$1 arcminute. | The thermal emission of cool dust, at temperatures of $\sim$20-60K, is a major constituent of the spectral energy distribution (SED) of all star forming galaxies. Normal spirals, like the Milky Way Galaxy, emit $\sim$ 30\% of their energy through cool dust emission (eg. \cite{2012MNRAS.427.2797D}) in the far-IR, at wavelengths peaking near to 100$\mu$m. Since the dust is heated through absorbing optical/UV emission from a galaxy's stellar population and/or AGN, this implies that roughly 30\% of the light from stars in a typical galaxy is absorbed by dust. Galaxies with higher star formation rates have a greater fraction of their energy output in the far-IR, with the most extreme objects in the local universe, the Ultraluminous IRAS Galaxies (ULIRGs with L$_{fir}>10^{12}$M$_{\odot}$) emitting over 90\% of their energy in the far-IR (e.g. \cite{1984Natur.309..430W}, \cite{1984ApJ...283L...1S}), with the vast majority of the energy generated by their starbursts obscured by dust. While the most luminous, ULIRG-class, objects are rare in the local universe, this population evolves rapidly with redshift, with their higher redshift equivalents contributing 3-5 orders of magnitude more to the co-moving luminosity density at $z>1$ than they do locally (\cite{2005ApJ...632..169L}), possibly becoming the dominant source of far-IR luminosity at the epoch of peak star formation around $z\sim2$ (\cite{p12}, \cite{2013A&A...553A.132M}, \cite{2013MNRAS.432...23G}). Understanding the nature of the far-IR emitting dust and the powers sources driving the far-IR emission in all galaxies, and especially in the most luminous ULIRG sources, is thus an important task for far-IR astronomy. Since the dust SED peaks at wavelengths around $\sim$100$\mu$m, observations in the far-IR are required to determine the properties of the dust emission. Observations at wavelengths close to this peak are especially important for accurate temperature determination and to see if there are any signs that the dust obscuration might be optically thick (see eg. \cite{2011ApJ...743...94R}) and to search for evidence of multiple populations of dust at different temperatures (see eg. \cite{2001MNRAS.327..697D}). Because far-IR observations are largely impossible from the surface of the Earth, this work requires data from space based observatories. {\em IRAS} observations have been the mainstay for this work for many years since it provides an all-sky survey at 12, 25, 60 and 100$\mu$m. Observations with {\em ISO} expanded this dataset and extended fluxes out to 170$\mu$m (\cite{2003AJ....125.2361B}, \cite{2007A&A...466..831S}) whilst those with {\em Spitzer} went much deeper and added 70 and 160$\mu$m fluxes (eg. \cite{2003PASP..115..928K}). However, {\em ISO} and {\em Spitzer} covered only a small fraction of the sky, so relatively limited samples of targets are available that have these data. More recently the large area {\em Herschel} surveys have covered between them $\sim$1000 sq. deg. of sky at 250, 350 and 500$\mu$m to 1$\sigma$ sensitivities of a few mJy (eg. \cite{2012MNRAS.424.1614O}, \cite{2010PASP..122..499E}). Samples of specific sources of interest, such as ULIRGs (eg. \cite{2013ApJ...776...38F}) were additionally targeted at shorter wavelengths adding flux points at 70, 100 and 160$\mu$m. However, two things are lacking from the compendium of data currently available to far-IR astronomers: improved coverage of the wavelength region between $\sim$90 and 160$\mu$m where the SED peaks, which is important in determining optical thickness and/or the presence of dust at multiple temperatures; an all sky survey comparable to {\em IRAS} but adding data at wavelengths longer then 100$\mu$m. The {\em AKARI} mission provides both of these requirements, so it potentially has a key role to play in the analysis of far-IR SEDs of galaxies in the local universe, and especially the local ULIRGs. However, in order for this potential to be realised, we must make sure that the {\em AKARI} fluxes can be accurately combined with data from {\em IRAS}, {\em Herschel} and other space missions with any systematic photometric offsets corrected. In this paper we search the {\em AKARI} Bright Source Catalog version 2 (BSCv2 \cite{y18}) for evidence that beam corrections are needed to account for any far-IR flux missed from extended sources, and then combine the corrected fluxes with other far-IR data to examine the far-IR SEDs of local ULIRGs from the HERUS survey (\cite{2013ApJ...776...38F}, \cite{2016ApJS..227....9P}, \cite{2013ApJ...775..127S}, \cite{2018MNRAS.475.2097C}). The rest of the paper is structured as follows: in section 2 we describe our detection and determination of beam corrections in the {\em AKARI} BSCv2 catalog; in section 3 we summarise the other data and fitting methods used with the corrected {\em AKARI} fluxes to fit model SEDs to the HERUS ULIRGs and present the results of these fits; in section 5 we discuss these results and further test our beam correction scheme by analysis using the closer and more extended {\em Herschel} Reference Sample galaxies \cite{2014MNRAS.440..942C}. Our conclusions are summarised in section 5. We assume a Hubble constant of $H_0 = 70$ kms$^{-1}$ Mpc$^{-1}$ and density parameters of $\Omega_M = 0.3$ and $\Omega_L = 0.7$. | We have derived beam corrections for {\em AKARI} BSCv2 sources by correlating the ratio of {\em AKARI} fluxes to {\em IRAS} fluxes, in appropriate bands, to the difference in magnitude in the $J$ band between the 2MASS point source and extended source catalogs. We find that beam corrections are needed in both the 65 and 90$\mu$m bands, but no corrections are needed for the 140 and 160$\mu$m bands, which have larger beams on the sky. We then compared the results of simple MBB SED fitting models to the HERUS ULIRGs derived from {\em IRAS} and {\em Herschel} data alone and those found when combining this with corrected and uncorrected {\em AKARI} data. For the simple MBB fits we find no significant change in the SED properties, but the reduced $\chi^2$ values are best for the dataset that includes the beam corrected {\em AKARI} data. We also attempt to fit an optically thick SED model. We find that in most cases this yields degenerate fits that cannot distinguish between solutions that are optically thick or optically thin at far-IR wavelengths. However, for some of our ULIRGs the optically thick models appear to be favoured. This includes Arp220, for which we recover optically thick dust SED parameters very similar to those found by \cite{2011ApJ...743...94R}. This demonstrates the value of including the beam-corrected {\em AKARI} data in such studies. We also test our beam correction method on the {\em Herschel} Reference Sample galaxies \cite{2014MNRAS.440..942C} which have larger angular extents than those of the HERUS ULIRGs or the majority of the 2MASS sources used in our beam correction measurement. For these sources we find evidence that additional beam corrections are needed at 140 and 160$\mu$m and that our simple approach used for beam correction at 90 and 65$\mu$m may not be sufficient. For nearby galaxies, extended on scales of 1 arcminute or more, specific extended source processing appears to be be needed beyond what currently exists in the {\em AKARI} pipeline. Improvements to the pipeline to allow the full recovery of extended source fluxes, or the possible addition of a Small Extended Source Catalog would significantly enhace the usefulness of the {\em AKARI} data in the context of multiwavelength and multimission photometric studies. A reduction in the currently quite large calibration errors at 140 and 160$\mu$m would also be very helpful for such projects. \begin{ack} This research is based on observations with {\em AKARI}, a JAXA project with the participation of ESA. SPIRE has been developed by a consortium of institutes led by Cardiff Univ. (UK) and including: Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); and Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC, UKSA (UK); and NASA (USA). This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. E.GA is a Research Associate at the Harvard-Smithsonian Center for Astrophysics, and thanks the Spanish Ministerio de Econom\'{\i}a y Competitividad for support under projects FIS2012-39162-C06-01 and ESP2015-65597-C4-1-R, and NASA grant ADAP NNX15AE56G. JBS wishes to acknowledge the support of a Career Integration Grant within the 7th European Community Framework Program, FP7-PEOPLE-2013-CIG-630861-FEASTFUL. J.A. acknowledges financial support from the Science and Technology Foundation (FCT, Portugal) through research grants PTDC/FIS-AST/2194/2012 and UID/FIS/04434/2013. DLC and JG acknowledge support from STFC, in part through grant numbers ST/N000838/1 and ST/K001051/1. The authors wish to thank A. Jaffe for useful conversations, and DLC would like to thank the {\em AKARI} project, Tokyo University and K. Kohno for their generous hospitality. \end{ack} | 18 | 8 | 1808.03563 |
1808 | 1808.04175_arXiv.txt | Compact interferometers, called phasemeters, make it possible to operate over a large range while ensuring a high resolution. Such performance is required for the stabilization of large instruments dedicated to experimental physics such as gravitational wave detectors. This paper aims at presenting the working principle of the different types of phasemeters developed in the literature. These devices can be classified into two categories: homodyne and heterodyne interferometers. Improvement of resolution and accuracy has been studied for both devices. Resolution is related to the noise sources that are added to the signal. Accuracy corresponds to distortion of the phase measured with respect to the real phase, called non-linearity. The solutions proposed to improve the device resolution and accuracy are discussed based on a comparison of the reached resolutions and of the residual non-linearities. | Relative motion between two points can be measured by a number of transducers, converting the variation of a physical quantity into some useful voltage. Some examples of commonly used sensors are capacitive sensors, linear variable differential transformers (LVDT) and eddy current sensors. For each application, the adequate choice depends on many criteria, including resolution, dynamic range, space available, price, and compatibility with operating environment. While based on very different working principles, all of theses sensors are fundamentally limited by a trade-off between resolution and dynamic range. In other words, none of them can process both small and large quantities. Moreover, even the most sensitive of these techniques have limited resolution, and are not reliable in operating environments with stray magnetic fields. These two aforementioned limitations prevent them from being used in many applications like high precision machine tools or production chains. Interferometers are an excellent alternative due to their high sensitivity, non-contact measurement, and immunity to magnetic couplings. Conventional interferometers have a small working range, but when the optical phase is measured in two quadratures, the output can be unwrapped creating a large working range optical-phasemeter. Compact optical-phasemeters are of increasing interest to physics and precision engineering communities. In this paper we review a range of devices that can be called `compact', which implies that the interferometer is an enabling tool and that either the complete system or an optical head can be deployed onto an apparatus. While not all reviewed works clearly specify the size and form of the interferometer, we have attempted to apply these two criteria to determine their relevance. For convenience, we often refer to the complete signal chain from the interferometer to the unwrapped phase readout simply as a phasemeter. Many prototypes of compact interferometers have been developed for two principal types of applications. The first application is as a simple position sensor. Such sensors have been used in gravitational wave detectors for local damping~\cite{Aston2011} or on the ISI~\cite{strain2012damping,voyager}. The second application is in the development of high-resolution inertial sensors, where one mirror is fixed on an inertial mass~\cite{Zumberge04}. These sensors are useful for the stabilization of gravitational wave detectors \cite{MatichardLantz2015, Venkateswara15}, gravimeters \cite{Pena13,Harms16Notes, Zhou15, Zhou12, McGuirk01} or particle accelerators \cite{Collette12}. The objective of this paper is to provide a comparison of compact interferometers in terms of resolution, dynamic range, and linearity. The focus is on devices with a working range of more than one fringe. The paper starts with a brief section explaining the working principle and limitations of conventional two-beam and resonant interferometers. It is followed by Sections \ref{sec:homodyne} and \ref{sec:heterodyne} dedicated to homodyne- and heterodyne-phasemeters. For each of them, the working principle is presented and several examples from relevant literature are described. Sections \ref{sec:nonlinearities} and \ref{sec:noisesources} discuss problems that are common to all types of phasemeters, and counter measures that mitigate these problems. The first is the limited accuracy due to the non-linearities in the phase measurement. The second is a short review of the main noise sources in interferometers. The paper concludes with historical trends, and a discussion on the dimensions of compact interferometers.% | Chronological evolution of the resolution of heterodyne interferometers. The area corresponds to the surface occupied by the interferometer, without the laser source and the data acquisition system.} \begin{ruledtabular} \begin{tabular}{ccccccc} Year & Device & Resolution (ASD) & Resolution Meas-& Heterodyne & Wavelength & Area\\ & & pm / \(\sqrt{\rm{Hz}}\) & urement Frequency & Frequency & nm & cm$^2$\\ \hline 1970 & HP \cite{hpinterferometer70} & 10,000 & - & 2\,MHz & 632 & 38x28\\ \hline 1972 & De la Rue \cite{delaRue72} & 0.2 & 2\,Mhz & 22.5\,MHz & 632 & -\\ \hline 1984 & Monachalin \cite{Monchalin84} & 0.06 & 1\,Hz & 40\,MHz & 632 & - \\ \hline 1986 & Royer \cite{royer86} & 0.1 & $>$100\,kHz & 70\,MHz & 632 & 8x5\footnotemark[1] \\ \hline 1987 & Willemin \cite{Willemin87} & 10 & 1\,kHz & 1\,MHz & 632 & - \\ \hline 2002 & Wu \cite{Wu2002} & 2 & 1.7\,kHz & 80\,kHz & 632 & 40x40\footnotemark[1] \\ \hline 2007 & Martinussen \cite{Martinussen07} & 2 & 3.3\,Hz & 31\,MHz & 532 & -\\ \hline 2009 & Schuldt \cite{Schuldt09} & 10 & 0.01\,Hz & 10\,kHz & 1064 & 30x40\\ \hline 2010 & Hsu \cite{Hsu10} & 0.5 & 10\,Hz & 1.65\,MHz & 632 & - \\ \hline 2012 & Weichert, Pisani \cite{weichert12,Pisani15} & 0.03 & 1\,kHz & 1.5625\,MHz& 532 & - \\ \hline 2013 & Leirset \cite{Leirset13} & 0.071 & 21\,MHz & 0-1.3\,GHz & 532 & - \\ \hline 2016 & LPF \cite{Armano16} & 1 & 10\,mHz & - & 1064 & 20x20\footnotemark[1] \\ \end{tabular} \end{ruledtabular} \footnotetext[1]{Interferometer plate only} \end{table*} \subsection{Conclusion} While operating at the beat-note is an advantage in terms of simplicity of the readout, interferometers that use this technique are still subject to the same fundamental noise sources. These include but aren't limited to: shot noise and length noise coupling into the readout. The effective contribution of laser frequency noise can be reduced as shown in Ref.~\cite{Heinzel03} and Ref.~\cite{Wu2002}. In terms of low frequency resolution, the phasemeter on board of the LISA pathfinder spacecraft represents the best heterodyne phasemeter in terms of linearity and sensitivities below 1\,Hz, however the interferometer is expensive when compared to other devices. The most compact interferometer reviewed here, with a specified size is developed by Royer \textit{et al.}\cite{royer86}, this device has excellent resolution of 30\,fm / \(\sqrt{\rm{Hz}}\) at 70\,MHz, however the device does not specify its linearity. The most linear interferometer is that presented by Weichert \textit{et al.} \cite{weichert12} with non-linearities less than 5\,pm and a noise floor of 30\,fm / \(\sqrt{\rm{Hz}}\) above 150\,Hz, though it lacks the simplicity of devices such as the one presented in Leirset \textit{et al.} \cite{Leirset13}. A summary of the interferometers reviewed and their subsequent sensitivities are shown in chronological order in Table \ref{tab:hetSummary}. \label{sec:discussion} \paragraph*{} This paper has presented a review of `compact' interferometers that employ different methods to increase the dynamic range compared with that of a simple interferometer. All techniques are based on the same principle: create a phasemeter by generating two (or more) quadrature signals from which the phase, and as such the displacement, can be extracted over more than one fringe by unwrapping the outputs with a 4-quadrant arctangent. To determine the size of systems, we searched for their dimensions in the literature. From Table~\ref{tab:resolutionEvolutionHomodne}, we see that in average the optical homodyne interferometer occupies an area of approximately 17x17~cm$^2$, with some substantial variation in size. Heterodyne phasemeters are somewhat larger, typically 30x30\,cm$^2$, but in both cases the `size' often neglects the input beam preparation optics and data acquisition system. Heterodyne devices typically require more space as either an additional laser source or an AOM is required. In most homodyne systems, two polarization states are used to sample the target mirror with different phase shifts, creating the quadrature outputs. For heterodyne interferometers, two beams with different laser frequencies pass through the interferometer and the quadrature signals are most commonly created by demodulating the beat signal with a sine and a cosine. \paragraph*{}The resolution of homodyne phasemeters has improved considerably since their inception, largely due to decreasing technical noises. Table~\ref{tab:resolutionEvolutionHomodne} shows that several devices in the last 10 years have reached a sensitivity at or below 1\,pm$/\sqrt{\rm{Hz}}$ at 1\,Hz. To improve sensitivity it is possible to increase the number of reflections in one or both arms of an interferometer (Section~\ref{sec:MultipassHomodyne}). Several experiments have employed additional photodiodes to reduce intensity-noise coupling (Section~\ref{sec:HomodyneExtraphotodiodes}). \paragraph*{}Heterodyne phasemeters push (much of) the optical complexity onto the input beam preparation and the signal analysis. It is difficult to make a fair comparison between devices to the very large range of design parameters, including the heterodyne frequency and the measurement frequency, but Table~\ref{tab:hetSummary} shows that resolutions less than 1\,pm$/\sqrt{\rm{Hz}}$ are routinely achieved. Many of the devices reviewed were not very compact, and it is difficult to determine the size scale of the complete apparatus, including the input-beam preparation. \paragraph*{} Overall, heterodyne interferometers are larger and more complex than their homodyne cousins, but they are less susceptible to low-frequency readout noise. A brief summary of the common noise sources that limit the resolution of interferometers is included in Section~\ref{sec:noisesources}. \paragraph*{} A significant advantage of all phasemeters is that they are inherently calibrated to the wavelength of the laser. There are, however, several sources of non-linearity that affect their accuracy, and these have also been reviewed. Non-linearities can be reduced in several ways. Ellipse fitting algorithms, section~\ref{sec:EllipseFittingAlgorithm}, are widely used to transform the phasemeters output into a unitary circle centred at the origin, removing both the leading order of non-linearity and the offsets inherent to measuring intensity with photodiodes. Additional sensors, section~\ref{sec:AdditionalSensors}, can reduce the residual non-linearity by reducing the effect of power fluctuations or by subtracting large input phase-shifts. Polarization mixing (homodyne) and phase mixing (heterodyne) can be reduced thanks to spatially separating beams, section~\ref{sec:PhaseMixing}. The most promising heterodyne technique for reducing non-linearity is to employ a `phase lock' to hold the signal in a single quadrature and read out the phase-shift required to keep it there. From figure~\ref{fig:NonLinearitiesComparison}, it is clear that non-linearities have been improving consistently during the last decades and that modern interferometers can consistently achieve single-digit picometer accuracy. \paragraph*{} Heterodyne interferometers achieve a good resolution and can inherently reject any DC component. However, there is still room for improvement before they reach the level of compactness of homodyne devices. As explained in this Section, the size of the beam preparation equipment limits the compactness. Therefore, reducing the size of these devices should be the concern of future research if they want to compete with the size of the homodyne devices. \paragraph*{} In addition, many designs have already been developed that offer significant improvement in resolution and sensitivity. Nevertheless, whether for homodyne or heterodyne interferometers, a lot of parameters change from one device to another (wavelength, type of photodetectors, etc.). In order to have a fair comparison between the different solutions proposed, they should be tested on the same setup. Such a prototype should also allow to see if certain solutions can be combined and whether the performance of the combined interferometer corresponds to the combination of the performance obtained with the two solutions independently. | 18 | 8 | 1808.04175 |
1808 | 1808.07495_arXiv.txt | Infrared Imaging Spectrograph (IRIS) is the first light instrument for the Thirty Meter Telescope (TMT) that consists of a near-infrared (0.84 to 2.4 micron) imager and integral field spectrograph (IFS) which operates at the diffraction-limit utilizing the Narrow-Field Infrared Adaptive Optics System (NFIRAOS). The imager will have a 34 arcsec x 34 arcsec field of view with 4 milliarcsecond (mas) pixels. The IFS consists of a lenslet array and slicer, enabling four plate scales from 4 mas to 50 mas, multiple gratings and filters, which in turn will operate hundreds of individual modes. IRIS, operating in concert with NFIRAOS will pose many challenges for the data reduction system (DRS). Here we present the updated design of the real-time and post-processing DRS. The DRS will support two modes of operation of IRIS: (1) writing the raw readouts sent from the detectors and performing the sampling on all of the readouts for a given exposure to create a raw science frame; and (2) reduction of data from the imager, lenslet array and slicer IFS. IRIS is planning to save the raw readouts for a given exposure to enable sophisticated processing capabilities to the end users, such as the ability to remove individual poor seeing readouts to improve signal-to-noise, or from advanced knowledge of the point spread function (PSF). The readout processor (ROP) is a key part of the IRIS DRS design for writing and sampling of the raw readouts into a raw science frame, which will be passed to the TMT data archive. We discuss the use of sub-arrays on the imager detectors for saturation/persistence mitigation, on-detector guide windows, and fast readout science cases ($<$ 1 second). | \label{sec:intro} % IRIS is the first light instrument for TMT, operating at the diffraction-limit with a near-infrared (0.84 to 2.4 micron) imager and IFS. The imager will have four 4k$\times$4k Hawaii-4RG detectors with a 34 arcsec$\times$34 arcsec field of view (FoV) using a 4 mas pixel scale. The IFS will have one Hawaii-4RG which will have four spatial scales (lenslet:4 mas, 9 mas; slicer: 25 mas, 50 mas). The IFS will have 12 gratings with resolutions R=4000, 8000, and 10000. With the sequential optical design, both the imager and IFS will use the same set of 68 filters. There will be hundreds of configurations that IRIS will support. The next generation of large telescopes will provide unique challenges for data reduction pipelines. IRIS is designed to operate its imager and IFS simultaneously, and therefore the DRS needs to support hundreds of modes with real-time reduction for acquisition and analysis. The DRS will handle both lenslet and slicer IFS data supporting all resolutions. Another key feature of the DRS is that it communicates with TMT, NFIRAOS, and IRIS, storing the relevant metadata into each of the individual readout FITS headers. This paper describes the current design for the IRIS DRS and some of the instrument and telescope design challenges. Section 2 gives an overview of the DRS. Section 3 describes the DRS data rates and storage requirements. Section 4 discusses the On-Instrument Wavefront Sensors potential vignetting of IRIS and how the DRS will mitigate it. Section 5 discusses the imager sub-array use cases and the DRS plans to handle them. Finally, Section 6 discuses the potential use of the Atmospheric Dispersion Corrector (ADC) to implement a low dispersion mode on the imager. \begin{figure}[ht!] \begin{center} \caption{\label{fig:drs_flow}DRS flow chart showing its interactions with the instrument sequencer, the detector HCDs, the TMT DMS and the storage disks.} \includegraphics[width=.8\linewidth]{DRS_Flow_Diagram_v3.png} \end{center} \end{figure} | We have presented the updated DRS design for IRIS for the TMT. One of the biggest changes to the design is that the detectors now send the individual readouts through a socket to the readout processor. Additionally, the readout processor will write the readouts and perform sampling (e.g., UTR) on an exposure (series of reads and ramps). In this updated design, the science pipeline director acts as the interface between the instrument sequencer and the various DRS pipelines, as well as sending raw and reduced images to the visualization tools. The science pipeline director will also request all of the telemetry from the relevant TMT, IRIS and NFIRAOS subsystems. We also investigate the data storage and computation requirements for running the DRS during an observing campaign, and find that they are modest. The storage requirements only become challenging when we want to store all of the individual readouts to the archive. The OIWFS can vignette the IRIS imager and IFS (even slightly outside the FoV), which can be mostly solved either with the TMT planning tool or with ODGWs. If the imager does get vignetted, the DRS will need to investigate masking based on the OIWFS telemetry. With TMT's 30-meter aperture, saturation will be a significant issue for IRIS. One of the ways to mitigate it is through the use of sub-arrays. However, resetting the detector or read out a much faster rate within the sub-array will heat the detector. The DRS will employ masking beyond the reset regions to account for the ``blooming" effects from the heating. Finally, we investigate a potential new mode, utilizing the IRIS ADC as a low dispersion prism with the IRIS imager, which has the advantage of the imager throughput and the potential disadvantage of the high sky background. | 18 | 8 | 1808.07495 |
1808 | 1808.02565_arXiv.txt | This paper presents a photometric and spectroscopic study of the short-period binary star Cl*~Melotte~111~AV~1224. Measurements in the $B$, $V$, and $R$ passbands obtained during three observing runs between 2014 and 2017 and medium-resolution spectra secured in 2014, are analyzed together with public data from the SuperWASP and LAMOST projects. Our light curves show marked asymmetry with a variable O'Connell effect. The SuperWASP photometry is used to derive a mean binary period of 0.345225 days. The analysis of the $(O-C)$ diagram reveals that the orbital period is decreasing at a rate of $dP/dt = -3.87 \times 10^{-6}$ days yr$^{-1}$, which may be caused by mass transfer from the more-massive component to the less-massive one. The system is found to be a single-lined spectroscopic binary with a systemic velocity, $\gamma = 1 \pm 3$ Km s$^{-1}$, and a semi-amplitude, K$_{1}$ = 21 $\pm$ 5 Km s$^{-1}$. The spectral classification and the effective temperature of the primary component are estimated to be K0V $\pm$ 1 and $5200 \pm 150$ K, respectively. The photometric and spectroscopic solutions reveal that Cl*~Melotte~111~AV~1224 is a low-mass ratio ($q=m_{2}/m_{1} \sim 0.11$), low-inclination ($\sim ~ 38^{\circ}$) near-contact system. The masses, radii and luminosity for the primary and secondary are estimated to be $1.02 \pm 0.06\, M_\odot$, $1.23 \pm 0.05\, R_\odot $, $1.01 \pm 0.06\, L_\odot$ and $0.11 \pm 0.08\, M_\odot$, $0.45 \pm 0.05\, R_\odot$, $0.10 \pm 0.06\, L_\odot$, respectively. The marginal contact, together with the period decrease, suggests that this binary system may be at a key evolutionary stage, as predicted by the theory of thermal relaxation oscillations. | \label{sect:1} Near-contact binaries (NCBs) are a important source in understanding the formation and evolution of the binary systems, as they are assumed to be lying in a key evolutionary stage. They have been defined by \citet{shaw} as a subclass of close binaries in which both components fill, or nearly fill, their critical Roche lobe and are near enough to each other as to have strong proximity effects like those of W~UMa type systems without being in contact. NCBs have periods of less than a day, display strong tidal interaction, and show EB-type variations. Such systems may be the evolutionary precursors to the A-type W~UMa systems, and are probably in the early stages of mass transfer. Indeed, some NCBs exhibit asymmetric light curves and orbital period decrease caused probably by mass transfer or/and by angular momentum loss via magnetic braking. As the period decreases, the separation between both components will be reduced, and the system will evolve into an A-type overcontact binary. Therefore, it is suggested that the NCBs could be the missing link between detached binaries and W~UMa systems. There is a considerable number of newly discovered near-contact systems brighter than 12 mag $(V)$ since the early work by \cite{shaw}, which are divided into two subclasses: V1010 Ophiuchi systems and FO Virginis binaries. The study of fainter NCBs ($V > 13$ mag) is challenging, but also important, because they may provide new insights into underlying mechanisms of the mass transfer in close binaries. \begin{table*} \caption{Log of Photometric and Spectroscopic Observations.} \begin{tabular}{|lccccr|ccc|} \hline \hline \noalign{\smallskip} \multispan6 \hfill Photometric Observations \hfill \vline & \multispan3 \hfill Spectroscopic Observations \hfill \\ \noalign{\smallskip} \hline UT Date & Start Time & End Time & Filter &OAN-SPM &XL & Start Time &Number of spectra & Exp. Time \\ &(HJD 2454900+)&(HJD 2454900+)& & (hr) & (hr) & (HJD 2454900+) & & (sec) \\ \hline 2014 April 05 & 1853.00 & 1853.34 & $BVR$ & - & 8.1 & - & - & - \\ 2014 April 06 & 1854.00 & 1854.34 & $BVR$ & - & 8.3 & - & - & - \\ 2014 April 08 & 1855.63 & 1856.01 & $BVR$ & 9.2 & - & 1855.70 & 6 & 1800 \\ 2014 April 09 & 1856.63 & 1857.25 & $BVR$ & 7.6 & 7.6 & 1856.70& 8 & 1800 \\ 2014 April 10 & 1857.90 & 1857.99 & $BVR$ & 2.3 & - & 1857.90& 4 & 1800 \\ 2016 April 15 & 2593.75 & 2594.00 & $BVR$ & 7.9 &- & - & - & - \\ 2016 April 17 & 2595.64 & 2595.86& $BVR$ & 5.3 &- & - & - & - \\ 2016 April 18 & 2596.67 & 5597.00& $BVR$ & 7.6 &- & - & - & - \\ 2016 April 20 & 2598.68 & 2598.98& $BVR$ & 7.3 &- & - & - & - \\ 2017 March 28 & 2940.64 & 2940.93 & $BVR$ & 6.9 & - & - & - & - \\ 2017 April 13 & 2956.64 & 2967.00& $BVR$& 8.7 & - & - & - & - \\ \hline \end{tabular} \label{tab:log} \end{table*} In this paper, we present a detailed analysis of Cl*~Melotte~111~AV~1224\footnote{Star designation in the Simbad astronomical database. AV~1224 refers to the star running number in the astrometric catalog by \cite{abad}.} (GSC 01989-00964, 2MASS~J12214222+2500569, and \av hereater ) --a NCB candidate target ($V \sim 13.4$ mag) identified as a short-period eclipsing binary by \citet{fox}. The light curves in the $V$ band show $\beta$ Lyrae type variations and a remarkable O'Connell effect. Because the preliminary photometric solutions reported by \citet{fox} were unable to discriminate between overcontact and semi-detached configurations, we have conducted follow-up spectroscopic and photometric optical observations of \av to shed more light on its nature. We incorporated publicly available SuperWASP and LAMOST data into our study. The absolute physical parameters of the components are derived, and the evolutionary status of this rare binary system is discussed. | \label{sect:4} We have presented follow-up photometric and spectroscopic observations of the short-period binary \av. The photometric solutions were derived for three sets of multi-color light curves: $B$, $V$ and $R$ obtained in 2014, 2016, and 2017 years. The results indicate that \av is a near-contact binary with both the primary and the secondary almost filling their Roche lobes, and therefore is a marginal contact NCB system. The spectroscopic solutions suggest that \av is a single-lined spectroscopic binary, in which the primary component, partially eclipsed at phase 0.0, is hotter than less-massive companion by $\sim$ 500 K. A single radial-velocity measurement reported by the LAMOST team is in good agreement with our measurements. The photometric light curves show remarkable asymmetries and variations from one year to another, suggesting that the system is strongly active. The three sets of photometric solutions consider spots to account for the variations observed in the light curves. Combining photometric and spectroscopic solutions, we have derived the parameters listed in Table~\ref{tab:abspar}. The positions of both components are shown with filled symbols in the mass-luminosity diagram of Figure~\ref{fig:evol}, where open circles and triangles represent, respectively, the primaries and the secondaries of NCBs from \citet{yakut}. The solid and dotted lines refer, respectively, to the zero-age main sequence (ZAMS) and the terminal-age main sequence (TAMS), which were constructed by the binary stars evolution (BSE) code \citep{hurley}. It is found that the primary component of \av is near the ZAMS line, in agreement with our spectral classification. Meanwhile, the less-massive component lies above the TAMS line. \begin{figure}[] \includegraphics[width=8.5cm]{fig8.eps} \caption{Positions of the components of \av on the $M-L$ diagram. The filled circle denotes the primary, and the filled triangle represent the secondary. The other symbols denote the sample of NCBs from \citet{yakut}, where open circles and open triangles represent the primary and secondary components. The ZAMS and TAMS are the continuous and dotted lines, respectively.} \label{fig:evol} \end{figure} A possible secular decrease in its orbital period has been found from a few times of minima over eight years time interval. From the quadratic term of the Equation (2), it follows that the orbital period may be decreasing with a rate of $dP/dt = -3.87 \times 10^{-6}$ days yr$^{-1}$ or 0.3 s yr$^{-1}$. Such a rapid period decrease has been noticed before in others NCBs (e.g., \citealp{lohr}, or Table~5 of paper by \citealp{zhu}). With the orbital period decrease, the primary component transfer mass to the secondary one, the mass ratio increases and eventually the NCB systems evolve into the contact phase. Period decrease might be caused by mass or angular momentum loss (AML) due to magnetic stellar wind (magnetic breaking) and/or mass transfer from the more to the less-massive component. According to the formula given by \citet{bradstreet}: \begin{equation} \begin{split} (\dot P)_{\rm AML} \sim -1.1 \times 10^{-8} q^{-1} (1 + q)^{2} \times \\ (M_{1} + M_{2})^{-5/3} k^{2} (M_{1}R_{1}^{4} + M_{2}R_{2}^{4})P^{-7/3}. \end{split} \end{equation} \noindent Using $M_{1}$, $M_{2}$, $R_{1}$ and $R_{2}$ from Table~\ref{tab:abspar}, $q=0.11$ and gyration constant $k^{2} = 0.1$, the theoretical AML is estimated to be $(\dot P)_{\rm Theoretical} \sim -2.0\times10^{-7}$ days yr$^{-1}$. This suggests that magnetic breaking is not the main cause of period decrease. The mass transfer from $M_{1}$ to $M_{2}$ or the mass loss from the system can be evaluated using the equations given by \citet{hilditch}: \begin{equation} \frac {\dot P}{P} = \frac {3 \dot M_{1}(M_{1} - M_{2})}{M_{1}M_{2}}, \end{equation} \begin{equation} \frac {\dot P}{P} = 3{\dot M_{1}}\left[ \frac{(M_{1} + M_{2})}{M_{1}M_{2}} \frac{d^{2}}{a^{2}} - \frac{M_{2}}{M_{1}(M_{1} + M_{2})} \right ] , \end{equation} \noindent for conservative and non-conservative mass loss, respectively. $P$ and ${\dot P}$ are the orbital period and its change rate, $d$ is the distance from the binary centre of mass to the Lagrange point, $L_{2}$. Calculating the necessary values of $\dot M_{1}$ to explain the observed ${\dot P}/P$, we obtain for \av $\dot M_{1} \sim -4.60 \times 10^{-7}$ $M_{\odot}$ yr$^{-1}$ and $\dot M_{1} \sim -3.15 \times 10^{-7}$ $M_{\odot}$ yr$^{-1}$ for conservative and non-conservative mass transfer, respectively. The timescale of the conservative mass transfer can be estimated to be approximately $2.2\times 10^{6}$ yr. On the other hand, the thermal timescale of the massive component can be estimated as $\tau_{th} \approx 3.0 \times 10^{7} (M/M_{\odot})^{2} (R/R_{\odot})^{-1} (L/L_{\odot})^-1$ $\sim$ $1.62 \times 10^{7}$ yr \citep{hilditch}, which is longer than the conservative mass-transfer duration. This suggests that the primary component cannot stay in thermal equilibrium and the mass transfer in \av is unstable. Possible mechanisms for unstable mass transfer leading to rapid period decrease in contact or NCB systems are discussed in \cite{rasio} and \cite{jiang}. \begin{table} \caption{Absolute Parameters of \av.} \begin{tabular}{lcr} \hline \hline Parameter & Primary & Secondary \\ & & \\ \hline Mass $(M_{\odot}$) & $1.02 \pm 0.06$ & $0.11 \pm 0.08$\\ Radius $(R_{\odot}$) & $1.23 \pm 0.05$ & $0.45 \pm 0.05$ \\ Luminosity $(L_{\odot}$) & $1.01 \pm 0.06$ &$0.10 \pm 0.06$ \\ \hline \hline \end{tabular} \label{tab:abspar} \end{table} An independent estimation of the distance to \av can be obtained from the empirical relation by \citet{gettel}: \begin{equation} \log D= 0.2V_{\rm max} - 0.18 \log(P) - 1.60 (J-H) + 0.56, \end{equation} \noindent assuming $V_{\rm max}= 13.338$, the distance is estimated to be $\sim$ 387 pc which is in agreement with the distance estimated from Str\"omgren photometry by \citet{fox}, and is consistent with the fact that \av is not a cluster member of Melotte~111 ($d \sim$ 87 pc). As it is known, Melotte~111 open cluster presents an apparent deficit of low mass stars, in particular {\rm K}-dwarfs (e.g. \citealp{garcia}; \citealp{randich}; \citealp{terrien}). Our analysis indicates that \av could be in a very important evolution phase of the thermal relaxation oscillation theory (TRO theory; e.g. \citealp{lucy}; \citealp{flannery}; \citealp{robertson}; \citealp{lucyandwilson}). According to TRO, contact systems must undergo oscillations around the state of marginal contact as a result of being unable to achieve thermal equilibrium. They oscillate periodically between a contact and a near-contact phase and alternately show EW and EB light curves. Thus, each oscillation comprises a contact phase followed by a semi-detached phase. When a system lies on a near-contact phase and keeps expanding, the contact configuration will break and the system will reach the semi-detached phase with a more-massive component filling its Roche lobe. We note that our high-precision photometric observations were limited to a few nights every year. While the complete photometric light curve are presented, long-time continuous observations with additional precision timings of minimum light are still necessary to offer more information on this system. This work has received financial support from the Universidad Nacional Aut\'onoma de M\'exico (UNAM) under grant PAPIIT IN100918. J.N.F. acknowledges the support from the National Natural Science Foundation of China (NSFC) through the grant 11673003 and the National Basic Research Program of China (973 Program 2014CB845700 and 2013CB834900). Based upon observations carried out at the Observatorio Astron\'omico Nacional on the Sierra San Pedro M\'artir (OAN-SPM), Baja California, M\'exico, and Xinglong Station of the National Astronomical Observatories of Chinese Academy of Science. Special thanks are given to the technical staff and night assistants of the San Pedro M\'artir Observatory and Xinglong Station. L.F.M. would like to thank Profs. X.B. Zhang and J.N. Fu for their hospitality during his visit to the National Astronomical Observatory of China (NAOC) and Beijing Normal University (BNU). T.Q.C. and C.Q.L. would like to thank Drs. L. Fox-Machado and R. Michel for their support during a work visit in Mexico at Insituto de Astronom\'{\i}a, Campus Ensenada. This research has made use of the SIMBAD database operated at CDS, Strasbourg (France). {\it Facilities:} \facility{OAN-SPM 0.84 m (MEXMAN), 2.12 m (Boller \& Chivens)} \facility{XL 0.85 m} | 18 | 8 | 1808.02565 |
1808 | 1808.02279_arXiv.txt | Ever since its discovery \cite{Perlmutter:1998np,Riess:1998cb}, the current accelerating expansion of the universe has been one of the major puzzles of modern physics and its cause is often dubbed dark energy as its very nature is still an mystery. The simplest solution may be adding a pure cosmological constant to the Einstein-Hilbert action and indeed the $\Lambda$CDM model has described our universe quite well \cite{Aghanim:2018eyx}. Nevertheless, the physical origin of cosmological constant has remained obscured and the na\"ive theoretical expectation is about 120 orders of magnitude larger than the observed value \cite{Weinberg:1988cp}. To explain the value of cosmological constant, one may appeal to anthropic arguments \cite{Weinberg:1987dv,Garriga:1999bf}, whose recent resurgence stems from the string theory landscape \cite{Dasgupta:1999ss,Bousso:2000xa,Giddings:2001yu,Kachru:2003aw,Susskind:2003kw}. To date, cosmological constant problem remains one of the most challenging problem in fundamental physics. Cosmological constant problem aside, over the years many alternatives have been proposed to account for the accelerating expansion. Among various proposals, there is a class of models where the dark energy is attributed to a canonical scalar field named quintessence \cite{Ratra:1987rm,Wetterich:1987fm,Zlatev:1998tr}. For a review see \cite{Tsujikawa:2013fta}. Some early models of this kind posses tracker behavior where the evolution of the field at late time is insensitive to initial conditions and hence make them rather attractive. Yet, as the observations have significantly improved for the past decades, now such models are under strong pressure from the observational constraints \cite{Linder:2015zxa}. But regardless the initial condition problem and/or cosmic coincidence problem (why the energy density of matter and dark energy are comparable at present time) can be solved or not, one basic question we wish to know is whether dark energy is purely a constant or if it is dynamical and evolves over time. Thanks to the advancement in many cosmological observations like eBOSS, SuMIRe (HSC and PFS on Subaru), DESI, Euclid, WFIRST and many others in the near future, we will have better sensitivity to see if the equation of state parameter $w$ of dark energy has any deviation from $-1$, which is the case if dark energy is not a pure cosmological constant. From this perspective, quintessence models are phenomenological tools that help us describe dark energy if it is dynamical with $w>-1$ and very often the vacuum energy contribution is assumed to be zero due to other mechanism. Certainly, regardless dark energy is a pure cosmological constant or not, one still needs to answer if vacuum energy contributes to dark energy and if so, how large it should be. Yet, these are ambitious problems and very likely a full theory of quantum gravity is required to completely solve the cosmological constant problem. On the other hand, recently a constraint on scalar field potential from quantum gravity was proposed in \cite{Obied:2018sgi} which suggests that the de Sitter vacuum may belongs to the ``swampland", where models cannot be UV completed with consistent theory of quantum gravity, while the quintessence models are still safe \cite{Agrawal:2018own}. This gives another motivation to reexamine quintessence model-building. Obviously, the string theory requires supersymmetry and hence its low-energy limit must be studied within the supergravity (SUGRA) theory. Therefore, quintessence models must be formulated within SUGRA. For example, see \cite{Brax:1999gp,Copeland:2000vh,Brax:2006kg,Brax:2006dc,Brax:2009kd} for some previous works along this line. One particular point we would like to emphasize and is the focus of this paper is that when building quintessence model in supergravity, it is necessary to consider the effect of supersymmetry (SUSY) breaking on the quintessence sector because even if one successfully constructs a quintessence model alone, the SUSY breaking effect will spoil the flatness of the potential. In particular, the mass scale of quintessence is at the order of current Hubble parameter $H_0 \sim 10^{-33}$ eV. On the other hand, quite often quintessence will acquire a mass that has the same order as the gravitino mass $m_{3/2}$ which, for example, is about TeV in gravity mediation models, way much larger then the mass scale of quintessence. This steepens the quintessence potential, yielding the field settles at the minimum in early time and one cannot distinguish it from a pure cosmological constant. To be more concrete, let us consider a simple model where the hidden and quintessence sector are separated in the \Kahler potential with the canonical form, \beq K = z^* z + Q^* Q, \eeq where $z$ and $Q$ are the chiral superfields of the hidden and quintessence sector respectively. This is a natural assumption in the sense that one would expect the interaction between the hidden and quintessence sector is as minimal as possible so there should be no cross terms in the \Kahler potential. Similarly, we assume the two sectors are separated in the superpotential as well, \beq W= W_0(z) + W_1(Q). \eeq Given the \Kahler potential and superpotential, the F-term scalar potential then reads \beq V_F = e^{K/\Mp^2}\left[D_i W K^{i\bar{j}}D_{\bar{j}}W^* - \frac{3}{\Mp^2} |W|^2 \right], \label{SUGRAFpotential} \eeq where $i$ and $j$ sum over the two sectors and \beq D_i W \equiv \frac{\p W}{\p \Phi^i} + \frac{W}{\Mp^2} \frac{\p K}{\p \Phi_i}. \eeq Here $\Mp$ is the reduced Planck mass $\Mp \equiv 1/\sqrt{8\pi G}$. Among various terms in the potential, there is a quadratic term of quintessence that couples to the superpotential of the hidden sector, \beq V \supset \frac{|W_0|^2}{\Mp^4} |\p_Q K|^2 = \frac{|W_0|^2}{\Mp^4} |Q|^2. \label{PolonyiMassTerm} \eeq As the gravitino mass $m_{3/2}$ is related to the superpotential by $\braket{|W_0|^2} \sim m^2_{3/2} \Mp^4$, we see that \beq V \supset m^2_{3/2}|Q|^2. \eeq Due to the large hierarchy between the gravitino mass scale and the current Hubble scale, such term will make quintessence roll down to the minimum and stick at there at a very early time, regardless how flat the potential is in the quintessence sector \textit{alone}. Observationally, quintessence then acts like a non-dynamical cosmological constant. If we wish to construct a quintessence model that can be observationally distinguishable from a pure cosmological constant, for example having a time-varying equation of state in the present epoch, then one needs to prevent the quintessence sector from acquiring such gravitino mass. One known method is to impose shift symmetry to the quintessence sector \cite{Brax:2009kd}. We will review this in Sec.\ref{sec:ShiftQ}, emphasizing that one can incorporate quintessence with all kinds of potential into supergravity using shift symmetry. As a particular example, we will show that a hidden supersymmetric QCD axion \cite{ArkaniHamed:2000tc} can naturally play the role of quintessence and be embedded into SUGRA. The cosmic coincidence problem is also ameliorated in such scenario. After reviewing the case with shift symmetry, in Sec.\ref{sec:SequesteredQ} we will show our attempt to construct a quintessence model where the quintessence and hidden sector are \textit{sequestered}, inspired by the brane-world scenario \cite{Randall:1998uk}. In such sequestered scenario, quintessence is protected from the SUSY breaking at least at the tree level, and it is possible to construct quintessence models of the small field type where the quintessence was frozen by Hubble damping for most of the time and only thawed recently. Yet, the constraint from the fifth force remains strong in this case and quintessence field value is limited in a tiny range, rendering it challenging to observationally distinguish such model from cosmological constant. However, in the phenomenological allowed range, exactly because of the small field displacement, the quantum correction beyond the tree level is well suppressed and the model is consistent from the effective field theoretic point of view. On the other hand, in the case with shift symmetry, the fifth force constraint is avoided. We conclude in Sec.\ref{sec:Discussion}. | \label{sec:Discussion} In this note we have discussed quintessence model building in supergravity. We stressed that there are two main issues when trying to construct supergravity quintessence models that are observationally indistinguishable from a pure cosmological constant. Firstly, for any realistic models, it is necessary to consider the effect of SUSY breaking which often gives quintessence a mass at the scale of the gravitino mass, which is much larger than that of the current Hubble scale. This renders the potential too steep such that quintessence settles at the minimum in the very early time and acts like a pure cosmological constant. One can avoid this problem by imposing shift symmetry on the quintessence sector, and an advantage of this approach is that it is much easier to embed any quintessence potential in this framework -- the quintessence potential is simply proportional to the real part of the superpotential. In addition, even though the shift symmetry is broken by the superpotential, because of the hierarchy between the quintessence energy scale and the Planck scale, such effect does not affect the quintessence dynamics. As an application, we considered the scenario where SUSY is broken at the TeV scale and there is a hidden SQCD axion that plays the role of quintessence. In such scenario, the dark energy scale is given by the electroweak scale and Planck scale, and the cosmic coincidence problem can be ameliorated. We also proposed another way to circumvent this issue, namely by sequestering the SUSY breaking and quintessence sectors. This approach is based on the picture of a higher dimensional theory where two sectors live on different 3-branes and only communicate with each other through gravity. We showed that indeed quintessence does not acquire a gravitino mass at least at the tree level. Once higher order terms kick in this generally no longer hold and one needs to assume some mechanism in quantum gravity preserves the form of the \Kahler potential as Eq.(\ref{gQQ}). However, for models with small field displacement, these higher order terms are Planck suppressed and hence do not disturb the dynamics of the quintessence field. The second main issue one needs to consider is the observational constraints from various gravitational tests like the fifth force constraint. In particular, the strongest source of the coupling between quintessence and matter stems from the exponential factor in the fermionic mass term Eq.(\ref{fermionmass}). In most models, this gives a strong constraint on the quintessence field range, and as we showed in Eq.(\ref{wDeviation}), how much the equation of state parameter can deviate from -1 is constrained by the quintessence field displacement. Therefore, in order to build quintessence models that can be observationally distinguishable from pure cosmological constant, it seems that one needs to ensure quintessence field does not appear in the exponential factor. In the case with shift symmetry, because the \Kahler potential Eq.(\ref{ShiftKahler}) does not depend on the imaginary part of the quintessence superfield which plays the role of the slow-roll quintessence field, the quintessence field does not appear in the exponential factor and hence the observational constraint on matter-quintessence coupling, $\alpha$, defined in Eq.(\ref{alpha}) can be satisfied even for large field displacement. In the sequestered scenario, because the quintessence field still appear in the exponential factor, field displacement is strictly limited by the fifth force constraint and it would be a challenging task to observationally distinguish such models from pure cosmological constant through equation of state parameter. | 18 | 8 | 1808.02279 |
|
1808 | 1808.10236_arXiv.txt | {In this study, we investigated the differences between four commonly-used exoplanet catalogs (exoplanet.eu; exoplanetarchive.ipac.caltech.edu; openexoplanetcatalogue.com; exoplanets.org) using a Kolmogorov--Smirnov (KS) test. We found a relatively good agreement in terms of the planetary parameters (mass, radius, period) and stellar properties (mass, temperature, metallicity), although a more careful analysis of the \emph{overlap} and \emph{unique} parts of each catalog revealed some differences. We quantified the statistical impact of these differences and their potential cause. We concluded that although statistical studies are unlikely to be significantly affected by the choice of catalog, it would be desirable to have one consistent catalog accepted by the general exoplanet community as a base for exoplanet statistics and comparison with theoretical~predictions.} \keyword{methods: statistical; astronomical data bases: miscellaneous; catalogs; planetary systems; stars:~statistics} \makeatletter \DeclareRobustCommand*\textsubscript[1]{% \@textsubscript{\selectfont#1}} \def\@textsubscript#1{% {\m@th\ensuremath{_{\mbox{\fontsize\sf@size\z@#1}}}}} \makeatother \renewcommand{\highlight}[2][yellow]{\mathchoice% {\colorbox{#1}{$\displaystyle#2$}}% {\colorbox{#1}{$\textstyle#2$}}% {\colorbox{#1}{$\scriptstyle#2$}}% {\colorbox{#1}{$\scriptscriptstyle#2$}}}% \begin{document} | } Since the detection of ‘\emph{51 Peg b}’, the first exoplanet around a main sequence star~\cite{B1-geosciences-343110}, many more planets around other stars have been discovered. Currently, more than 3500 exoplanets have been detected in our galaxy. The diversity of these exoplanets in terms of orbital and physical properties is overwhelming. This diversity challenges planet formation and evolution theories, which were tuned originally to explain the planets in our Solar System~\cite{B2-geosciences-343110,B3-geosciences-343110}. Several groups took it upon themselves to label and classify the known exoplanets, and compile catalogs to provide the scientific community with a comprehensive working tool to access the data and perform statistical studies of the exoplanet sample (hereafter, exostatistics). These databases include information about the physical properties of the planets, as well as their host stars. Analysis of this information is constantly improving our understanding of planet formation mechanisms~\cite{B4-geosciences-343110}, protoplanetary disks~\cite{B5-geosciences-343110}, and planetary composition and internal structure~\cite{B6-geosciences-343110,B7-geosciences-343110}. At the moment, several exoplanet catalogs are available and are used by the community (see~\cite{B8-geosciences-343110}). The most widely-used exoplanet catalogs are: \begin{enumerate}[label=\arabic*.] \item The Extrasolar Planets Encyclopaedia, \url{www.exoplanet.eu} (\cite{B9-geosciences-343110}; hereafter, EU). \item \textls[-20]{The NASA Exoplanet Archive, \url{https://exoplanetarchive.ipac.caltech.edu} (\cite{B10-geosciences-343110}; hereafter, ARCHIVE).} \item The Open Exoplanet Catalogue, \url{www.openexoplanetcatalogue.com/} (hereafter, OPEN). \item The Exoplanet Data Explorer, \url{www.exoplanets.org} (\cite{B11-geosciences-343110}; hereafter, ORG). \end{enumerate} These catalogs include data from ground-based observations as well as space missions such as CoRoT, Kepler, and K2. The available data in these catalogs are comprehensive and include the physical properties of the host star, available information on the planetary physical properties, and the referenced confirmation paper or other mentioned~source. The different teams of each catalog use different criteria to include a planet, which are usually based on the physical properties of the planet or statistical thresholds (see \tabref{tabref:geosciences-343110-t001}). Furthermore, each catalog has a different approach to displaying the database. For example, ARCHIVE designates a set of default parameters for each planet. This set is extracted from a single published reference to ensure internal consistency. Additional values published in other papers can only be found by viewing the pages dedicated to individual planets, where multiple sets of parameters are displayed. As a result, the ARCHIVE table provides a self-consistent set of parameters for any system, with missing values when the information is unavailable. On the other hand, EU uses a table displaying information on specific planet extracted from different sources, thus making for a more complete parameter set, though not necessarily~self-consistent. Many exostatistics papers use one of these catalogs as their source of observational data. Nevertheless, so far, the different catalogs have not been compared in terms of their possible differences and potential biases and selection effects that might affect inferred results and~conclusions. \begin{table}[H] \tablesize{\small} \centering \caption{The exoplanets catalogs inclusion~criteria.} \label{tabref:geosciences-343110-t001} \setlength{\cellWidtha}{\textwidth/3-2\tabcolsep-1.2in} \setlength{\cellWidthb}{\textwidth/3-2\tabcolsep-0.4in} \setlength{\cellWidthc}{\textwidth/3-2\tabcolsep+1.4in} \scalebox{1}[1]{\begin{tabular}{>{\PreserveBackslash\centering}m{\cellWidtha}>{\PreserveBackslash\centering}m{\cellWidthb}>{\PreserveBackslash\centering}m{\cellWidthc}} \toprule \textbf{Catalog} & \textbf{Object Mass Criterion} & \textbf{Reference Criteria}\\ \cmidrule{1-3} EU & \textless{}$60M_{J} \pm 1\sigma $ & Published or submitted to peer-reviewed journals or announced in conferences by professional astronomers.\\ ARCHIVE & \textless{}$30M_{J} $ & Accepted, refereed paper.\\ OPEN & Not listed & Open-source.\\ ORG & \textless{}$24M_{J} $ & Carefully vetted, peer-reviewed journal papers.\\ \bottomrule \end{tabular}} \end{table} In this work, we present a simple statistical comparison between the different exoplanet catalogs. We mainly focus on the EU, ARCHIVE and OPEN catalogs. The database of the ORG catalog contains a single and reliable set of parameters for each planet. However, since it has not been updated for a couple of years now (see website and discussion in Reference~\cite{B8-geosciences-343110}), we perform only a coarse comparison. As discussed in Reference~\cite{B8-geosciences-343110}, there are plans to restart regular updates in the near~future. | } Our analysis suggests that, although the main exoplanet catalogs overlap significantly, which results in similar distributions for most astrophysical parameters, the small discrepancies between the subsets highlight some of the catalogs’ biases. These biases can best be seen in the extreme ends of the examined distributions of small mass, long orbital period planets or small stars (less than \mbox{our sun}). These biases do not only result from different numbers of confirmed planets in each catalog, but mainly from contributing factors, related to the data collection policy of each catalog, such as: \mbox{The process} each catalog uses to present and collect the properties of a specific planet, the decision whether to include a controversial object as a planet, or the routine maintenance each catalog team performs to its current listed~planets. Furthermore, in our analysis, we excluded planets with masses larger than $M_{p} > 10M_{J} $. However the different catalogs use different mass boundaries, which also adds to their different biases. Unfortunately, most of the biases we found are due to the use of various subjective criteria in compiling and maintaining the database. Although all catalogs usually include in their database planets announced in peer-reviewed publications, this should not be the only criterion for a confirmed planet. We suggest that the explosive growth in the known planet population in recent years once again highlights the need for a more rigorous and objective mechanism to tag planets as confirmed. The differences among the catalogs demonstrate that there are conflicting views in the community regarding such criteria. The International Astronomical Union (IAU) is an objective and well-accepted authority by the community, and we therefore suggest that a central catalog could be maintained by Division F (Planetary Systems and Bio-astronomy) of the IAU, and specifically its Commission F2 (Exoplanets and the Solar Systems). Discussions within the commission should resolve the various differences and arrive at a system that can be agreed~upon. After performing this analysis and scrutinizing the different calculated biases, we can carefully make the following statements: \begin{enumerate}[label=$\bullet$] \item The ARCHIVE catalog is the most up-to-date catalog, with recent Kepler and K2 planet discoveries. It is also the least biased catalog in terms of the interpretation of the mass upper limit, being the true value or the adoption of a model-based value instead of a genuine measurement. Another interesting feature the catalog has is a list of “removed targets” displaying objects that had been listed in the catalog but were removed, suggesting a more rigorous process applied by the ARCHIVE~team. \item The EU catalog is less restrictive when listing the planetary properties, and therefore could include imprecise estimates. The EU catalog differs the most with the \emph{overlap} subsets, probably due to its more permissive acceptance criteria and the use of mixed sources of information. However, it has the most wide and large coverage of~planets. \item The OPEN catalog is somewhere in the middle, between ARCHIVE and EU. In some cases, we find that it resembles the EU subsets, while in others the ARCHIVE. This might not be surprising, given that this catalog is an open-source catalog which is managed and updated by the astronomical community. Although its interface is elegant and user friendly, it has its drawbacks, especially the lack of detection reference and a smaller planet~population. \end{enumerate} Finally, while each catalog suffers biases, for an exostatistics work, there should not be too much difference among the databases, since the planet population (especially the one compared in \mbox{this work}) is large enough to wash out the small biases and discrepancies. Nevertheless, we find the fusion of catalogs (the \emph{overlap} subset) a powerful tool as a starting point for increasing the reliability of exostatistics research. A promising platform seems to be the Data \& Analysis Center for Exoplanets (DACE) database (\url{https://dace.unige.ch}), which includes a linked table to commonly-used exoplanet catalogs. DACE offers an accessible option to check the properties of a specific planet listed in different catalogs, and to compare its properties as they are displayed on the~catalogs. Besides a careful and detailed inspection of each exoplanet related paper confirmation, other useful techniques that can be used to increase the confidence of some exoplanet databases is to check other related parameters such as: Discovery date and update times, which can solve issues of “catch-up” times between catalogs and the rate by which they upload new exoplanets; a measure of the velocity semi-amplitude K parameter can suggest the mass measurement is truly deduced from a RV measurement and not derived from some theoretical model; a TTV flag with reported eccentricity parameter can suggest the reported mass measurement is probably not an upper limit, but some nominal~value. \vspace{6pt} \authorcontributions{Formal analysis, D.B.; Investigation, D.B., R.H. and S.Z.; Methodology, D.B., R.H. and S.Z.; Supervision, R.H. and S.Z.; Validation, D.B., R.H. and S.Z.; Writing--original draft, D.B.; Writing--review \& editing, D.B., R.H. and S.Z.} \funding{This research was supported by the Ministry of Science, Technology and Space,~Israel.} | 18 | 8 | 1808.10236 |
1808 | 1808.06616_arXiv.txt | The fraction of stars forming in compact, gravitationally bound clusters (the `cluster formation efficiency' or CFE) is an important quantity for deriving the spatial clustering of stellar feedback and for tracing star formation using stellar clusters across the Universe. Observations of clusters in nearby galaxies have revealed a strong dependence of the CFE on the local gas density, indicating that more stars form in star clusters when the star formation rate surface density is higher. Previously, it has not been possible to test this relation at very young ages and in clusters with individual stars resolved due to the universally-low densities in the cluster-forming regions in the Local Group. This has even led to the suggestion that the CFE increases with distance from the Sun, which would suggest an observational bias. However, the Central Molecular Zone of the Milky Way hosts clouds with densities that are orders of magnitude higher than anywhere else in the Local Group. We report a measurement of the CFE in the highest-density region in the Galaxy, Sgr~B2, based on ALMA observations of high-mass young stellar objects. We find that over a third of the stars ($37\pm7\%$) in Sgr~B2 are forming in bound clusters. This value is consistent with the predictions of environmentally-dependent models for the CFE and is inconsistent with a constant CFE in the Galaxy. \vspace{10mm} | Gravitationally bound stellar clusters are some of the most important objects in astronomy, providing both luminous probes of the star formation process at great distances \citep[e.g.,][among many others]{Brodie2006a,Adamo2013b,Kruijssen2018b,Kruijssen2018a} and large coeval and co-located samples of stars in the local universe. The prevalence of these clusters varies substantially with environment: the fraction of star formation occurring in bound, compact clusters, i.e.,~the \emph{cluster formation efficiency} (CFE) $\Gamma$ is not constant \citep{Adamo2015a,Johnson2016a,Messa2018a}. \citet{Kruijssen2012a} proposed a theory in which $\Gamma$ is a function of gas density\footnote{In galactic disks in hydrostatic equilibrium, this can be expressed as the observable gas surface density, whereas in non-equilibrium environments, the model depends on the gas volume density.}, with secondary dependences on other global environmental quantities. While this theory reasonably explains observations spanning many galaxies, it has not yet been directly tested in a high-density environment where both the unclustered and clustered stars are detected in a spatially resolved sense. In this Letter, we perform such a test in the Sgr~B2 cloud, a high-density region in the Galactic center in which both stars and compact clusters (taken to be a proxy for gravitational boundedness) are presently forming. | We have measured the cluster formation efficiency in the Galactic Center cloud Sgr~B2, resulting in $\Gamma=37\pm7\%$. This CFE is higher than that in the solar neighborhood, implying that the CFE varies within the Milky Way. Specifically, it changes with the galactic environment in a way that correlates with the local gas conditions. This observation is consistent with existing extragalactic observations. However, it additionally rules out the idea that the environmental dependence of the CFE is exclusively driven by an underlying dependence on the distance from the Sun, which affected previous work and would have been suggestive of an observational bias. Instead, our results show that the environmental variation of the CFE is a physical effect. The CFE model of \citet{Kruijssen2012a}, in which higher average gas densities yield higher CFEs due to shorter free-fall times and higher star formation efficiencies, successfully predicts the observed value to within the uncertainties. \begin{table*}[htp] \centering \begin{minipage}{130mm} \caption{Model parameters} \begin{tabular}{ccccccc} \label{tab:model} Quantity & Units & Median & Uncertainty & `Global model' & `Local model' & Reference \\ \hline $\log{\Sigma}$ & [$\msun~\pc^{-2}$] & 3.00 & 0.22 & \checkmark & & 4 \\ $\Omega$ & [$\myr^{-1}$] & 1.80 & 0.25 & \checkmark & & 6,8 \\ $\log{\rho}$ & [$\msun~\pc^{-3}$] & 2.84 & 0.22 & & \checkmark & 9 \\ $c_{\rm s}$ & [$\kms$] & 0.53 & 0.07 & & \checkmark & 3,5 \\ $\log{\sigma}$ & [$\kms$] & 1.00 & 0.07 & \checkmark & \checkmark & 7 \\ $\log{\Sigma_{\rm GMC}}$ & [$\msun~\pc^{-2}$] & 3.61 & 0.18 & \checkmark & \checkmark & 2,10 \\ $\log{\alpha_{\rm vir}}$ & [--] & 0.04 & 0.16 & \checkmark & \checkmark & 10 \\ $\log{\beta_0}$ & [--] & $-0.47$ & 0.32 & \checkmark & \checkmark & 1,2 \\ $t_{\rm view}$ & [$\myr$] & 0.74 & 0.16 & \checkmark & \checkmark & 6 \\ \hline $f_{\rm bound,global}$ & [\%] & 44.8 & 13.1 & \checkmark & & this work \\ $f_{\rm bound,local}$ & [\%] & 48.9 & 11.6 & & \checkmark & this work \\ \hline \end{tabular}\\ \tablerefs{ (1) \citealt{Barnes2017b}, % (2) \citealt{Federrath2016b}, % (3) \citealt{Ginsburg2016a}, % (4) \citealt{Henshaw2016b}, % (5) \citealt{Krieger2017a}, % (6) \citealt{Kruijssen2015a}, % (7) \citealt{Kruijssen2018c}, % (8) \citealt{Launhardt2002a}, % (9) \citealt{Longmore2013b}, % (10) \citealt{Walker2015a}. % } \end{minipage} \end{table*} \textit{Acknowledgements:} We thank the anonymous referee for a timely and helpful report that led to substantial improvement of the paper. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This paper makes use of the following ALMA data: ADS/JAO.ALMA\#2013.1.00269.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. JMDK gratefully acknowledges funding from the German Research Foundation (DFG) in the form of an Emmy Noether Research Group (grant number KR4801/1-1), from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme via the ERC Starting Grant MUSTANG (grant agreement number 714907), and from Sonderforschungsbereich SFB 881 ``The Milky Way System'' (subproject P1) of the DFG. \software{ The software used to make this version of the paper is available from github at \url{https://github.com/keflavich/SgrB2_ALMA_3mm_Mosaic/} and \url{https://github.com/keflavich/SgrB2_CFE}. Pyspeckit \citep{Ginsburg2011c} was used for the line fitting. Plots were made with matplotlib \citep{Hunter2007a}. } | 18 | 8 | 1808.06616 |
1808 | 1808.01941_arXiv.txt | We study the evolution of cosmological perturbations around a homogeneous and isotropic background in the framework of the non-minimal torsion-matter coupling extension of $f(T)$ gravity. We are concerned with the effects of the non-minimal coupling term on the growth of matter overdensities. Under the quasi-static approximation, we derive the equation which governs the evolution of matter density perturbations, and it is shown that the effective gravitational coupling 'constant' acquires an additional contribution due to the non-minimal matter-torsion coupling term. In this way, this result generalizes those previously obtained for the growth of matter overdensities in the case of minimal $f(T)$ gravity. In order to get a feeling of our results we apply them to the important case of a power-law coupling function, which we assume to be the responsible for the late-time accelerated expansion in the dark energy regime. Thereby, analytic solutions for the matter density perturbation equation in the regime of dark matter dominance and the dark energy epoch are obtained, along with a complete numerical integration of this equation. In particular, we show that this model predicts a growth index larger than those obtained for $\Lambda$CDM model, indicating therefore a smaller growth rate. Concomitantly, we show that the model at hand is potentially capable in alleviating the existing $\sigma_{8}$-tension, being that it can provide us a $f\sigma_{8}$ prediction which is $\sim 4-5$ per cent below the respective prediction of concordance model. | Dark energy is one of the most amazing findings in modern cosmology \cite{Riess:1998cb,Perlmutter:1998np,Ade:2013zuv,Aghanim:2018eyx}. This dark component is responsible by the accelerated expansion of the Universe and its nature is still one of the great mysteries of the Big Bang \cite{Copeland:2006wr,Frieman:2008sn1,Nojiri:2017ncd}. Furthermore, dark energy together with dark matter, another mysterious component \cite{Zwicky:1933gu}, constitute $68 \%$ and $27 \%$, respectively, of the total mass-energy of the present Universe, remaining only $5 \%$ for the normal baryonic matter \cite{AmendolaTsujikawa,FR-reviews7}. There are two principal ways to proceed in the study of the nature and properties of this dark energy entity. The first one is considering it as a new modified matter source which is described for example through a scalar field such as quintessence \cite{Ratra:1987rm,Copeland:1997et,Caldwell:1997ii,Barreiro:1999zs}, tachyon field \cite{Sen:2002nu,Sen:2002in,Padmanabhan:2002cp,Abramo:2003cp,Copeland:2004hq}, k-essence \cite{Chiba:1999ka,ArmendarizPicon:2000dh,ArmendarizPicon:2000ah}, or, dilatonic ghost condensate \cite{Piazza:2004df,Gasperini:2001pc,Szydlowski:2006pz}, etc, being that in all these models, the scalar field contributes with a negative pressure which drives the accelerated expansion. On the other hand, the second one alternative is represented by modified gravity theories, which are mainly based on quantum corrections to the Einstein-Hilbert action of General Relativity (GR), in the form of additional higher curvature terms such as $f(R)$ gravity theories \cite{FR-reviews1,FR-reviews2,FR-reviews3,FR-reviews4,FR-reviews5,FR-reviews6}. In this last approach, one may identify in the modified Friedmann equations an effective dark energy density and its corresponding pressure density, which comes to have an origin in quantum corrections to GR, and therefore it becomes conceptually different from a modified matter model \cite{AmendolaTsujikawa,FR-reviews7}. It is well known that gravity can be described in terms of curvature, as is usually done in GR and $f(R)$ gravity theories, or through torsion, in which case we have the so-called teleparallel equivalent of GR or simply Teleparallel Gravity (TG) \cite{Einstein,TranslationEinstein,Early-papers1,Early-papers2,Early-papers3,Early-papers4,Early-papers5,Early-papers6}. In TG the dynamical variables are the tetrad fields instead of the metric tensor $g_{\mu \nu}$, and the usual torsionless Levi-Civita connection of GR is replaced by the Weitzenb\"{o}ck connection, which has torsion but no curvature \cite{Aldrovandi-Pereira-book,JGPereira2,AndradeGuillenPereira-00}. So, TG is a classical gauge theory for gravitation based in the translation group, that due to existence of "soldering" between the Minkowski tangent space (fiber) and the spacetime (base space), it becomes a non-standard gauge theory, keeping nevertheless a remarkable similarity to electromagnetism, also a gauge theory for an abelian group \cite{Aldrovandi-Pereira-book,Arcos:2005ec}. It is worth noting that, the Lagrangian density of GR, the scalar curvature $R$, and the Lagrangian density of TG, the scalar torsion $T$, only differ in a total derivative term, and despite being conceptually speaking different theories they are equivalent in the level of field equations \cite{Aldrovandi-Pereira-book}. In the context of modified gravity theories one may also start to introduce modifications to gravity from this torsion-based formulation, in a similar fashion to the curvature-based one. Thus, in a close analogy with the $f(R)$, the $f(T)$ gravity theory is obtained by extending the Lagrangian density of TG, that is to say, the scalar torsion $T$, to an arbitrary function of the same scalar $T$ \cite{Bengochea:2008gz,Linder:2010py}. Although GR and TG are equivalent theories, the $f(R)$ and $f(T)$ gravity represent different modified gravity theories. In comparison with $f(R)$, whose field equations are of fourth-order, the $f(T)$ gravity has the advantage that its dynamics is given by second-order differential equations \cite{Ferraro:2006jd}. This remarkable characteristic, added to the fact that $f(T)$ gravity allow us to explain the currently observed accelerated expansion of the Universe, has given rise to a fair number of papers on these gravity theories, in which several features of $f(T)$ gravity have been examined, including observational solar system constraints~\cite{Iorio-Saridakis-2012,Iorio-2015,Farrugia-2016}, cosmological constraints~\cite{Bengochea-2011,Wei-Ma-Qi-2011,Capozziello-Luongo-Saridakis-2015,Oikonomou-Saridakis-2016,Nunes-Pan-Saridakis-2016}, dynamical behavior~\cite{Wu-Yu-b-2010}, cosmological perturbations~\cite{Chen:2010va,Dent-Duta-Saridakis-2011,Zheng-Huang-2011,Wu:2012hs,Izumi:2012qj,Li:2011wu,Cai:2015emx,Nesseris:2013jea,Basilakos:2016xob}, spherically symmetric solutions~\cite{Wang-2011,Atazadeh:2012am,Ruggiero:2015oka}, the existence of relativistic stars~\cite{Stars-in-f(T)}, cosmographic constraints~\cite{Cosmography-2011}, energy conditions bounds~\cite{Liu-Reboucas-2012}, homogeneous G\"{o}del-type solutions \cite{Liu:2012kka,Otalora:2017qqc} and gravitational waves (GWs) constraints \cite{Cai:2018rzd,Li:2018ixg}. For an excellent review on $f(T)$ gravity see Ref. \cite{Cai:2015emx} and for some others important aspects on it such as Local Lorentz invariance, see Refs. \cite{Li:2010cg,Krssak:2015oua}. A very important generalization of $f(T)$ gravity is obtained by allowing a non-minimal coupling between torsion and matter \cite{Harko:2014aja,Farrugia:2016pjh,Harko:2014sja,Carloni:2015lsa}. This non-minimal coupling arises in a close analogy with the curvature-matter coupling in $f(R)$ gravity \cite{Nojiri:2004bi,Allemandi:2005qs,Nojiri:2006ri,Bertolami:2007gv,Harko:2008qz,Harko:2010mv,Bertolami:2009ic,Bertolami:2013kca,Wang:2013fja,Bertolami:2011fz,Bertolami:2010cw,Gomes:2016cwj,Otalora:2018bso}, whose origin can have several motivations. It is well known for example that non-minimal coupling terms acting as counterterms are required when quantizing a self-interacting scalar field in curved spacetime \cite{Birrell:1982ix}. Thus, one may be tempted to relate the need for this non-minimal coupling between gravity and matter to the existence of scalar-tensor theories and the low-energy limit of string theory \cite{Asselmeyer-Maluga:2016mvv}. In Ref. \cite{Harko:2014sja} the authors have proposed a new extension of the $f(T)$ gravity by including the coupling with an arbitrary function of the scalar tensor $T$ to the matter Lagrangian density. For the Friedmann-Robertson-Walker (FRW) background geometry, they have shown that this novel theory allows us to obtain an effective dark energy sector whose equation-of-state (EOS) parameter can be quintessence- or phantom-like, or exhibit the phantom-divide crossing, being that for a large range of the model parameters the Universe undergoes a de Sitter, dark-energy-dominated, accelerating phase. Furthermore, it can provide an early-time inflationary solution too, and hence, it is also possible an unified description of the history of cosmological expansion. On the other hand, in Ref. \cite{Carloni:2015lsa} has been studied the cosmological applications for this model from the perspective of dynamical systems by extracting the fixed points corresponding either to dark-matter-dominated, scaling decelerated solutions, or to dark-energy-dominated accelerated solutions, and thus studying their cosmological properties. The investigation of small fluctuations around the FRW background in the framework of cosmological perturbation theory has become a cornerstone of modern cosmology. It allows us to confront any cosmological model with observations of cosmic microwave background (CMB) and large-scale structure (LSS) \cite{AmendolaTsujikawa}. Thereby, in order to reveal the full scope and predictive power of theory at hand, one must go beyond the background and enter in the perturbative level. The main goal of the present paper is to study the evolution of cosmological perturbations for this non-minimal matter-torsion coupling model \cite{Harko:2014sja,Carloni:2015lsa} in the FRW background. In particular, we are interested in the evolution of scalar perturbations and the growth of matter overdensities. The paper is organized as follows. In Section \ref{NMTC_Theories}, we introduce the non-minimal matter-torsion coupling theories together with the effective dark energy sector in the FRW background. In section \ref{LSPert} we define the perturbed tetrad field and obtain the corresponding linearised field equations along with the evolution equations for the matter density perturbations. In section \ref{Growth_Matter} we study the growth of matter overdensities under the quasi-static approximation for sub-horizon scales and for a power-law coupling function. Finally, in Section \ref{Concluding_Remarks}, we summarize our findings and present our main conclusions and final remarks. | In studying the dark energy problem of cosmology a very interesting class of modified gravity models are the so-called $f(T)$ gravity theories \cite{Bengochea:2008gz,Linder:2010py,Ferraro:2006jd}, that generalize the teleparallel equivalent of GR \cite{Einstein,TranslationEinstein,Early-papers1,Early-papers2,Early-papers3,Early-papers4,Early-papers5,Early-papers6}, in which gravity is described through torsion and not curvature \cite{Aldrovandi-Pereira-book,JGPereira2,AndradeGuillenPereira-00,Arcos:2005ec}. These torsion-based modified gravity theories constitute a good alternative to the conventional based-curvature modified gravity models \cite{Cai:2015emx}. Furthermore, in the same spirit of non-minimal $f(R)$ gravity theories \cite{Nojiri:2004bi,Allemandi:2005qs,Nojiri:2006ri,Bertolami:2007gv,Harko:2008qz,Harko:2010mv,Bertolami:2009ic,Bertolami:2013kca,Wang:2013fja,Bertolami:2011fz,Bertolami:2010cw,Gomes:2016cwj,Otalora:2018bso}, one may think in an attractive generalization of this framework, by allowing a non-minimal coupling between matter and torsion \cite{Harko:2014sja}. This generalized theory has proven to have very important features at background level, providing an explanation for both late-time accelerated expansion, and early-time inflationary phase, that in this way, it also could lead to a possible unified description of the cosmological expansion history \cite{Carloni:2015lsa}. Additionally, it is fundamental a study of cosmological perturbations in order to compare all the predictions and results obtained from the model with the observational data of cosmic microwave background (CMB) and large-scale structure (LSS) \cite{AmendolaTsujikawa}. In the present paper we have studied the evolution of scalar cosmological perturbations in these non-minimal torsion-matter coupling theories. In particular, by using the quasi-static approximation in sub-horizon scales we have obtained the evolution equation for matter density perturbations as written in Eq. \eqref{MatterEq}. Thus, we have found an effective gravitational coupling ’constant’ $G_{eff}$ given by Eq. \eqref{Geff}, which carries an additional contribution $G_{c}$, as defined in Eq. \eqref{G2}, whose origin is related to the non-minimal matter-torsion coupling function. This result constitutes a generalization of those previously obtained for the growth of matter overdensities in minimal $f(T)$ gravity, since the strength of the gravitational coupling, as given by $G_{eff}/G$, now depends on both functions, $f_{1}(T)$ and $f_{2}(T)$, and their derivatives. In Eq. \eqref{Geff2} we have rewritten $G_{eff}$ in terms of the relevant cosmological parameters, the deceleration parameter $q$ and the fractional matter density $\Omega_{m}$, and the new parameters $\Sigma(T)=f_{2}/(F/\kappa)$, $m_{1}(T)=T F'/F$ and $m_{2}(T)=T f'_{2}/f_{2}$, which encode all the information about the model in the functions $f_{1}(T)$ and $f_{2}(T)$. Clearly, in order to decide something about the behaviour of the ratio $G_{eff}/G$, it is necessary to know the specific functional form of this set of parameters. By applying our results to a particular model, we have considered the important case of a power-law coupling function $f_{2}(T)=1+(T/T^{*})^{n}$, with $n$ negative and $T^{*}$ a characteristic torsion scale, which we have assumed to be the dominant term in late-times, and hence leading the accelerated expansion of the Universe in the dark energy regime. Here, to isolate the effects of the non-minimal coupling between torsion and matter we have taken the pure gravitational sector to have the teleparallel equivalent form of GR, that is to say, $f_{1}(T)=T$. With these assumptions we have analytically solved the evolution equation of matter overdensities in Eq \eqref{delta2}, in two different asymptotic regimes through cosmological evolution, the dark matter-dominated era, $(T/T^{*})^{n}\ll 1$, and dark energy-dominated epoch, $(T/T^{*})^{n}\gg 1$. We have obtained the analytic solution for the matter overdensity \eqref{deltaDM} validates for the dark matter-dominated regime, which allows us to compare with the solution $\delta\sim a$ for the standard cold dark matter era. Clearly, our solution \eqref{deltaDM} reproduces this linear growth with the scalar factor in the limit $(T/T^{*})^{n}\rightarrow 0$, but more importantly, one can see that the factor $12 n/\left[5 \left(1-n\right)\left(1-3 n\right)\right]$ works in attenuating or enhancing the growth of matter overdensities. However, although for negative $n$ the growth rate $f=\delta'/\delta$ is increased, the deviation with respect to the standard matter model is very small and it becomes smaller yet for $\left|n\right|\gtrsim 1$. On the other hand, for the dark energy regime we have found the analytic solution \eqref{deltaDE}. This solution allows us to show that for $n<-1/2$ the growth of matter perturbations is approximately frozen, such that the growth rate decays as $f=\delta'/\delta \sim a^{(1+2 n)/(2 (1-n))}$. An interesting additional conclusion about this solution can be obtained if one puts the growth rate $f$ in terms of the ratio $G_{eff}/G$ as in Eq. \eqref{fGeff}. This relation implies that an effectively weakened gravitational coupling, i.e. $G_{eff}/G<1$, produces a growth of matter overdensities slower than in $\Lambda$CDM, as it has been further corroborated by our results in a subsequent numerical analysis. Since the transition between the dark matter and dark energy-dominated eras is not included in the above analytical analysis, a complete numerical analysis is also required. Thus, we have numerically solved the matter perturbation equation \eqref{delta3}. In this way, from the numerical solution for the matter perturbation $\delta$ (FIG \ref{FIG3} (upper graph)) we have computed the growth index $\gamma=\ln f/\ln \Omega_m$ (FIG \ref{FIG3} (lower graph)), for several different values of power $n$ at a scale $k=0.1 h$ $Mpc^{-1}$ of the linear regime. From this numerical analysis we have ratified all the results previously obtained in the analytical analysis. Also, we have found that our model predicts a growth index larger than those obtained for $\Lambda$CDM model, indicating therefore a smaller growth rate. In full agreement with these results, we also have found that the model at hand is potentially capable in relaxing the existing $\sigma_{8}$-tension, once that it can provide us a $f\sigma_{8}$ prediction which is $\sim 4-5$ per cent below the corresponding $\Lambda$CDM prediction. Furthermore, after comparing with its counterpart in $f(R)$ gravity, where a non-minimally coupling of power-law form between matter and curvature has been studied \cite{Bertolami:2013kca,Wang:2013fja}, we found that a non-minimal power-law coupling between matter and torsion in $f(T)$ gravity becomes more constrained than those in $f(R)$ gravity. The explicit coupling between the torsion scalar and the matter Lagrangian density has as consequence an energy exchange between matter and gravity, which manifests itself in the non-vanishing of the covariant divergence of the matter stress-energy tensor as it is shown in Eq. \eqref{NonConsLAw}. This non-conservation of energy can be interpreted as a failure of the theory in relation to the so-called metric postulates \cite{C_M_Will}, as it could generate a non-geodesic motion of test bodies, and therefore it also could imply a possible violation of Einstein equivalence principle (EEP) \cite{Bertolami:2007gv}. Furthermore, in Ref. \cite{Bertolami:2007gv} it was also suggested that for the parametrization $f_{2}(R)=1+\lambda \tilde{f}_{2}(R)$ of the non-minimal coupling function between curvature and matter one could potentially tune the parameter $\lambda$ with the purpose of reducing the effects of such violation below current experimental accuracy. Nevertheless, as it has also been shown in Ref. \cite{Sotiriou:2008it}, the metric postulates or the non-conservation of energy do not themselves provide quantitative estimates of the deviations from the EEP. So, in order to decide something with respect to the relationship between the values of the parameters $n$, and $T^{*}$, for the parametrization $f_{2}(T)=1+\left(T/T^{*}\right)^n$ of non-minimal torsion-matter coupling function, and the measured bounds of the EEP, a more detailed study must be performed in this direction. This necessary study lie beyond the scope of the present work, and is left for a separate project. Finally, it is important to highlight that due to the Local Lorentz violation in $f(T)$ gravity, and its extensions, one has an extra degree of freedom represented by the scalar perturbation $\chi$ in the perturbative framework developed for these theories \cite{Zheng-Huang-2011,Wu:2012hs,Izumi:2012qj,Li:2018ixg}. As it also happens in the miniminal case, we have shown that in the non-minimal extension of $f(T)$ gravity, this additional scalar perturbation $\chi$ does not have a significant contribution on the growth of matter overdensities at sub-horizon scales. However, as it has been shown in Refs. \cite{Wu:2012hs,Li:2011wu} for the $f(T)$ gravity theory, it is expected that at super-horizon scales this new scalar mode has an important effect on the evolution of matter perturbation. | 18 | 8 | 1808.01941 |
1808 | 1808.01170_arXiv.txt | We show that a nonsingular bounce, free of ghosts and gradient instabilities, can be realized in the framework of Horndeski or generalized Galileon cosmology. In particular, we first review that the theoretical {\it no-go} theorem, which states that the above is impossible, is based on two very strong assumptions, namely that a particular quantity cannot be discontinuous during the bounce, and that there is only one bounce. However, as we show in the present work, the first assumption not only can be violated in a general Horndeski/Galileon scenario, but also it is necessarily violated at the bounce point within the subclass of Horndeski/Galileon gravity in which $K(\phi,X)$ becomes zero at $X=0$. Additionally, concerning the second assumption, which is crucial in improved versions of the theorem which claim that even if a nonlinear free of pathologies can be realized it will lead to pathologies in the infinite past or infinite future, we show that if needed it can be evaded by considering cyclic cosmology, with an infinite sequence of nonsingular bounces free of pathologies, which forbids the universe to reach the ``problematic'' regime at infinite past or infinite future. Finally, in order to make the analysis more transparent we provide explicit examples where nonsingular bounces without theoretical pathologies can be achieved. | Nonsingular bouncing cosmologies may offer a potential solution to the problem of cosmological singularity \cite{Mukhanov:1991zn}. In particular, although inflation is considered to be a crucial part of the history of our universe \cite{inflation}, it is still accompanied by the above problem, since such a big bang singularity is unavoidable if inflation is driven by a scalar field in the framework of general relativity \cite{Borde:1993xh}. Hence, alongside the efforts to alleviate the initial singularity through quantum gravity effects, a significant amount of research directs towards its solution through the bounce realization. Bounce cosmology \cite{Novello:2008ra, Brandenberger:2012zb, Cai:2014bea, Battefeld:2014uga, Brandenberger:2016vhg, Cai:2016hea} can be realized by various modified gravity constructions \cite{Nojiri:2006ri, Capozziello:2011et, Cai:2015emx}, such as the Pre-Big-Bang \cite{Veneziano:1991ek} and the Ekpyrotic \cite{Khoury:2001wf, Khoury:2001bz} scenarios, higher-order gravity \cite{Tirtho1, Nojiri:2013ru}, $f(R)$ gravity \cite{Bamba:2013fha, Nojiri:2014zqa, Pavlovic:2017umo}, $f(T)$ gravity \cite{Cai:2011tc}, massive gravity \cite{Cai:2012ag}, braneworld models \cite{Shtanov:2002mb, Saridakis:2007cf}, non-relativistic gravity \cite{Cai:2009in, Saridakis:2009bv}, loop quantum cosmology \cite{Bojowald:2001xe, Cai:2014zga, Odintsov:2015uca}, Lagrange modified gravity \cite{Cai:2010zma} etc. Alternatively, nonsingular bouncing cosmology may be studied through the application of effective field theory techniques, and the introduction of matter sectors that violate the null energy condition \cite{Cai:2007qw, Cai:2008qw, Cai:2009zp, Nojiri:2015fia}, or of non-conventional mixing terms \cite{Saridakis:2009jq, Saridakis:2009uk}. Such constructions can additionally provide an explanation for the scale invariant power spectrum \cite{Wands:1998yp, Finelli:2001sr} and moderate non-Gaussianities \cite{Cai:2009fn, Li:2016xjb}. In summary, bouncing cosmology may be considered as a potential alternative to the big bang one. A general class of gravitational modification are the so-called galileon theories \cite{Nicolis:2008in, Deffayet:2009wt, Deffayet:2009mn, DeFelice:2011bh}, which are a re-discovery of the general scalar-tensor theory constructed by Horndeski under the requirement of maintaining the equations of motion second-ordered \cite{Horndeski:1974wa}. Application of the Horndeski/Galileon theory at a cosmological framework proves to be very interesting and thus it has been investigated in detail in the literature. In particular, one can study the late-time acceleration \cite{Silva:2009km, Gannouji:2010au, DeFelice:2010pv, Tretyakov:2012zz, Leon:2012mt}, inflation \cite{Creminelli:2010ba, Kobayashi:2010cm, Ohashi:2012wf} and non-Gaussianities \cite{Mizuno:2010ag, Gao:2011qe, RenauxPetel:2011uk}, cosmological perturbations \cite{Kobayashi:2009wr, DeFelice:2010as, Barreira:2012kk}, or use observational data to extract constraints on various sub-classes of the theory \cite{Ali:2010gr, Iorio:2012pv, Appleby:2012ba}. One interesting feature of Horndeski/Galileon theories is that they offer the framework for the realization of bouncing cosmology. In particular, one can obtain bouncing solutions in various sub-classes of the theory, describing both the background evolution as well as the generation of perturbations \cite{Qiu:2011cy, Easson:2011zy, Cai:2012va, Osipov:2013ssa, Qiu:2013eoa, Battarra:2014tga, Qiu:2015nha, Banerjee:2016hom, Ijjas:2016tpn, Ijjas:2016vtq, Ijjas:2017pei, Saridakis:2018fth}. Despite the success of Horndeski/Galileon theories in generating nonsingular bouncing solutions, there is a discussion on whether these solutions are stable. In particular, in \cite{Kobayashi:2016xpl, Akama:2017jsa, Kolevatov:2016ppi, Kolevatov:2017voe} the authors presented a theoretical {\it no-go} theorem stating that nonsingular models with flat spatial sections suffer in general from gradient instabilities or pathologies in the tensor sector. The proof of this theorem is based on two strong assumptions, namely that a specific non-observable quantity related to the tensor perturbation remains finite at the bounce point, and that there is only one bounce. However, this is not the general case, and indeed one can show that in successful and stable bounces the above assumption(s) are violated. Hence, the above theorem can be evaded and stable nonsingular bounces can be safely realized in the framework of Horndeski/Galileon cosmology. For instance, with the correspondence between the effective field theory (EFT) formalism and Horndeski/Generalized Galileon theories made in \cite{Gleyzes:2013ooa}, one may avoid this issue in bounce cosmology by modifying the dispersion relation for cosmological perturbations with the help of certain EFT operators \cite{Cai:2016thi, Cai:2017tku, Cai:2017dyi}. In the following we explicitly show how the theoretical {\it no-go} theorem on nonsingular bounces in Horndeski/Galileon cosmology can be evaded. We mention here that there is another {\it no-go} theorem from the observational perspective, which indicates that the parameter space for single-field nonsingular bounces is extremely limited due to the severe tension between tensor-to-scalar ratio and primordial non-gaussianity \cite{Quintin:2015rta, Li:2016xjb} (which in turn needs additional mechanisms to amplify the scalar perturbations \cite{Fertig:2016czu}). In the present work we refer only to the theoretical no-go theorem, namely our goal is to show that there is not a ``theoretical no-go theorem'', in the sense of a mathematically proven theorem of general validity, that forbids a non-singular bounce, and not to construct a bounce in perfect agreement with every observational requirement (which would require the thorough incorporation of background (SNIa, BAO, CMB shift parameter, $H_0$ measurements, etc) as well as perturbation ($f\sigma_8$) related data). Hence, even if a nonsingular bounce is difficult to be constructed from the observational point of view, it is not mathematically impossible. The plan of the manuscript is as follows: In Section \ref{nogotheorem} we review the theoretical {\it no-go} theorem, mentioning the assumptions on which it is based. In Section \ref{Evading} we show that the aforementioned theorem is based on two strong assumption which for general sub-classes of the theory can be violated, and thus offering a safe evading of the theorem. Additionally, we provide explicit examples where nonsingular bounces free of ghost and gradient instabilities can be realized in Horndeski/Galileon cosmology. Finally, in Section \ref{Conclusions} we summarize the obtained results. | \label{Conclusions} In this work we showed that a nonsingular bounce, free of ghosts and gradient instabilities, can be realized in the framework of Horndeski or generalized Galileon cosmology. This result was known through specific models \cite{Easson:2011zy, Cai:2012va, Battarra:2014tga, Qiu:2015nha, Banerjee:2016hom, Ijjas:2016tpn, Ijjas:2016vtq, Ijjas:2017pei, Saridakis:2018fth}, however in this work we proved why the theoretical {\it no-go} theorem which states that such a realization is impossible \cite{Kobayashi:2016xpl,Akama:2017jsa} can be evaded. In particular, we first reviewed that this theoretical {\it no-go} theorem is based on two very strong assumptions, namely that a particular quantity, $\xi$ in \eqref{xi}, cannot be discontinuous, and that there is only one bounce. Concerning the first assumption we showed that not only can be violated in a general Horndeski/Galileon scenario, but that it is necessarily violated at the bounce point in the subclass of Horndeski/Galileon gravity in which $K(\phi,0)=0$ (as for instance in the kinetic choices where $K$ is a polynomial of $X$). In order to make the analysis more transparent, and without loss of generality, we provided an explicit example where a nonsingular bounce is realized, with all stability conditions being satisfied. Concerning the second assumption, which is also crucial in improved versions of the theoretical no-go theorem which claim that even if a nonsingular bounce free of pathologies can be realized it will lead to pathologies in the infinite past or infinite future, we showed that it can be evaded by considering cyclic cosmology, with an infinite sequence of nonsingular bounces free of pathologies, which forbids the universe to reach to the ``problematic'' regime at infinite past or infinite future. In this case we also provided a specific example with the above behavior, with all stability conditions being satisfied eternally. In conclusion, stable nonsingular bounce realizations are not mathematically impossible in Horndeski/Galileon cosmology, which may serve as an additional advantage for this class of gravitational modification. | 18 | 8 | 1808.01170 |
1808 | 1808.05565_arXiv.txt | Cosmological perturbations are considered in $f(T)$ and in scalar-torsion $f(\varphi)T$ teleparallel models of gravity. Full sets of linear perturbation equations are accurately derived and analysed at the relevant limits. Interesting features of generalisations to other teleparallel models, spatially curved backgrounds, and rotated tetrads are pointed out. | Teleparallel gravity in its initial formulation is an equivalent way to describe general relativity (GR) in terms of torsion instead of curvature \cite{Aldrovandi:2013wha}. A. Einstein's attempts to incorporate electromagnetism in the reformulation, in spaces characterised by parallelism at distance and originally envisaged by E. Cartan, were unsuccessfull \cite{Goenner:2004se}, but concrete advantages of the reformulation were later clarified by C. M{\o}ller who found a covariant gravitational energy-momentum complex \cite{MOLLER1961118} and discussed the possible resolution of singularities in the tetrad framework\footnote{One may remind that rather than something ''extra'' or ''alternative'', the tetrad is {\it necessary} to couple matter to gravitation. Only currently, a new preprint claims to consistently supersede the tetrad by a more minimal structure, which may shed new light on the singularity resolution and, surprisingly, the nature of dark matter \cite{Zlosnik:2018qvg}.} \cite{moller1978crisis}. In the last decade we have a great resurgence of interest towards such alternative formulations. The reasons range from the need to revisit the foundations of GR \cite{Koivisto:2018aip} to the phenomenological interests in new approaches to modify gravity \cite{Cai:2015emx}. In this paper we take the latter viewpoint. In modern cosmology it has become commonly accepted that the existence of the three well-known unknowns, the agents that are supposed to cause inflation, dark matter and dark energy, could serve as a good motivation for modifying the gravitational interaction. In particular, the problems of the early Universe \cite{Ferraro:2006jd,Ferraro:2008ey} and the search for a self-acceleration mechanism in the present day Universe \cite{Bengochea:2008gz,Linder:2010py} have led to one of the simplest modified teleparallel models, the $f(T)$. Since then, many variations of this model have been proposed to the same aims, with (e.g. \cite{Geng:2011aj,Jarv:2015odu,Hohmann:2018rwf}) and without (e.g. \cite{Kofinas:2014owa,Bahamonde:2015zma,Bahamonde:2017wwk}) additional scalar fields in the action (see \cite{Cai:2015emx} and Section \ref{generalisations}). Obviously, a model which aims at solving any of the mentioned cosmological puzzles must be tested against all available sets of cosmological data \cite{Cardone:2012xq}. One of the first steps to be taken is the theory of linear cosmological perturbations. We will take the classical route of working with zero spin connection in $f(T)$ (pure tetrad formulation) because the covariantisation \cite{Krssak:2015oua,Golovnev:2017dox}, though important at the foundational level, introduces new variables without changing physical predictions which is impractical for our current purposes. The cosmological perturbation equations for $f(T)$ were given first in the Ref. \cite{Li:2011wu}. It was done in the covariant 1+3 language which might be unfamiliar for many workers in the field, and only in the Appendix were they specified to the Newtonian gauge\footnote{Unfortunately, with a typo (missing prime for the extra perturbation in the space-space field equation) inherited in \cite{Nunes:2018xbm}.}. Recently these equations were used \cite{Nunes:2018xbm} in the first actual comparison with the data. Note that, working with pure tetrad formalism, it is very important to consider the most general perturbation of the tetrad and to take care of all equations of motion including the antisymmetric part self-consistently. Many works\footnote{Both before and after the correct derivations of the linear field equations \cite{Li:2011wu} and the quadratic action \cite{Izumi:2012qj}.} in the field lack consistency in these and other respects, e.g. \cite{Li:2018ixg}. We feel it very timely to give a detailed account of cosmological perturbation analysis, with accurate and consistent derivations directly in the Newtonian gauge, in a formalism that is straightforward to generalise to the many variations of the $f(T)$ model. In the next Section \ref{ft} we briefly review the $f(T)$ model, emphasising the crucial point of the presence of antisymmetric field equations which is generic to teleparallel modified gravity models. The following Section \ref{parameterisation} introduces a parameterisation of the tetrad perturbations and their gauge transformations. These considerations are totally independent of the particular model under study ($f(T)$ or otherwise). We shall then derive the perturbed components of torsion and apply them to check the $f(T)$ model sector by sector: tensors, vectors, pseudoscalars and scalars in Sections \ref{tensor}, \ref{vector}, \ref{pseudoscalar} and \ref{scalar}, respectively. The latter are the most non-trivial and the most interesting ones. We apply these scalar perturbations in Section \ref{comoving} to study the structure formation in the presence of arbitrary matter sources. Generalised models are then discussed in Section \ref{generalisations}, where a more complete study is presented for the case of the scalar-torsion model. Finally, we briefly comment on the generalisation to curved cosmology in Section \ref{nonflat} and point out the issue of inequivalent choices of ''good'' tetrads. In the conclusions of Section \ref{conclusions}, we list the new results obtained along the way. | \label{conclusions} We analysed cosmological perturbations in teleparallel $f(T,\varphi)$ models of gravity. Taking carefully into account the 16 independent perturbative components in a generic tetrad, classified into scalar, pseudoscalar, vector, pseudovector and tensor perturbations, we confirmed the previous conclusion of the absence of extra degrees of freedom in the flat FRW background in $f(T)$ models. This conclusion was generalised to the scalar-torsion models, which may appear surprising due to the explicit kinetic term that is added into the action (\ref{action2}). Other new results in this paper include the exact Poisson equation (\ref{poisson}) and the exact evolution equation for dust perturbations (\ref{delta}), where the latter is a special case of the second order differential equation (\ref{bardeen}) that makes no assumptions about the cosmological sources. The quasistatic equation governing the evolution of the matter spectrum was given also for the scalar-torsion theories as (\ref{delta}), which now allows to easily include the structure formation constraints when confronting these models with the available (e.g. SDSS \cite{Tegmark:2003uf}) and forthcoming (e.g. Euclid \cite{Amendola:2016saw}, SKA \cite{Carilli:2004nx}) cosmological precision data. We also checked the behaviour of the linear perturbations at the critical turnover points in bouncing and recollapsing cosmologies. We deduced from (\ref{bounce}) that homogeneous perturbations are divergent unless the action is contrived such that $f_{TT}(0)=0$, which rules out realistic bounces. The scalar-torsion case scrutinised in Section \ref{scalar-torsion} is a simple example amongst the various more general teleparallel modified gravity actions that have been proposed in the literature during the past few years. In Section \ref{generalisations} we sketched the general recipe to obtain the linearised cosmological equations for various classes of models. We believe this will facilitate the more extensive analysis that is necessary to carry out for each of the models to sort out the viable ones according to their degrees of freedom and to understand their implications to cosmology beyond the time-dependence of the scale factor. Finally, we pointed out a new feature of the perturbation system, which occurred with tetrads that were rotated to be compatible with spatial curvature. There was a discrepancy in the predictions of the modified models at the limit $K \rightarrow 0$. The conclusions can crucially depend upon the tetrad that is chosen to represent a given metric, spatially curved or otherwise. That is intriguing and calls for further studies. \paragraph{Acknowlegdments.} The authors are grateful to the organisers of the TeleGrav2018 workshop in Tartu, and AG acknowledges Saint Petersburg State University travel grant 27801255 which made his participation possible. We thank Yi-Fu Cai, Stefano Camera and especially the authors of \cite{Bejarano:2017akj} for useful comments on the manuscript. | 18 | 8 | 1808.05565 |
1808 | 1808.00340_arXiv.txt | The tests of the deviations from Newton's or Einstein's gravity in the Earth neighbourhood are tied to our knowledge of the shape and mass distribution of our planet. On the one hand estimators of these ``modified" theories of gravity may be explicitly Earth-model-dependent whilst on the other hand the Earth gravitational field would act as a systematic error. We revisit deviations from Newtonian gravity described by a Yukawa interaction that can arise from the existence of a finite range fifth force. We show that the standard multipolar expansion of the Earth gravitational potential can be generalised. In particular, the multipolar coefficients depend on the distance to the centre of the Earth and are therefore not universal to the Earth system anymore. This offers new ways of constraining such Yukawa interactions and demonstrates explicitly the limits of the Newton-based interpretation of geodesy experiments. In turn, limitations from geodesy data restrict the possibility of testing gravity in space. The gravitational acceleration is described in terms of spin-weighted spherical harmonics allowing us to obtain the perturbing force entering the Lagrange-Gauss secular equations. This is then used to discuss the correlation between geodesy and modified gravity experiments and the possibility to break their degeneracy. Finally we show that, given the existing constraints, a Yukawa fifth force is expected to be sub-dominant in satellite dynamics and space geodesy experiments, as long as they are performed at altitudes greater than a few hundred kilometres. Gravity surveys will have to gain at least two orders of magnitude in instrumental precision before satellite geodesy could be used to improve the current constraints on modified gravity. | The efforts to test Newton and Einstein gravity have been continuous in the last hundred years and lie at the crossroads between theoretical and experimental physics, laboratory and space physics. Celestial mechanics has historically been crucial in that respect, motivated mostly by the imperfect understanding of the shape of the Earth, the stability of the Solar system and the long lasting Newtonian problem of the anomalous drift of the perihelion of Mercury. A main difficulty arises from the fact that gravity is a long range interaction that cannot be screened. Hence, the knowledge of our environment (Earth gravitational field and its evolution, Solar system structure, cosmological model) is a limitation to these tests. In that respect, the developments of dedicated satellite missions have brought new insights on both possible deviations from General Relativity (GR) and the Earth gravitational field. Today, GR is well-tested on local scales~\cite{Will:1993ns,will14} whilst the need to improve the existing constraints is partly motivated by cosmology. The accelerated cosmic expansion and other evidences, such as the dynamics of spiral galaxies, have led to the conclusion that there should exist a dark sector, composed of dark matter and dark energy, representing respectively 26\% and 70\% of the energy budget of the universe. This dark sector can also be interpreted as a sign that GR may not be a good description of gravity on cosmological scales and on low acceleration regimes. Many extensions of GR have been proposed (see e.g. Refs.~\cite{adelberger03,clifton12, joyce15} for reviews) and many tests of GR and of Einstein's equivalence principle on cosmological scales have been designed to test them~\cite{Uzan:2000mz,Uzan:2003zq,Uzan:2006mf,Uzan:2010ri,Jain:2013wgs}. No deviations from GR have been detected so far (see e.g. Refs.~\cite{will14,adelberger03, jain10,safronova17,delva17} for general reviews of laboratory and cosmological scales tests). Concerning the recent experimental tests of GR, let us mention the Lense-Thirring effect \cite{everitt11, ciufolini13a, ciufolini13b}, the pericentre anomaly \cite{iorio02a, lucchesi03, lucchesi10, lucchesi14, li14}, the gravitational redshift \cite{bertotti03}, the universality of free fall \cite{williams04,schlamminger08,wagner12,touboul17,viswanathan18} and the constancy of fundamental constants~\cite{Uzan:2002vq,Uzan:2010pm,Uzan:2004qr}, the last two involving Einstein's equivalence principle. To these standard tests, let us add the new window opened by the detection of gravitational waves~\cite{abbott16}. In particular, the detection of an electromagnetic counterpart to the gravitational-wave signal emitted by a binary neutron star merger~\cite{abbott17} put severe constraints on a whole class of alternatives to GR~\cite{baker17,creminelli17,ezquiaga17,sakstein17}. Among all the extensions of GR, scalar-tensor theories~\cite{damour92}, in which a scalar long range interaction that may be composition dependent, is added to the standard spin-2 interaction mediated by the gravitons, are still among the open alternatives. In particular, if this scalar is light, they may enjoy sizeable cosmological imprints. As a long range fifth force would then appear on Solar system scales, they need to include a screening or a decoupling mechanism~\cite{vainshtein72,Damour:1992kf,damour94,khoury04a, khoury04b, babichev09,hinterbichler10, brax13}. While the parameter space of these models has been severely reduced (see e.g. Ref.~\cite{burrage18} for up-to-date tests), they remain ideal candidates for extensions of GR. Even if the scalar field is heavy on Solar system scales, it is still responsible for a fifth force described, in the Newtonian regime, by a Yukawa potential (see e.g. the Supplemental material of Ref.~\cite{berge18} and references therein). Many constraints on the mass and the amplitude of this extra-potential have been obtained so far (e.g. Refs.~\cite{adelberger03,fischbach99} and references therein, and Refs.~\cite{kapner07,masuda09,sushkov11,klimchitskaya14,berge18} for more recent works). The goal of this article is to revisit the constraints on such a Yukawa interaction drawn from the analysis of geodetic data. As already emphasised, it is a tautology to say that local GR tests are limited by our knowledge of the Earth gravitational field. Nevertheless, there have been extensive studies under the assumption of Newton's gravity whilst the tests of Yukawa gravity have all been performed assuming at best a spherical and homogeneous Earth, but most often, assuming that the Yukawa interaction is sourced by a point-like Earth. We develop a method to describe the effects of such a modified gravity on the orbits of dedicated satellites in a realistic description of the Earth. Clearly, in that case our ignorance of the properties of the fifth force does limit our reconstruction of the property of the mass distribution of the Earth, while the latter limits the constraints on this fifth force. We propose to analyze these interactions and provide tools to test GR in our terrestrial neighbourhood. The shape and mass distribution of the Earth, and their variability, have so far been reconstructed from local measurements of the gravitational field (on-ground or airborne) and global satellite models of the full gravitational field. Recent satellite geodesy missions have allowed geophysicists to map the Earth gravity model with an exquisite precision: e.g. GOCE \cite{rummel11, pail11} or GRACE \cite{tapley04, tapley05, reigber05} and combinations of (satellite) missions~\cite{pail10, mayer06}. GOCE and GRACE provide measurements of the spherical harmonics coefficients up to degree and order 250, whereas the EGM2008 model goes up to degree and order 2159 \cite{pavlis12}. The uncertainties on the shape of the Earth add up to other systematic errors (such as Solar radiation pressure, atmospheric drag, Earth tides, Earth magnetic field, thermal instabilities --for discussions of systematics in both laboratory and space, see e.g. Refs.~\cite{lucchesi14,touboul17,hoyle04}). Then, they must be either shielded or corrected for during the data analysis process (see e.g. Refs.~\cite{touboul17, hoyle04,fischbach86, toth18}). This article focuses on satellite tests of gravity so that the main sources of gravitational error come from the zonal terms, and especially the first one, $J_2$ (which describes the Earth flattening)~\cite{ciufolini13a, lucchesi14}. Before the advent of the precise satellite measurements from GRACE and GOCE, the large uncertainty on $J_2$ was considered a show-stopper for precise tests of gravity. Techniques were then elaborated to cancel its effect. For instance, by empirically combining the perigee shift and precession of the line of nodes of LAGEOS and LAGEOS II, it was shown that the contribution of $J_2$ (and the associated error) to the perigee shift and to the Lense-Thirring effect could be cancelled \cite{ciufolini96}. The GRACE and GOCE missions changed the situation thanks to their remarkably precise measurements, giving the parameter $J_2$ to a $10^{-8}$ relative precision level when combined with LAGEOS data. In the case of the perigee shift measurement of the LAGEOS II satellite, Lucchesi \& Peron \cite{lucchesi14} evaluate that using the errors on $J_2$ provided by the EIGEN-GRACE02S gravitational field model \cite{reigber05} allows for a percent level test of GR's perigee shift with no further empirical correction. However, correcting for the shape of the Earth when testing gravity in space relies on two pillars: (i) a model of the Earth gravitational field and (ii) accurate and precise values of the coefficients of the model. To the best of our knowledge, the model is always described as a spherical harmonics expansion derived from Laplace equation to solve for the Newtonian gravitational field sourced by the shape of the Earth. The values of the spherical harmonics coefficients are provided by Earth gravity surveys, such as GRACE, GOCE, LAGEOS, or local on-ground surveys. The evaluation of the accuracy of coefficients estimator and of robust uncertainties is a highly non-trivial part of the data analysis needed to make a model of the gravitational field. Errors on spherical harmonic coefficients are commonly separated between formal and calibrated errors \cite{lucchesi14,reigber05}. Formal errors come from the data regression method and mainly include statistical errors as well as possible numerical uncertainties linked to the data analysis method itself. For instance, because of its Sun-synchronous orbit, GOCE never flew over the poles; the resulting polar gaps (whereby no data can constrain the spherical harmonics model in the polar regions) causes the least-square regression on spherical harmonics coefficients to be ill-conditioned, thus requiring a regularization technique. With no regularization, estimating the (near)-zonal terms is particularly difficult. These coefficients come with large error bars; after regularization, the error bars can be seen to shrink \cite{pail11, metzler05} (for $J_2$, the error shrinks from a few $10^{-9}$ to a few $10^{-12}$). However, there does not seem to be any investigation about the possible bias introduced by the regularization technique. Under the Newtonian gravity hypothesis (i.e. the static part of spherical harmonics coefficients should be consistent between different data subsets along the experiment's time span, or between different experiments), formal errors are a posteriori calibrated to account for systematic errors: for a single satellite model, subset solutions are generated from data covering different time periods, and the scattering of subset solutions is used as the calibrated error (see e.g. Ref.~\cite{reigber05} for GRACE). The same method is applied to calibrate multi-satellite models, where an upper bounds for the systematic errors is derived from the difference between estimates of several satellite data \cite{lucchesi14}. In this case, it is implicitly assumed that any tension between different data sets comes from imperfectly controlled systematic errors. Although this is true if the underlying hypothesis (the Earth gravity is described by Newton's theory) is true, any tension may also provide a smoking gun for physics beyond Newton's inverse square law and GR. Indeed, a modified gravity model may very well predict non-universal spherical harmonics coefficients, e.g. coefficients whose value depends on the distance to the centre of the Earth (in this paper, we show that it is indeed the case). Along this line, it should be noted that despite very precise measurements of the static $J_2$ zonal term, the GRACE-only, GOCE-only and EIGEN-6C (combining LAGEOS, GOCE, GRACE and ground measurements) models provide inconsistent values (as was already noted by Wagner \& McAdoo \cite{wagnermcadoo12}), which differ by at least 700 $\sigma$; see Table~\ref{t.tab1}. \begin{table}% \caption{Constraints on the $J_2$ parameter by several experiments.} \begin{center} \begin{tabular}{ccc} \hline GRACE & $J_2=1.0826354309122197\times10^{-3}\pm3.5263625612834223\times10^{-12}$ & \cite{mayer06}\\ GOCE & $J_2=1.0826265326404513\times10^{-3}\pm1.2127946116555258\times10^{-11}$ & \cite{pail11}\\ EIGEN-6C & $J_2=1.0826263376893369\times10^{-3}\pm2.477786925867517\times10^{-13}$ &\cite{shako14}\\ \hline \end{tabular} \end{center}\label{t.tab1} \label{default} \end{table}% Whether this tension is due to largely underestimated errors, to biases introduced by regularization techniques, to uncontrolled systematics, to inconsistent data sets, or to new physics beyond GR is not clear. However, it should invite us to extreme caution when using gravity surveys and geodesy results to model and correct for the Earth gravitational field when testing GR or looking for deviations to Newton's inverse square law. This article investigates the effects of modified gravity on the Earth gravitational field and our ability to reconstruct the shape of the Earth and, in turn, the effect of an imperfect knowledge of the Earth gravitational field on searches for modified gravity. As explained, we base our discussion on phenomenological deviations from Newton gravity described by a Yukawa potential. In particular, we shall show that although we can still describe the Earth gravitational field with a spherical harmonics expansion, a Yukawa interaction modifies the meaning of the expansion coefficients. They mix properties of the Earth and of gravity and get an explicit dependence on the distance to the centre of the Earth. As a consequence, they are not simply related to the Earth geometry any more, and should not be used to map the Earth mass distribution and geoid. For instance, the $J_2(r)$ zonal term does not only describe the Earth flattening, but is impacted by the Yukawa interaction. Furthermore, we should not expect coefficients measured by different satellites at different altitude (or even by a single satellite at different times, provided that satellite's orbit is not circular) to be consistent; combining different data sets should also be performed with great care. Therefore, using geodesy results derived under the assumption that no deviation to GR (or to Newton's law) exists is prone to errors when constraining modified gravity, just because the Earth gravity model used to correct for the Newtonian contribution may be incorrect. This may be the case if using (possibly inconsistent) multi-satellite models, or a model set with a satellite at an altitude other than the altitude of the gravity test. The underlying question is that of the model to use. When looking for modified gravity in terms of a Yukawa interaction, two parameters are added to the Newtonian gravity sector (the strength and range of the interaction), {\it de facto} changing the model --which is not simply Newtonian any more. Using geodesy results derived assuming a simple Newtonian model must then be seen as inconsistent with the task at hand, and will introduce biases and uncertainties that must be quantified and accounted for in the modified gravity constraints. The way out of this difficulty is, as usual, to set all analyses within the same theoretical framework to ensure consistency. The Earth gravitational field should be measured under the assumption that a Yukawa interaction may exist. The Earth gravitational field models would then explicitly contain information about the Yukawa interaction, either explicit or marginalised upon. In the former case, they would provide constraints on modified gravity; in the latter case, their estimated coefficients would have larger uncertainty, but would be unbiased and could safely be used by modified gravity experiments. This paper is organised as follows. In Sect. \ref{sect_general} and \ref{sect_emodel}, we derive the spherical harmonic expansion of the Earth gravitational field in presence of a Yukawa interaction and give expressions for the gravitational acceleration and for the Gauss-Lagrange equations of motion. Sect. \ref{sect_impact} provides a general discussion of the entanglement between geodesy and modified gravity measurements, and order-of-magnitude estimates derived with a simple Earth model are given in Sect. \ref{sect_ofm}. This formalism provides a consistent framework to derive constraints on fifth force from space-borne experiments. | We have investigated the entanglement between the shape of the Earth and modified gravity. Describing deviations to Newtonian gravity with a Yukawa interaction, we showed that the Earth gravitational field potential can still be expanded in spherical harmonics, just like in the pure Newtonian realm. We derived explicit expressions for the spherical harmonic coefficients, that we used to compute the (modified) gravity acceleration. We finally considered the Lagrange-Gauss equations, that describe the effect of a perturbing force on a satellite's orbital dynamics, in the case where the Yukawa interaction is sourced by the complex shape of the Earth. To perform those calculations, we introduced a new method to compute a multipolar decomposition of the gravity acceleration with spin-weighted spherical harmonics, which greatly simplifies the required algebra. We showed that although formally the coefficients of the spherical harmonic expansion keep the same form as in the Newtonian case, they acquire a new meaning and are not universal to the Earth system anymore, since they become explicitly dependent on the distance from the centre of the Earth. Consequently, the gravitational acceleration and the perturbing force due to the shape of the Earth also acquire a new radial dependence. This behavior has many implications both in geodesy and in modified gravity experiments: \begin{itemize} \item in presence of a non-zero Yukawa interaction, measurements of the Earth gravitational field performed at different altitudes inevitably provide inconsistent results (up to measurement errors). \item in presence of a non-zero Yukawa interaction, using a Newtonian gravity model to map the Earth mass distribution by inverting the spherical harmonic coefficients measured for the gravitational field is prone to be biased; using a prior on modified gravity, considered as a systematic error, should help to minimise the bias, although the uncertainty on the mass distribution estimator will increase. \item Earth-model-dependent measurements of a Yukawa interaction are inevitably affected by any bias or uncertainty on the Earth model (e.g. coming from geodesy data). Model-independent estimators might be constructed but require that gravity surveys go beyond the implicit assumption that the underlying field is Newtonian. \item even experiments that rely only on the measured Earth gravitational field (with no need to detail its source) are prone to errors if they are performed at an altitude different from that where the gravitational field was measured. \end{itemize} We proposed to combine gravitational surveys to define a new estimator of the Yukawa interaction strength $\alpha$. Taking advantage of the radial dependence of the spherical harmonic coefficients in presence of a Yukawa interaction, we can simply take the difference of the values of a given coefficient as measured by two satellites at different altitude. We discussed the limitations caused by our imperfect knowledge of the Earth. Despite a significant bias in $\alpha$ if the model of the Earth is too simplistic, we found that we can increase the instrumental precision by several orders of magnitude before being limited by our knowledge of the Earth. However, we restrained from deriving new constraints on the Yukawa interaction from the strong tension in the $J_2$ zonal term as measured by GOCE and GRACE, since we find it dubious and its most probable cause is underestimated measurement errors. Although the limitations listed above seem profound, we showed that they are subdominant compared to other usual gravitational and non-gravitational perturbations. We based our conclusion on order-of-magnitude estimates using a simple Earth model and taking into account those values of the Yukawa interaction that are still allowed by experiments but that give the strongest effects. For instance, the strength of the perturbation imparted by the coupling of the Earth quadrupole with a Yukawa interaction on a satellite is smaller than that due to Jupiter. Very-low-altitude satellites could be affected by a mid-range, still undetected Yukawa interaction, at the level of usual relativistic effects. Thus, it is from low-altitude experiments that it seems most likely to improve our knowledge about a possible Yukawa interaction, provided that the atmospheric drag can be correctly taken into account (e.g. through a drag-free system). We can therefore expect that although we should rigorously take into account the complex shape of the Earth when constraining modified gravity in orbit, especially for experiments performed in a low-Earth orbit, considering the Earth as a sphere remains a very good approximation for high-altitude satellites. Nevertheless, it would be sound to gather geodesy and modified gravity to minimise any modeling limitation. This can be done by performing geodesy experiments with modified gravity in mind (i.e. using a beyond-Newton gravity model), or even by designing experiments aiming to measure the shape of the Earth and modified gravity simultaneously. \appendix | 18 | 8 | 1808.00340 |
1808 | 1808.02035_arXiv.txt | We analyse 2,015 mid-infrared (MIR) spectra of galaxies observed with \textit{Spitzer}'s Infrared Spectrograph, including objects with growing super-massive black holes and objects where most of the infrared emission originates from newly formed stars. We determine if and how accreting super-massive black holes at the centre of galaxies -- known as active galactic nuclei (AGN) -- heat and ionize their host galaxies' dust and molecular gas. We use four MIR diagnostics to estimate the contribution of the AGN to the total MIR emission. We refer to galaxies whose AGN contribute more than 50 per cent of the total MIR emission as AGN-dominated. We compare the relative strengths of PAH emission features and find that PAH grains in AGN-dominated sources have a wider range of sizes and fractional ionizations than PAH grains in non-AGN dominated sources. We measure rotational transitions of \molh\ and estimate \molh\ excitation temperatures and masses for individual targets, \molh\ excitation temperatures for spectra stacked by their AGN contribution to the MIR, and the \molh\ excitation temperature distributions via a hierarchical Bayesian model. Using the hierarchical Bayesian model, we find an average 200K difference between the excitation temperatures of the \molh S(5) and \molh S(7) pure rotational molecular hydrogen transition pair in AGN-dominated versus non-AGN dominated galaxies. Our findings suggest that AGN impact the interstellar medium of their host galaxies. | \label{sec:intro} The evolution of central supermassive black holes (SMBHs) appears connected to the histories of the host galaxies that harbour them. Observations suggest that there are SMBHs in all galaxy bulges and their masses are proportional to the masses of the host bulges \citep[see][for reviews]{fabian12,kormendyaa,heckmanaa}. Furthermore, star-formation and SMBH growth have similar evolutions \citep[see][for a review]{madau}. Theory suggests that feedback from growing SMBHs/active galactic nuclei (AGN) is able to successfully reproduce the properties of local massive galaxies \citep[see][for review]{silk12}, and explain the observed galaxy scaling relations and the quenching of star-formation in massive galaxies \citep{silk98,fabian99,king03,hopkins06,illustris2017}. There is mounting observational evidence for AGN interacting with the gas and dust of their host galaxies. Some AGN appear to ionize the interstellar medium (ISM) up to several kiloparsecs away from the central black hole \citep{greene2011,greene2012,liu2013,cresci,villar,karouzos,dominika}. Strong radio galaxies have been observed injecting energy into the molecular gas of their host galaxies \citep[e.g.][]{appleton,ogle,nesvadba,guillard}. Molecular outflows have been observed in powerful quasars \citep{feruglio10,cicone12,stone16}. Evidence for feedback effects in host galaxies that harbour lower luminosity AGN has been mixed, but these surveys were on relatively small numbers of AGN \citep[e.g.][]{petric,Hill,stierwalt,petric18}. In this paper, we use mid-infrared (5.2--38.0~\micronm) spectra of a sample of 2,015 galaxies, 942 of which are galaxies whose IR emission comes predominantly from the AGN, to investigate the impact of the AGN on the warm molecular gas and dust components of the ISM in their host galaxies. \newpage The ISM fuels star-formation and AGN activity. The primary sources for heating the ISM in AGN host galaxies are newly formed stars and supernovae \citep[e.g.][]{weedman}, AGN \citep{sanders,elvis,elitzur12}, and old stars \citep{buat,rowan,sauvage92,sauvage94}. To estimate the impact AGN have on the ISM, we first estimate how much the AGN contributes to the total mid-infrared (MIR) emission. We use a range of diagnostics developed from studies of normal galaxies, luminous AGN, and luminous infrared galaxies using data from the \textit{Infrared Space Observatory} \citep[for a review]{genzel} and the \textit{Spitzer Space Telescope}'s Infrared Spectrograph \citep{armus07,spoon,petric}. Optical diagnostics \citep[e.g.][]{baldwin,kauffmann} can provide distinctions between star-formation (SF) and accretion processes, but are not ideal for objects with significant dust obscuration or for composite objects with both significant AGN and SF activity \citep{trump15}. MIR diagnostics are less sensitive to dust obscuration. MIR empirical methods that can be used to disentangle an AGN-dominated from an SF-dominated galaxy include the ratio of the continuum to dust emission features, the relative fluxes of high- to low-ionization emission, and the slope of the MIR continuum. These diagnostics were derived using observations of pure star-formation and pure-AGN samples \citep{genzel,laurent,armus06,smith07,spoon}. In this paper we use the 6.2~\micronm\ polycyclic aromatic hydrocarbon (PAH) equivalent width, hereafter \eqwpah, to quantify AGN activity. PAHs are organic compounds whose emission features in physics laboratories are similar to MIR features in astronomical spectra \citep{leger,allamandola}. PAH emission features are ubiquitous in MIR spectra of regions with recent star-formation \citep{tielens05}. PAHs radiate through IR fluorescence after being excited by a single ultraviolet photon and may play an important role in the energy balance of the ISM. Several models predict the impact of radiation on the ionization and grain sizes of PAHs \citep{li2001,draine07}. % Although the relations between the PAH features and their environments are not completely understood \citep{sadjadi,zhang}, empirically we measure low \eqwpah\ in galaxies with AGN \citep{smith07,sales}. This property is a powerful diagnostic of the AGN's contribution to the MIR emission. % In star-forming galaxies, \molh\ and PAH emission are tightly correlated \citep{roussel}. \molh\ is the dominant component of the warm, dense, star-forming molecular gas of galaxies. \molh\ can be excited through three primary mechanisms: (1) far ultraviolet heating, in which photons radiatively pump the \molh\ into its electronically excited states; (2) inelastic collisions, in which collisions maintain the lowest pure rotational levels in thermal equilibrium in regions where the gas density and temperature is high enough; and (3) X-ray heating, in which hard X-ray photons penetrate into UV-opaque zones and radiatively excite \molh. In normal galaxies, \molh\ is predominantly heated by far-ultraviolet photons in photon-dominated regions (PDRs) \citep{hollenbach}. For PDRs with $n_\mathrm{H} \gtrsim 10^{4} \,\mathrm{cm}^{-3}$, collisions maintain the lowest rotational levels ($J\lesssim5$), keeping the PDRs in thermal equilibrium \citep{burton}. This makes their populations consistent with Boltzmann distributions, which makes the \molh\ emission a robust thermal probe. Other sources of \molh\ excitation include small-scale shocks \citep{neufeld}, extra-nuclear large-scale shocks from galactic gravitational interactions \citep{appleton,cluver,ogle12}, and X-ray heating \citep{roussel}. % Some AGN host galaxies appear to have more \molh\ emission relative to that of other coolants such as PAHs or [\ion{Si}{ii}] emission, suggesting that at least some of the \molh\ does not originate in PDRs. This may indicate that AGN impact the molecular component of their host's ISM \citep{rigopoulou,higdon,zakamska,petric,shipley,Hill}. While observational studies have provided evidence of some AGN injecting the additional energy required to heat the molecular gas, the small sample size of these studies makes it difficult to assess whether this scenario is representative. Our large catalogue of AGN resolves this.% In galaxies where the AGN contributes most of the IR emission, there is an excess of warm \molh\ emission relative to PAH emission \citep{rigopoulou}. Subsequent studies using \Spitzer's Infrared Spectrograph confirmed the trend of excess \molh\ emission in Ultra Luminous InfraRed Galaxies (ULIRGs) with IR luminosities above $10^{11}\,\mathrm L_{\sun}$, and a subset of slightly less luminous LIRGs \citep{zakamska,Hill,stierwalt, petric18}. \citet{ogle12} find excess \molh\ emission in over 30 per cent of the their sample of radio galaxies. However, \citet{higdon} analyse a similar sample of ULIRGs, and do not find a relationship between the warm \molh\ mass and the \textit{IRAS} 25 to 60~\micronm\ flux density ratio (an empirical AGN contribution diagnostic), despite finding an excess of warm \molh\ relative to the PAH emission. In this paper we present \molh\ and PAH emission measurements in active galaxies observed with the \Spitzer\ IRS low resolution ($R= {{\lambda}/{\Delta \lambda}} \sim 60$) modules. Our sample consists of a wide range of infrared luminosities ($\nu L_\nu[24\ \micron]\sim10^{8}\textrm{--}10^{12}\, \mathrm L_{\sun}$), which allows us to test if the \molh\ to PAH ratio increases as a function of the AGN's contribution to the total IR emission of the galaxy, and if the temperatures of the warm \molh\ are different in AGN host galaxies versus SF dominated galaxies. We use the pure rotational transitions of \molh\ observed in the MIR to estimate the masses and temperatures of 100--1000~K molecular gas. We then look for differences between \molh\ in AGN-dominated galaxies and \molh\ in SF-dominated systems. In \autoref{sec:sample} we describe the data acquisition, reduction, and analysis algorithms. In \autoref{sec:results} we present our AGN selection methods, PAH properties of our sample, and molecular hydrogen properties of our sample. We show a significant difference between the temperatures of the higher \molh\ transitions in AGN and SF-dominated systems via three independent analysis methods. In \autoref{sec:discussion} we discuss the implications of AGN host galaxies containing higher \molh\ temperature distributions than galaxies dominated by SF processes, and we summarize our findings in \autoref{sec:conclusion}. We use an $h = 0.7$, $\Omega_{m} = 0.3$, $\Omega_{\Lambda} = 0.7$ cosmology throughout this paper. To evaluate the statistical significance of correlations, we use the Spearman rank test ($r_{s}$), and report the probability of a null hypothesis as $p_{s}$, the probability of two sets of data being uncorrelated. We use the two-sample Kolmogorov--Smirnov test ($D_\mathrm{KS}$) to evaluate if two underlying distributions come from the same distribution, and report the probability of the two distributions being the same as $p_\mathrm{KS}$. | \label{sec:conclusion} We use MIR spectroscopy to evaluate the relationship between AGN and the ISM of their host galaxies. We analyse 2,015 objects low-resolution spectra \citep{cassis} with published spectroscopic redshifts \citep{ideos}. We correct mismatches between the different spectral orders and check the flux calibration of the spectra using \WISE\ photometry. We measure rotational \molh\ transitions, PAH emission in the 6.2, 7.7, and 11.3~\micronm\ bands, and summarize our results as follows: \begin{enumerate} \item We use the \eqwpah\ to separate our sample galaxies where the AGN contributes more than 50 per cent of the MIR luminosity and galaxies where star-formation contributes more than 50 per cent of the MIR luminosity. \item We find that the PAHs in AGN-dominated galaxies have a wider range of ionizations and sizes, and the effect of silicate absorption on grain size tracers is different for AGN dominated galaxies vs SF dominated galaxies. This may imply that the ISM in AGN hosts is more complex than the ISM of SF-dominated galaxies; without analysing the host morphologies we cannot separate the impact of the AGN on the ISM from that of any gravitational interactions. \item In AGN-dominated systems, we find an excess of molecular \molh\ emission relative to what we would measure if the molecular \molh\ originated solely from PDRs. \item We assess the properties of the warm molecular gas applying Bayesian interpretations of the most commonly implemented techniques, and find statistically different temperature distributions between MIR SF dominated (\eqwpah\ > 0.54 \micronm) and MIR AGN dominated (\eqwpah\ < 0.27 \micronm) galaxies. \item We construct a hierarchical Bayesian model, and we find a 120~K temperature difference in $T_{5,3}$ between AGN-dominated galaxies and SF-dominated targets with greater than 3$\sigma$ significance. The difference in $T_{7,5}$ between these targets increases to 200 K with greater than 3$\sigma$ significance. This may suggest that the AGN heats the molecular gas in the inner $\sim5$ kpc probed by the IRS observations. \end{enumerate} | 18 | 8 | 1808.02035 |
1808 | 1808.02580_arXiv.txt | \label{sec:intro} Pulsar glitches are thought to result from either quakes in the neutron star crust \citep{baym69}, or by a transfer of angular momentum between the superfluid interior and the outer crust \citep{anderson75}. The event manifests as a sudden increase in the observed spin period and spin-down of the pulsar, which can be followed by a recovery phase where the period exponentially returns to its pre-glitch evolution. We report here the detection of a glitch event in the pulsar J1709$-$4429 (also known as B1706$-$44) during regular monitoring observations with the Molonglo Observatory Synthesis Telescope (MOST). MOST is an aperture synthesis radio telescope located $40$\,km East of Canberra, Australia, operating in the 820--850\,MHz frequency range. The UTMOST backend upgrade to the MOST \citep{bailes17} has enabled study of the dynamic radio sky on millisecond timescales, and is well suited to pulsar timing, pulsar searches, observing single pulses from pulsars and discoveries of Fast Radio Bursts (FRBs) \citep{caleb17, farah18}. The glitch was found during timing operations, in which we regularly observe over 400 pulsars with up to daily cadence, while commensally searching for Rotating Radio Transients, pulsars, and FRBs. | 18 | 8 | 1808.02580 |
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1808 | 1808.03779_arXiv.txt | We present an interdigitated capacitor trimming technique for fine-tuning the resonance frequency of superconducting microresonators and increasing the multiplexing factor. We first measure the optical response of the array with a beam mapping system to link all resonances to their physical resonators. Then a new set of resonance frequencies with uniform spacing and higher multiplexing factor is designed. We use simulations to deduce the lengths that we should trim from the capacitor fingers in order to shift the resonances to the desired frequencies. The sample is then modified using contact lithography and re-measured using the same setup. We demonstrate this technique on a 112-pixel aluminum lumped-element kinetic-inductance detector array. Before trimming, the resonance frequency deviation of this array is investigated. The variation of the inductor width plays the main role for the deviation. After trimming, the mean fractional frequency error for identified resonators is \num{-6.4e-4}, with a standard deviation of \num{1.8e-4}. The final optical yield is increased from 70.5\% to 96.7\% with no observable crosstalk beyond $\SI{-15}{dB}$ during mapping. This technique could be applied to other photon-sensitive superconducting microresonator arrays for increasing the yield and multiplexing factor. | 18 | 8 | 1808.03779 |
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1808 | 1808.09377.txt | \baselineskip 17pt \noindent The seesaw and leptogenesis commonly depend on the masses of same particles, and thus are both realized at the same scale. In this work, we demonstrate a new possibility to realize a TeV-scale neutrino seesaw and a natural high-scale leptogenesis. We extend the standard model by two gauge-singlet scalars, a vector-like iso-doublet fermion and one iso-triplet Higgs scalar. Our model respects a softly broken lepton number and an exactly conserved $\ZZ_2^{}$ discrete symmetry. It can achieve three things altogether: (i) realizing a testable type-II seesaw at TeV scale with two nonzero neutrino mass-eigenvalues, (ii) providing a minimal inelastic dark matter from the new fermion doublets, and (iii) accommodating a thermal or nonthermal leptogenesis through the singlet scalar decays. We further analyze the current experimental constraints on our model and discuss the implications for the dark matter direct detections and the LHC searches. \\[2mm] PACS numbers: {98.80.Cq, 14.60.Pq, 95.35.+d}\\[1mm] Phys.\ Rev.\ D, in Press [arXiv:1808.09377] % | \vspace*{1.5mm} \label{sec:1} The seesaw\,\cite{minkowski1977,mw1980} extensions of the standard model (SM) naturally explain the tiny neutrino masses\,\cite{olive2014}, while accommodating a leptogenesis\,\cite{fy1986,lpy1986,LG-Rev} mechanism to generate the observed cosmic baryon asymmetry\,\cite{olive2014}. In the conventional seesaw-leptogenesis scenarios, the scales of generating neutrino masses and baryon asymmetry are tied together, and determined by the masses of the same particles. The leptogenesis could not be realized at the TeV scale unless it invokes a large fine-tuning to resonantly enhance the required CP asymmetry. This means that a natural leptogenesis is achieved at high scale, and the conventional scenarios link the seesaw to the same leptogenesis scale, which prevent the realization of testable seesaw at the TeV scale. \vspace*{1mm} The strong evidence for non-baryonic dark matter (DM) poses another great challenge to the modern particle physics and cosmology\,\cite{olive2014}. There have been interesting ideas explaining the DM puzzle. For instance, the minimal DM models \cite{cfs2006,cst2007,chpst2015} can give testable predictions for DM properties including the DM mass and the DM-nucleon scattering. However, for models with the new weak multiplet of nonzero hypercharge, its neutral DM component will have gauge interactions with $Z^0$, and thus is excluded by the direct DM searches\,\cite{cfs2006}. This calls for viable extensions. Besides, the DM particle may also play an important role in the generation of neutrino masses \cite{knt2003,ma2006,typeII2009,lugu2015,chsvv2017} and the realization of baryon asymmetry \cite{typeII2009}. \vspace*{1mm} In this work, we propose an attractive possibility that new physics for generating a testable TeV-scale seesaw can accommodate a thermal or inflationary baryogenesis at a very high scale. At the same time, we provide a viable minimal inelastic DM candidate at the TeV scale. In our construction, we will construct a realistic model including two gauge-singlet scalars, a vector-like iso-doublet fermion and one iso-triplet Higgs scalar besides the SM fields. Our model has a softly broken lepton number and an exactly conserved $\ZZ_2^{}$ discrete symmetry, so it differs from other models\,\cite{gu2017} with a spontaneous breaking lepton or baryon number. Under such softly broken lepton number and the exact $\ZZ_2^{}$ symmetry, our model can achieve three things altogether: (i) realizing a testable type-II seesaw at TeV scale with two nonzero neutrino mass-eigenvalues, (ii) providing a minimal inelastic dark matter from the new fermion doublets, with the mass-splitting induced by interactions related to the neutrino mass-generations, and (iii) accommodating a thermal or inflationary leptogenesis at high scale through the scalar-singlet decays. Although the leptogenesis scale is high, realizing the DM relic density in our scenario requires the DM mass to be about $1.2$\,TeV. As we will show, the present minimal inelastic DM is a stable Majorana fermion, and depends on two new parameters: the DM mass and the mass difference between the DM and another particle. (This differs from the previous minimal DM model\,\cite{cfs2006,cst2007,chpst2015} where the DM is either a scalar or Dirac fermion, and its tree-level mass is the only new physics parameter.) The predicted Higgs triplet and DM fermion of our model can be searched at the LHC and future high energy $pp$ colliders. The same DM particle can be probed by the direct and indirect DM detection experiments\,\cite{DM-DirectExp-Rev}. \vspace*{1mm} This paper is organized as follows. In Section\,\ref{sec:2}, we present the model setup. Then, we study the minimal inelastic DM in Section\,\ref{sec:3} and the radiative type-II neutrino seesaw in Section\,\ref{sec:4}. The realization of high scale leptogenesis is presented Section\,\ref{sec:5}. Finally, we conclude in Section\,\ref{sec:6}. %\vspace*{3mm} | \vspace*{1.5mm} \label{sec:6} Understanding the origins of the neutrino masses, the baryon asymmetry, and the dark matter altogether poses an important challenge to the particle physics today. In the conventional seesaw framework, the neutrino mass generation and the leptogenesis for baryon asymmetry are tied to the same high energy scale. This means that a low-scale neutrino mass generation could not be consistent with a high-scale leptogenesis. In the present work, we demonstrated an attractive new possibility that a radiative neutrino mass generation can be achieved at the TeV scale, while a thermal or inflationary leptogenesis naturally happens at the high scale. Furthermore, our model realizes a viable minimal inelastic dark matter (DM) at the TeV scale, where the mass-splitting between the DM particle and its heavier partner can be naturally generated by the interactions related to the neutrino mass generation. \vspace*{1mm} In section\,\ref{sec:2}, we presented the model construction, which extends the standard model with two gauge-singlet scalars $(\sigma_1^{},\,\sigma_2^{})$,\, a vector-like iso-doublet fermion $(\psi_L^{},\,{\psi_L'}^c)$, and one iso-triplet Higgs $\Delta$\,. This model holds a softly broken lepton number and an exactly conserved $\ZZ_2^{}$ discrete symmetry. Then, in section\,\ref{sec:3}, we demonstrated that the lighter Majorana fermion $\chi_1^{}$ can serve as a stable DM candidate and provide the observed relic density in the present universe with its mass $M_{\chi_1^{}}^{}\!\!\simeq 1.24$\,TeV.\, This fermionic DM $\chi_1^{}$ can be searched by the current direct/indirect DM detection experiments\,\cite{DM-DirectExp-Rev} and by the on-going LHC experiments as well as the future high energy $pp$ colliders\,\cite{pp}. In Section\,\ref{sec:4}, we further demonstrated how our model can naturally realize the minimal type-II seesaw and radiatively generate the light neutrino masses $\,m_\nu^{}=\mathcal{O}(0.1\text{eV})\,$ at TeV scale [cf.\ Fig.\,\ref{fig:1} and Eq.\eqref{numassform2}]. Finally, in Section\,\ref{sec:5}, we studied the realization of a natural thermal or inflationary leptogenesis through decays of the lightest singlet scalar $\sigma^{}_{1I}$ at a high scale around $\mathcal{O}(10^{13})$GeV. %\newpage \vspace*{5mm} \noindent {\bf\large Acknowledgements} \\[1mm] We thank Alessandro Strumia for discussing the minimal dark matter models. PHG was supported by the National Natural Science Foundation of China under Grant No.\, 11675100 and the Recruitment Program for Young Professionals under Grant No.\, 15Z127060004. HJH was supported in part by the National NSF of China (under grants 11675086 and 11835005) and the National Key R\,\&\,D Project of China (under grant 2017YFA0402204); he was also supported in part by the Shanghai Laboratory for Particle Physics and Cosmology (under grant 11DZ2260700), and the Office of Science and Technology, Shanghai Municipal Government (under grant 16DZ2260200). %\addcontentsline{toc}{section}{Acknowledgments\,} %\newpage %\vspace*{2mm} %\begin{appendix} %\appendix \baselineskip 17pt \vspace{5mm} %\newpage % | 18 | 8 | 1808.09377 |
1808 | 1808.02828_arXiv.txt | We present Atacama Large Millimeter/submillimeter Array measurements of the `Cosmic Seagull', a strongly magnified galaxy at $z=2.7779$ behind the Bullet Cluster. We report CO(3-2) and continuum 344~$\mu$m (rest-frame) data at one of the highest differential magnifications ever recorded at submillimeter wavelengths ($\mu$ up to $\sim50$), facilitating a characterization of the kinematics of a rotational curve in great detail (at $\sim 620$~pc resolution in the source plane). We find no evidence for a decreasing rotation curve, from which we derive a dynamical mass of $(6.3 \pm 0.7) \times 10^{10} \rm M_{\odot}$ within $r=2.6\pm0.1$~kpc. The discovery of a third, unpredicted, image provides key information for a future improvement of the lensing modeling of the Bullet Cluster and allows a measure of the stellar mass, $1.6^{+1.9}_{-0.86} \times 10^{10} \, M_{\odot}$, unaffected by strong differential magnification. The baryonic mass is is expected to be dominated by the molecular gas content ($f_{gas}\leq80\pm20$\%) based on an $M_{H_2}$ mass estimated from the difference between dynamical and stellar masses. The star formation rate is estimated via the spectral energy distribution ($\rm SFR=190\pm10 M_{\odot} / yr$), implying a molecular gas depletion time of $0.25\pm0.08$\,Gyr. | \label{sec:intro} The flat rotation curves in local spiral galaxies \citep{2001ARA&A..39..137S} are a basic piece of evidence for the existence of dark matter in the Universe. At high redshift, studies of rotation curves are limited by the low surface brightnesses of the outskirts of galaxies. The recent report of decreasing (as a function of galactocentric radius) rotation curves in massive galaxies at redshift $z=0.9-2.4$ has suggested that dark matter does not dominate the dynamical mass at distances larger than 1.3--1.5 times the galaxy effective radius \citep{2017Natur.543..397G}. These findings are influenced by the effect of pressure support in turbulent disks, which is more commonly seen in high-$z$ gas-rich galaxies. Probing rotation curves is difficult at high $z$, however this can be aided by strong gravitational lensing \citep{2008Natur.455..775S}. \begin{figure*}[thb] \begin{center} \plotone{fig1} \caption{HST composite (WFPC2/F814W, WFPC3/F110W and F160W) image and contours (intervals from 20\% to 80\% of 1.16~mJy~beam$^{-1}$ peak intensity) from ALMA band-6 continuum centered at 231~GHz showing the structure of the `Cosmic Seagull' near a bright galaxy (GalB). The orange line represents the critical line in the lens plane. Inserts at the top exhibit the CO(3--2) rotation velocity map (in colours) and line intensity (in contours in intervals from 20\% to 80\% of 0.49~Jy~beam$^{-1}$~km~s$^{-1}$ peak intensity) for $\mathscr{A}0$, $\mathscr{A}1$, and $\mathscr{A}2$. The ALMA synthesized beam (0.5''$\times$0.7'', PA$=43^{\circ}$) is shown as black ellipses. Notice the inverted rotation curves for $\mathscr{A}2$ and $\mathscr{A}1$, caused by the lensing effect. The central velocity, as well as the velocity range, of $\mathscr{A}0$ is the same as for $\mathscr{A}2$ and $\mathscr{A}1$. The insert at the bottom shows the observed spectra from all of the 'Cosmic Seagull' images ($\mathscr{A}0$, $\mathscr{A}1$ and $\mathscr{A}2$). The spectra have been smoothed to a final resolution of 23.4~MHz (78 km~s$^{-1}$) to improve visualization. The effect of differential magnification (not corrected in the figure) can be significant in the low-frequency (high-velocity) side of the CO(3--2) emission line, as it approaches the critical line. The similarity of the profiles confirms that $\mathscr{A}0$, $\mathscr{A}1$ and $\mathscr{A}2$ are three images of the same object. \label{co32} } \end{center} \end{figure*} Strong gravitational lensing by massive clusters can boost the signal of background galaxies, offering the opportunity to probe in great detail their internal structures \citep{2010Natur.464..733S}. The Bullet Cluster (1ES 0657-558) consists of two merging galaxy clusters at z = 0.296 \citep{1998ApJ...496L...5T}, and its high mass \citep{2006ApJ...652..937B,2016A&A...594A.121P} causes the field to display an exceptionally high number of lensed sub-millimeter galaxies \citep{2010A&A...514A..77J}. Here we present the physical properties of galaxy SMM~J0658 at z=2.7793 \citep{2012A&A...543A..62J}, hereafter called the `Cosmic Seagull' (figure \ref{co32}). This galaxy lies close to a lensing critical line, resulting in one of the largest magnifications reported to date ($\mu$ up to $\sim 50$), which facilitates precise measurements of the kinematics at the outskirts of this dusty, star-forming galaxy. Through the text we assume a $\Lambda$CDM cosmology with $\Omega_m=0.3$, $\Omega_{\Lambda}=0.7$ and $h=0.7$. | These new ALMA observations reveal for the first time an exquisite system with multiple images behind the Bullet Cluster (the `Cosmic Seagull'). The kinematic information at the outskirts of a galaxy at $z\simeq2.8$ is facilitated by one of the largest gravitational magnifications ever recorded ($\mu \lesssim 50$). The strong lensing of this system reveals its internal dynamics, stellar mass, and gas distribution, providing an excellent opportunity to explore the internal structure of a normal (following the 'main sequence') high-z galaxy which would otherwise be undetectably faint. This exceptionally magnified object observed at sub-arcsecond resolution provides us with a detailed rotational curve near the peak of the galaxy mass assembly epoch. Our ALMA data have shown that SMM~J0658 is a disk-like galaxy, with a dynamical mass of $(6.3\pm0.7) \times 10^{10} \rm M_{\odot}$ inside $r=2.6 \pm 0.1$~kpc, where most of its baryonic mass is probably dominated by the molecular gas content ($f_{gas}\leq80\pm20$\%). The correspondence between the peak of the cold dust emission and the center of the velocity dispersion profile shows that this galaxy is not involved in a major merging event, supported also by the relatively long molecular gas depletion time, $\geq 0.25\pm0.08$\,Gyr. | 18 | 8 | 1808.02828 |
1808 | 1808.02533_arXiv.txt | The Radio Ammonia Mid-Plane Survey (RAMPS) is a molecular line survey that aims to map a portion of the Galactic midplane in the first quadrant of the Galaxy ($l= 10^{\circ} - 40^{\circ}$, $\lvert b\rvert \leq 0.4^{\circ}$) using the Green Bank Telescope. We present results from the pilot survey, which has mapped approximately 6.5 square degrees in fields centered at $l=10^{\circ}, \ 23^{\circ},\ 24^{\circ}, \ 28^{\circ}, \ 29^{\circ}, \ 30^{\circ}, \ 31^{\circ}, \ 38^{\circ}, \ 45^{\circ}, \ \mathrm{and} \ 47^{\circ}$. RAMPS observes the $\mathrm{NH_{3}}$ inversion transitions $\mathrm{NH_{3}(1,1) - (5,5)}$, the $\mathrm{H_{2}O} \ 6_{1,6}-5_{2,3}$ maser line at 22.235 GHz, and several other molecular lines. We present a representative portion of the data from the pilot survey, including $\mathrm{NH_{3}(1,1)}$ and $\mathrm{NH_{3}(2,2)}$ integrated intensity maps, $\mathrm{H_2O}$ maser positions, maps of $\mathrm{NH_{3}}$ velocity, $\mathrm{NH_{3}}$ line width, total $\mathrm{NH_{3}}$ column density, and $\mathrm{NH_{3}}$ rotational temperature. These data and the data cubes from which they were produced are publicly available on the RAMPS website\footnote{\url{http://sites.bu.edu/ramps/} \label{foot:ramps}}. | \label{sec:intro} Although high-mass stars ($\mathrm{M > 8 \ M_{\odot}}$) are rare, they dominate the chemical and energetic input into the interstellar medium (ISM). Gaining a detailed understanding of the formation of high-mass stars is thus important for theories of stellar cluster formation and galactic evolution. The current theoretical picture of high-mass star formation is that high-mass molecular clumps ($\mathrm{M > 200 \ M_{\odot}, \ R\sim1 \ pc}$) are the nurseries of high-mass stars and star clusters. Density enhancements within clumps (here we define a $\lq \lq$clump" as a molecular clump), called cores \citep[$\mathrm{M \sim 1 - 10 \ M_{\odot}, \ R\sim0.05 \ pc}$; ][]{S17}, are initially devoid of stars, and are thus referred to as $\lq \lq$prestellar" cores. Their ensuing collapse forms deeply embedded, accreting $\lq \lq$protostellar" cores, where a high-mass star or multiple stellar system may form. High-mass protostars quickly enter the main sequence and ionize their surrounding material to form an H II region. Despite this broad theoretical understanding, the details of high-mass star formation are not well understood compared to the formation of low-mass stars, especially with regard to the early fragmentation history, turbulent support of cores, and to the physical and dynamical evolution of protostars, as well as their physical and dynamical evolution. This difference is in part due to the difficulty of observing high-mass star-forming regions (SFRs), especially at early evolutionary stages. In contrast to low-mass stars, high-mass stars are rarer, form more quickly, and form in regions that are more deeply embedded in gas and dust. To make progress in the face of the observational challenges, large surveys are necessary to observe a statistically significant sample of high-mass SFRs. As high-mass stars form predominantly in the Galactic plane, surveys of high-mass SFRs typically focus their observations in the plane. Recently, continuum surveys of the Galactic plane, such as the 1.1 mm Bolocam Galactic Plane Survey \citep[BGPS;][]{2011ApJS..192....4A}, the 870 $\mathrm{\mu m}$ APEX Telescope Large Area Survey of the Galaxy \citep[ATLASGAL;][]{2009A&A...504..415S}, the 70 $-$ 500 $\mathrm{\mu m}$ $Herschel$ Infrared Galactic Plane Survey \citep[HiGAL;][]{2010A&A...518L.100M}, the Red MSX Source \citep[RMS; ][]{2009A&A...501..539U}, and the Coordinated Radio and Infrared Survey for High-Mass Star Formation \citep[CORNISH; ][]{2012PASP..124..939H}, have identified thousands of dense, high-mass, star-forming clumps from their dust emission. In addition to the position and structure of star-forming clumps, continuum surveys have contributed important information that helps characterize these clumps. In particular, modeling the dust continuum spectral energy distribution (SED) of a clump allows one to derive its dust temperature and column density. From the column density, one can estimate the dust mass of a clump at a known distance. With the dust-to-gas mass ratio, one can then determine the total mass of the clump. This information is crucial for determining whether a clump or core will go on to form high-mass stars and exactly how the clumps evolve. Although continuum surveys are essential, they do have significant limitations. Continuum emission may be blended owing to multiple clumps or unrelated diffuse dust along the line of sight, both of which will hinder the estimation of parameters from the dust SED. In addition, assumptions about the dust-to-gas mass ratio, the dust emissivity coefficient $\kappa$, and the dust emissivity index $\beta$ are uncertain, with the combination of such uncertainties affecting the accuracy of derived column densities and temperatures. Furthermore, the derivation of temperatures from graybody dust SEDs usually assumes optically thin emission at all far-IR to millimeter wavelengths. While this assumption is reliable for the majority of high-mass SFRs, it may not be true for the densest, coldest clumps. Many of the limitations of dust continuum surveys can be overcome by a focused molecular line survey. The main advantage of molecular line data is their ability to provide kinematic information, such as the velocity dispersion $\sigma$, a crucial parameter in all theories of high-mass star formation. The velocity dispersion, measured from the turbulent line width, sets the turbulent pressure $\mathrm{(\propto \rho \sigma^2)}$, the mass accretion rate (isothermal sphere: $\dot{M} \propto \sigma^3$ \citep{1980ApJ...241..637S}; Bondi-Hoyle: $\dot{M} \propto \sigma^{-3}$ \citep{1952MNRAS.112..195B}), the dynamical timescale $(\propto R / \sigma)$, and the virial parameter $(\alpha = M_{vir} / M \propto \sigma^2 R / G M)$. Using the kinematic distance method \citep{O58}, the velocity of a line can provide an estimate of distance, which is necessary to calculate the size, mass, luminosity, and Galactic location of a clump. Additionally, velocity fields can be used to separate multiple clumps along the line of sight and reveal bulk flows and rotation. Molecular line surveys that target transitions with large Einstein A-coefficients have an additional important advantage over continuum surveys. Such transitions have large critical densities, and thus they primarily trace regions with dense ($n > 10^3 \ \mathrm{cm^{-3}}$), star-forming gas, rather than unrelated diffuse gas along the line of sight. Spectral lines can also provide a robust estimate of the gas temperature. In local thermodynamic equilibrium (LTE), the gas temperature of an emitting medium may be determined by observing spectral lines of the same species that are well separated in excitation energy. The excitation temperature sets the level populations, and the excitation temperature is equal to the gas temperature when the gas is sufficiently dense. In LTE, measuring the relative intensity of the lines thus provides the temperature of dense gas. In addition, spectral lines can help to determine optical depth by comparing two or more spectral lines that have a known intensity ratio. This estimation is often done with a molecule and its isotopic counterpart, since the ratio of their optical depths is equal to their relative abundance. A similar method is available for spectral transitions that exhibit hyperfine splitting. In LTE, the ratio of the optical depths in various hyperfine lines is proportional to the ratio of their quantum statistical weights, which are constant, unlike relative abundance. This feature allows for a more reliable determination of optical depth and can be accomplished by observing a single set of hyperfine lines. The $\mathrm{H_{2}O}$ Southern Galactic Plane Survey \citep[HOPS;][]{2011MNRAS.416.1764W,P12} is a previous molecular line survey of dense gas. HOPS observed 100 deg$^{2}$ of the Galactic plane and primarily targeted several $\mathrm{NH_3}$ inversion lines and the 22.235 GHz $\mathrm{H_{2}O} \ 6_{1,6}-5_{2,3}$ maser line using the 22 m Mopra telescope. HOPS and similar surveys have provided a wealth of data for the high-mass star formation community. These data have helped advance our understanding of the complex kinematics, chemistry, and evolution of high-mass clumps \citep{2017MNRAS.470.1462L}. To further probe these SFRs, we must exploit new advancements in instrumentation. To this end, we are undertaking the Radio Ammonia Mid-Plane Survey (RAMPS). RAMPS is a new Galactic midplane molecular line survey, which employs the $K$-band Focal Plane Array on the Green Bank Telescope (GBT) to image several $\mathrm{NH_3}$ inversion lines and the 22.235 GHz $\mathrm{H_{2}O}$ line. In this paper, we describe the survey and highlight its first results. {We begin by discussing the survey and its observations (Section~\ref{sec:surv}). Subsequently, we present the results of the RAMPS pilot survey (Section~\ref{sec:results}). We then present a preliminary analysis of the data (Section~\ref{sec:analysis}) and a comparison of the features of the RAMPS survey to those of previous surveys (Section~\ref{sec:comparison}). Finally, we summarize our conclusions (Section~\ref{sec:conclusion}). | \label{sec:conclusion} RAMPS is an ongoing molecular line survey in the first quadrant of the Galactic midplane. In this paper, we have reported on the pilot survey, which mapped approximately 6.5 square degrees of the RAMPS survey region. RAMPS is a significant improvement on previous large molecular line surveys owing to advancements in instrumentation on the GBT. While the GBT provides excellent sensitivity and spatial resolution, the KFPA receiver array and the VEGAS spectrometer make a large $K$-band survey possible. The KFPA's seven receivers can map large areas in a relatively short amount of time, while VEGAS is able to observe simultaneously a large number of spectral lines over a wide frequency range. This combination gives RAMPS a distinct advantage in fast mapping at $K$-band frequencies. An important consequence of the new instrumentation is our ability to map simultaneously a suite of useful lines, namely, the $\mathrm{NH_{3}}$ inversion transitions, $\mathrm{NH_{3}}$(1,1)$-$(5,5), and the 22.235 GHz $\mathrm{H_{2}O}$ maser line. Not only do the $\mathrm{NH_{3}}$ inversion lines trace the dense molecular clumps where high-mass stars can form, but they also provide robust estimates of the gas temperature and column density. Furthermore, measured line widths allow us to determine the virial state of molecular clumps, while their velocities can help determine their distances. Among other things, $\mathrm{H_{2}O}$ masers can be used as an indicator of active star-formation; thus, an $\mathrm{H_{2}O}$ maser associated with $\mathrm{NH_{3}}$ can help indicate whether stars are forming within a molecular clump. RAMPS is a leap forward in large surveys of $\mathrm{NH_{3}}$ and $\mathrm{H_{2}O}$ masers; thus, the RAMPS dataset is an important step toward a better understanding of high-mass star formation. We have presented integrated intensity maps of $\mathrm{NH_{3}(1,1)}$ and $\mathrm{NH_{3}(2,2)}$, $\mathrm{H_{2}O}$ positions, and associations for six fields within the Galactic plane. In addition, we have presented representative maps of $\mathrm{NH_{3}}$ velocity, $\mathrm{NH_{3}}$ rotational temperature, total $\mathrm{NH_{3}}$ column density, and $\mathrm{NH_{3}}$ line width for the L23 and L24 fields. The data cubes and maps for the entire RAMPS pilot survey are now available on the RAMPS website (see footnote \ref{foot:ramps}). With the successful results from the pilot survey, we have shown that RAMPS works as expected. Following the pilot survey, RAMPS has been awarded additional observing time on the GBT to extend the survey. We plan to release RAMPS data publicly after calibration and verification. We anticipate that the full RAMPS dataset will support numerous scientific investigations in the future. | 18 | 8 | 1808.02533 |
1808 | 1808.09621_arXiv.txt | By using a non-local and time-dependent convection theory, we have calculated radial and low-degree non-radial oscillations for stellar evolutionary models with $M=1.4$--3.0\,$\mathrm{M}_\odot$. The results of our study predict theoretical instability strips for $\delta$ Scuti and $\gamma$ Doradus stars, which overlap with each other. The strip of $\gamma$ Doradus is slightly redder in colour than that of $\delta$ Scuti. We have paid great attention to the excitation and stabilization mechanisms for these two types of oscillations, and we conclude that radiative $\kappa$ mechanism plays a major role in the excitation of warm $\delta$ Scuti and $\gamma$ Doradus stars, while the coupling between convection and oscillations is responsible for excitation and stabilization in cool stars. Generally speaking, turbulent pressure is an excitation of oscillations, especially in cool $\delta$ Scuti and $\gamma$ Doradus stars and all cool Cepheid- and Mira-like stars. Turbulent thermal convection, on the other hand, is a damping mechanism against oscillations that actually plays the major role in giving rise to the red edge of the instability strip. Our study shows that oscillations of $\delta$ Scuti and $\gamma$ Doradus stars are both due to the combination of $\kappa$ mechanism and the coupling between convection and oscillations, and they belong to the same class of variables at the low-luminosity part of the Cepheid instability strip. Within the $\delta$ Scuti--$\gamma$ Doradus instability strip, most of the pulsating variables are very likely hybrids that are excited in both p and g modes. | \label{sec1} On the Hertzsprung--Russell (H--R) diagram, the diagonal Cepheid instability strip intersects with the main sequence (MS) at A--F dwarfs, along which a few types of classical variables are confined: Cepheids of Population I and II are located in the upper parts, RR Lyrae stars are in the middle part, while $\delta$ Scuti stars and white dwarfs are found near the MS and even lower luminosities. All of them are excited by $\kappa$ mechanism due to the ionization zones of hydrogen and helium. The overall properties for variables in the strip are that giants tend to pulsate in a single or a few low-order radial modes with large amplitudes, while dwarfs usually oscillate in multiple radial and non-radial modes with rather small amplitudes. Only very few of $\delta$ Scuti stars have relatively large amplitude in radial modes. High-amplitude $\delta$ Scuti stars are normally slow rotators ($v\sin i\leq 30\,\mathrm{km\,s}^{-1}$), while other stars in the same region of H-R diagram normally have high rotation speed ($v\sin i \sim 150\,\mathrm{km\,s}^{-1}$). It is believed that rotation is responsible for the existence of two types of $\delta$ Scuti stars. Whether this is true or not is still to be solved. The study of $\delta$ Scuti stars has a history of over a hundred years since the first discovery by Campbell \& Wright (1900). Research in this field has been largely boosted since the success of helioseismology, and it has then been the first objective in asteroseismology therefore has made a great leap forward. The earlier work has been intensively reviewed by Breger (2000). In contrast to that, the study of $\gamma$ Doradus stars, only 20 yr after officially classified (Balona, Krisciunas \& Cousins 1994), has just been started. On the H--R diagram, they are a group of variable stars attached to the low-temperature region outside the red edge of the $\delta$ Scuti strip, being the first class of g-mode oscillators in the Cepheid instability strip. The so-called convective blocking (Guzik et al. 2000) is believed to be the excitation mechanism of oscillations, with the original idea coming from the hypothesis of `frozen-in convection' (Warner, Kaye \& Guzik 2003). However, such a hypothesis can never give rise to the red edge of the instability strip theoretically. Using a time-dependent mixing-length theory, it was confirmed that convective flux blocking could be the driving mechanism in $\gamma$ Doradus stars, given the mixing-length parameter was fine-tuned (Dupret et al. 2005). Convection drives transportation of energy and momentum in stellar interiors, and therefore affects the structure and oscillation stabilities. For low-temperature stars with extended convective envelope, convection overtakes radiation and becomes the major play of energy transportation. The dynamic and thermodynamic couplings between convection and oscillations become the major mechanisms of excitation and damping against oscillations. Based on hydrodynamic equations and the theory of turbulence, we developed a non-local and time-dependent theory of convection (Xiong 1989; Xiong, Cheng \& Deng 1997; Xiong, Deng \& Zhang 2015). The current paper is the second of the series of work on turbulent convection and stability of pulsating stars. In this work, we calculated linear non-adiabatic oscillations in radial and low-degree ($l=1$--4) non-radial modes for evolutionary models of $1.4$--3.0\,$\mathrm{M}_\odot$ stars, from which the blue and red edges of $\delta$ Scuti and $\gamma$ Doradus stars were defined. Such calculations also facilitate studies of the properties of the strips and the relation between them. The {\it Kepler} mission provided high-precision photometry in continuous long time baseline for over 100 000 stars, among which about 1500 new $\delta$ Scuti and $\gamma$ Doradus stars were discovered. This high-precision sample not only greatly enhanced the data base, but also offers a best opportunity for the studies of the two types of variables (Balona \& Dziembowski 2011; Balona et al. 2011). In Section~\ref{sec2}, we discuss the general properties of linear non-adiabatic oscillations in our theoretical scheme. The theoretical instability strips of $\delta$ Scuti and $\gamma$ Doradus stars defined based on our calculations are presented in Section~\ref{s3}. Thorough statistics of the observational properties of these variables and comparison with existing theoretical results are given in Section~\ref{s4}. The excitation and stabilization mechanisms and related problems are discussed in Section~\ref{s5}. The work is then summarized together with some discussions in the last section. | \label{s6} Using a non-local and time-dependent convection theory, we have calculated radial and low-degree non-radial oscillations of the convective envelopes of stellar evolutionary models with $M=1.4$--$3.0\,\mathrm{M}_\odot$. The main results can be summarised as follows. \begin{enumerate} \item The linear stability analysis of g20--p29 modes is almost complete for stellar models with $M=1.4$--$3.0\mathrm{M}_\odot$. In the $\log Q - \log T_\mathrm{e}$ plane, unstable g and p modes form two detached areas. The boundary between them is at $Q \approx 0.04$. For all unstable g modes, $Q \gtrsim 0.04$, while for all unstable p modes, $Q \lesssim 0.04$. \item On the H-R diagram, unstable g and p modes are located in two partially overlapped instability strips. The g-mode instability strip is systematically redder than the p-mode instability strip. \item From the analysis of accumulated work, we find no distinct difference in the excitation and stabilization between p and g modes. They are both due to the combination of $\kappa$ mechanism and the coupling between convection and oscillations. Radiative $\kappa$ mechanism plays a major role in the excitation of warm $\delta$ Scuti and $\gamma$ Doradus stars, while the coupling between convection and oscillations is responsible for excitation and stabilization in cool stars. \end{enumerate} Based on aforementioned results of linear non-adiabatic oscillations, it seems reasonable to believe that there is no essential difference between $\delta$ Scuti and $\gamma$ Doradus stars. They are just two subgroups of one broader type of $\delta$ Scuti-$\gamma$ Doradus stars below the Cepheid instability strip: $\delta$ Scuti is a p-mode subgroup, while $\gamma$ Doradus is a g-mode subgroup. Within the instability strip, most of the variable stars may be hybrids pulsating in both p and g modes. The connections among them are very similar to that among RR$_\mathrm{ab}$, RR$_\mathrm{c}$, and RR$_\mathrm{d}$. Clearly, our interpretation of the excitation and stabilization is quite different from the mainstream view as a result of the employment of the non-local time-dependent convection theory. Moreover, we used non-local convection envelope models in the calculation of non-adiabatic oscillations, instead of full stellar models. This is because the effects of non-local convection is crucial to the pulsational stability of stars. These effects exist not only in the time-dependent convection in the calculation of oscillations, but also in the calculation of equilibrium structure of convective envelopes. We carefully studied the influences of radiative damping in stellar deep interior and the depth of bottom boundary on the stability calculations of stellar oscillations. Our results show that: \begin{enumerate} \item The excitation and damping of all p modes and unstable g modes come mainly from the outer layers of stars. Radiative damping in the deep region is relatively small, therefore its influence on the pulsational stability is negligible. \item Radiative damping in stellar deep regions is the main damping mechanism of stable g modes. \item As the bottom boundary moves outward, the amplitude growth rates of unstable modes decrease, and the damping of stable modes increases. However, the pulsational stability or instability of a specific mode does not change. If the bottom boundary is not set deep enough, the excitation from the outer layers will be underestimated in the calculations of non-adiabatic oscillations, while the radiative damping in the deep region will be overestimated. As a result, the amplitude growth rates decrease, but do not change signs. \end{enumerate} All the results in this work are based on chemically uniform envelope models. Unless there exists unusually large damping in the nuclear-reacting core, it is not likely that the use of full stellar models will cause radial changes to the results. We expect to carry out further examinations with full models in near future. | 18 | 8 | 1808.09621 |
1808 | 1808.03645_arXiv.txt | Molecular species in planetary atmospheres provide key insights into their atmospheric processes and formation conditions. In recent years, high-resolution Doppler spectroscopy in the near-infrared has allowed detections of H$_2$O and CO in the atmospheres of several hot Jupiters. This method involves monitoring the spectral lines of the planetary thermal emission Doppler-shifted due to the radial velocity of the planet over its orbit. However, aside from CO and H$_2$O, which are the primary oxygen- and carbon-bearing species in hot H$_2$-rich atmospheres, little else is known about molecular compositions of hot Jupiters. Several recent studies have suggested the importance and detectability of nitrogen-bearing species in such atmospheres. In this Letter, we confirm potential detections of CO and H$_2$O in the hot Jupiter HD 209458b using high-resolution spectroscopy. We also report a cross-correlation peak with a signal-to-noise ratio of $4.7$ from a search for HCN. The results are obtained using high-resolution phase-resolved spectroscopy with the Very Large telescope CRyogenic high-resolution InfraRed Echelle Spectrograph (VLT CRIRES) and standard analysis methods reported in the literature. A more robust treatment of telluric contamination and other residuals would improve confidence and enable unambiguous molecular detections. The presence of HCN could provide constraints on the C/O ratio of HD~209458b and its potential origins. | \label{sec:intro} Exoplanets orbiting nearby sun-like stars provide a unique opportunity to study the diversity of planetary processes that can arise from primordial environments like our own, particularly through chemical species in their atmospheres. Due to the low temperatures ($\lesssim$ 100~K) of solar system giant planets, several important chemical tracers of planetary origins are either condensed out, such as H$_2$O \citep{wong2004, atreya2016}, or present in trace quantities such as CO and HCN, making their origins ambiguous \citep{moreno2003, cavalie2008, cavalie2010, moses2010}. It has been predicted that atmospheres of hot giant exoplanets should contain copious amounts of these molecules and can, therefore, provide critical insights into planetary formation \citep{madhu2016}. \begin{figure*}[ht!] \centering \includegraphics[width=150mm,trim={10cm 15cm 10cm 15cm},clip]{detrendinghcn2.pdf} \caption{Stages of detrending, for detectors 1--4 and the set of observations taken on 2011 July 25. The x-axis corresponds to wavelength, and the y-axis corresponds to frame number, increasing in time. Panel (a): spectra immediately after reduction of nodding frames. Heavy telluric contamination is evident (e.g. at 3.1915 $\mu$m). Poor seeing conditions manifest as dark horizontal bands. Panel (b): reduced spectra after wavelength calibration, alignment, additional cleaning, normalization, and masking. This image (excluding masked regions) is the input of our detrending algorithm. Panel (c): data subject to column-wise mean subtraction, the optimal number of SYSREM iterations, a 15-pixel standard-deviation high-pass filter, and column-wise standard-deviation division. Panel (d): the same as in Panel (c), but with the injection of our planet model at 40x its nominal strength prior to detrending. The preserved planetary absorption features appear as dark trails that stretch over $\sim0.0008 \: \mu$m.} \label{fig:detrendinghcn2} \end{figure*} The hot Jupiter HD~209458b is one of the most favorable targets for atmospheric characterization. The planet orbits a bright ($V$=7.65) sun-like (G0V) star in a 3.5 day period and has mass 0.69$\pm$0.017 $M_J$ and radius 1.38$\pm$0.018 $R_J$ \citep{knutson2007}. However, with its close-in orbit and mass between those of Saturn and Jupiter, the planet has no analog in the solar system, even though it likely originated in similar conditions to the primordial solar nebula given its solar-like host star. Low-resolution spectra of the planet obtained using the \textit{Hubble Space Telescope} (HST) revealed the presence of H$_2$O in its atmosphere, albeit with significantly weaker spectral features than originally anticipated \citep{deming2013, line2016}. While the H$_2$O abundance has been found to be significantly sub-solar at the day--night terminator region of the atmosphere \citep{madhu2014b, barstow2017,macdonald2017a}, the same is not well constrained in the dayside atmosphere \citep{line2016}. Efforts to detect any other molecule using low-resolution spectra of the planet have proved elusive to date. Recently, an HST transmission spectrum of the planet initially suggested strong evidence for NH$_3$ and/or HCN at the terminator \citep{macdonald2017a}. However, a subsequent study including additional data and modeling lowered the detection significances, leaving HCN undetected \citep{macdonald2017}. In recent years, high-resolution Doppler spectroscopy has enabled detections of key molecules in the atmospheres of hot Jupiters \citep{snellen2010,brogi2012}. This method involves monitoring numerous (10$^2$-10$^3$) individual molecular lines in the planetary spectrum being Doppler shifted as the planet traverses its orbit, leading to a high-fidelity detection of the molecule. Such observations have led to detections of H$_2$O and CO in several hot Jupiters \citep{snellen2010, brogi2012, birkby2013, rodler2013, birkby2017}, and TiO in one \citep{nugroho2017}. The previous application of this method to HD 209458b has led to the detection of CO at the day--night terminator region of its atmosphere \citep{snellen2010} as well as both CO and possibly H$_2$O in the dayside \citep{brogi2017}. In this Letter we use this method in search of molecular signatures in the dayside atmosphere of the planet. | \label{sec:results} We find evidence for the presence of the species CO, H$_2$O, and HCN in the dayside atmosphere of HD 209458b. The cross-correlation S/N for each molecule over a grid of $K_p$ and $V_{sys}$ are shown in Figure \ref{fig:test}. In the 2.3$\mu$m window, cross correlation with H$_2$O and CO templates yield S/Ns of 4.4 ($K_p$ = 143$^{+15}_{-13}$ km s$^{-1}$ and $V_{sys}$ = -14$^{+6}_{-5}$ km s$^{-1}$) and 5.1 ($K_p$ = 136$^{+15}_{-14}$ km s$^{-1}$ and $V_{sys}$ = -16$^{+6}_{-5}$ km s$^{-1}$), respectively. Using a combined H$_2$O + CO model gives a boosted 5.3 S/N at $K_p$ = 141$^{+11}_{-14}$ km s$^{-1}$ and $V_{sys}$ = -15$^{+4}_{-4}$ km s$^{-1}$. Cross correlation with an HCN template yields a peak S/N of 4.7 at $K_p$ = 142$^{+21}_{-13}$ km s$^{-1}$ and $V_{sys}$ = -14$^{+5}_{-7}$ km s$^{-1}$ in the 3.2$\mu$m spectral window. The cross-correlation signal for HCN is made possible by its high molecular opacity in the 3.2$\mu$m band, and a dense forest of deep absorption lines. The presence of CO is consistent with previous detections, both in transmission \citep{snellen2010} and emission \citep{schwarz2015, brogi2017}. The $K_p$ and $V_{sys}$ of the detections are in precise agreement with those determined in previous studies $K_p$ = 140$^{+10}_{-10}$ km s$^{-1}$ and $V_{sys}$ = -14.8 km s$^{-1}$ from orbital parameters as well as previous studies \citep{snellen2010}. \begin{figure} \centering \centering \includegraphics[width=.49\linewidth]{CO_in_out_hist2.pdf} \includegraphics[width=.49\linewidth]{H2O_in_out_hist2.pdf} \centering \includegraphics[width=.49\linewidth]{H2O_CO_in_out_hist2.pdf} \includegraphics[width=.49\linewidth]{HCN_in_out_hist2.pdf} \caption{ In-trail and out-of-trail samples of the cross-correlation function for each molecular detection. Note the consistent shift of the in-trail samples toward higher means and the strong Gaussian nature of the out-of-trail samples. Welch $T$-tests suggest that the samples are drawn from different distributions with greater than 5.7$\sigma$ confidence for all cases.} \label{fig:in_vs_out} \end{figure} The S/N provides a metric for how well the model template matches intrinsic features. However, the cross correlation is sensitive to the stages of detrending outlined in section~\ref{sec:obs}, and the choice of model template. SYSREM does not exactly remove systematics and environmental effects, but rather approximations of them that best match a linear trend in the data. These approximations depend on the choice of columns that are masked. For example, masking less data around tellurics increased the number of spectral features with which we cross correlate, but requires more SYSREM iterations to fully remove higher-order residuals, which in turn can degrade the intrinsic planet signal. In optimizing the number of SYSREM iterations using model-injection, a commonly accepted method \citep{birkby2013,birkby2017,nugroho2017}, one potentially introduces the possibility of `overfitting' the detrending hyperparameters and obtaining a higher S/N than truly reflected within the data, especially given the fact that each detector is treated separately. We perform a series of tests, concentrating on the 3.2$\mu$m data, in which we (1) change the amount of masking around telluric zones; (2) optimize and search for other models including H$_2$O, CH$_4$, NH$_3$, and ones where the lines are randomly shuffled; and (3) optimize the number of SYSREM iterations to other local maxima in $K_p$-$V_{sys}$ space. As expected, varying the amount of masking has a significant effect on the detection significances and optimum number of SYSREM iterations. HCN is the only model with which we obtain a robust cross-correlation S/N$>4$ at the planetary $K_p$ and $V_{sys}$ in the 3.2$\mu$m band. However, when optimizing to other local maxima in $K_p$-$V_{sys}$ space, in some cases we are able to obtain cross-correlation S/N values $\geq 4$ at locations inconsistent with the expected $K_p$ and $V_{sys}$ of the planet. This raises some concern about potential false positives, especially if such a signal should happen to coincide with the known $K_p$ and $V_{sys}$ or if $K_p$ and $V_{sys}$ are not known from another method. It could be that the variation of hyperparameters effectively allow enough draws to be taken from the noise distribution, thus giving an S/N of $\geq 4$. We find that the Welch $T$-test metric commonly used in the literature \citep{birkby2017,nugroho2017} is similarly vulnerable and returns even higher estimates of the signal significances (see Figure 5), hence we choose to use the more conservative S/N metric. An ideal statistical metric for detection significances should account for false positives and the sensitivities inherent to the method. We perform an independent test using the detrending approach detailed in \cite{schwarz2015} and \cite{brogi2014}. This detrending approach is not generally used for data in the 3.2$\mu$m spectral window as SYSREM is preferred \citep{birkby2013,birkby2017,birkby2018}, which tends to be better at removing the stronger tellurics at these wavelengths. Nevertheless, we apply the alternative detrending method masking only a small percentage of the data on the ends of the detectors. We then linearly fit the logarithmic flux of each column with airmass variation, and remove the trend by division. Next, we sample columns from deep telluric features and linearly regress them to each column. We divide by the fit, which removes the second-order residuals. Finally, we normalize each column by its variance to suppress noisy pixels. Unlike the SYSREM approach, this method does not involve optimization, and reduces the chance of overfitting. However, we are unable to obtain an S/N $\geq 3$ from cross correlation with the HCN template in the 3.2$\mu$m band. This may be partly due to the presence of remaining systematics which SYSREM is more effective at removing. The best method to remove telluric contamination is yet to be determined \citep{birkby2018}. We find that, applied to the 2.3$\mu$m, the alternative detrending method produces a similarly inconclusive ($\sim3\sigma$) potential CO detection in agreement with \cite{schwarz2015}; it also suggests that it is indeed worse at removing telluric systematics. We consider a range of abundances when performing the cross correlation. We obtain the strongest CO signal using a mixing ratio of 10$^{-3}$ based on previous high-resolution analysis \citep{snellen2010}. We obtain the H$_2$O signal using a mixing ratio of 10$^{-4}$ that is consistent with previous estimates using low-resolution spectra of HD~209458b \citep{madhu2014b,brogi2017,macdonald2017}. Given no previous detection of HCN, we explore a range of mixing ratios between $\log$(HCN) of -8.0 and -3.0, and subsequently use a best-fit mixing ratio of 10$^{-5}$. We also obtain a lower bound on the atmospheric abundance of HCN in the planet for obtaining an HCN signal with a S/N $\geq3$. For this purpose, we first subtract the best-fit model planetary signal from the data, which reduces the cross-correlation detection significance to zero. We subsequently inject an artificial planetary signal corresponding to model spectra for each explored abundance, and try to detect it by cross-correlating with the same model. We find that a minimum $\log$(HCN) of -6.5 in the planet is required to obtain a SNR of $\geq3$. If confirmed, such a minimum atmospheric abundance of HCN could imply a high C/O ratio ($\sim$1 or higher) in the dayside atmosphere of the planet \citep{madhu2011,madhu2012,moses2013}. | 18 | 8 | 1808.03645 |
1808 | 1808.08761_arXiv.txt | KM3NeT is a research infrastructure housing the next generation neutrino detectors in the depths of the Mediterranean Sea. The ARCA detector, which is currently under construction, is optimized for searches for neutrinos from astrophysical sources as well as measurements of the diffuse astrophysical flux. The unambiguous detection of neutrinos of extraterrestrial origin by IceCube has led to the first measurement of a high energy astrophysical neutrino flux. The properties of sea water allow for a measurement of the neutrino direction with an excellent angular resolution for both track and cascade events. Here a method to differentiate track and shower events and a method to reject the atmospheric muon background from starting track-like events are combined in one analysis. The analysis on the discovery potential of KM3NeT/ARCA for a diffuse astrophysical neutrino flux using events with the reconstructed vertex inside the detector volume will be presented. | KM3NeT~\cite{LOI} is a research infrastructure housing the next generation neutrino detectors in the depths of the Mediterranean Sea. The main science objectives of the ARCA (Astroparticle Research with Cosmics in the Abyss) telescope are the detection of neutrinos from astrophysical sources and the measurement of the diffuse astrophysical flux. ARCA is currently under construction at a depth of 3500m, approximately 80km off-shore Portopalo di Capo Passero in Sicily. When completed, ARCA will consist of two building blocks of 115 vertical detection units, each hosting 18 Digital Optical Modules (DOMs), providing an instrumented volume of about 1 km${^3}$.The DOM is a high pressure resistant glass sphere containing 31 3-inch PhotoMultipliers (PMTs) and the related electronics. \par The unambiguous detection of neutrinos of extraterrestrial origin by IceCube~\cite{Ice_HESE} has led to the first measurement of a high energy astrophysical neutrino flux. Atmospheric muons, which constitute the most prominent and high-rate background to the astrophysical neutrino signal, can be suppressed either by selecting upgoing events, or by requiring that the reconstructed vertex lies inside the instrumented volume. The properties of sea water combined with the cutting-edge technology used for the multi-PMT Digital Optical Modules in the KM3NeT detectors, allow for a measurement of the neutrino direction with an excellent angular resolution for both track and cascade events. Taking advantage of this accuracy, tools to differentiate track-like from shower-like events and to reject incoming track events to the ARCA detector were developed. | Two BDT based tools were developed to identify the HEST and the contained shower-like events. These tools were combined to perform a HESE analysis for the KM3NeT/ARCA detector. The effect of vetoing atmospheric neutrinos accompanied by muons created at the same atmospheric shower was explored using the HEST sample, where the contribution of atmospheric neutrino is more prominent. The total number of atmospheric neutrino background events can be reduced by 32\% by exploiting self vetoes. Without including the self-veto effect for atmospheric neutrinos, the ARCA detector is expected to make a 5$\sigma$ discovery of the IceCube flux with 50\% and 90\% probability in less than 0.5 and 0.8 years, respectively. \par | 18 | 8 | 1808.08761 |
1808 | 1808.03190_arXiv.txt | We introduce a design for a tip-tilt sensor with integrated \acl{SMF} coupling for use with the front-end prototype of the iLocater spectrograph at the \acl{LBT} to detect vibrations that occur within the optical train. This sensor is made up of a \acl{MLA} printed on top of a fiber bundle consisting of a central \acl{SMF} and six surrounding \aclp{MMF}. The design in based on a previous prototype that utilized a \acl{MCF} with seven \aclp{SMF} \cite{Dietrich2017}. With this updated design, we are able to achieve a better sensing throughput. We report on the modeled performance: if the beam is perfectly aligned, $69\%$ light is coupled into the central \acl{SMF} feeding the scientific instrument. When the beam is not aligned, some of the light will be coupled into the outer sensing fibers, providing the position of the beam for tip-tilt correction. For this design we show that there is a linear response in the sensing fibers when the beam is subject to tip-tilt movement. Furthermore we introduce an adaptive optics testbed, which we call the Koenigstuhl Observatory Opto-mechatronics Laboratory (KOOL), this testbed currently simulates vibrations at the \acl{LBT}, and in collaboration we have extended it to allow \acl{SMF} coupling tests. | \label{sec:introduction} \acresetall % For many years the image quality of ground based telescopes was limited by the atmosphere, known as the seeing limit. However, recent advances in modern adaptive optics (AO) systems are allowing 8-10~m class telescopes to achieve better imaging quality, leading to new and exciting discoveries. In particular \ac{ExAO} can allow diffraction limited imaging in certain circumstances. Examples of these systems include FLAO at the Large Binocular Telescope (LBT, 2x8.4~m)\cite{Esposito2011}, GPI at the Gemini South Observatory (8.2~m)\cite{Macintosh2014}, SCExAO at the Subaru Telescope (8.2~m)\cite{Jovanovic2015} and SPHERE at the Very Large Telescope (VLT, 8.2~m)\cite{Beuzit2008}. Conventional fiber-fed spectrographs use \acp{MMF} as the different modes of the telescopes \ac{PSF} need to be propagated. Yet, improved developments in \ac{AO} open up the new possibility to use spectrographs fed by \ac{SM} fibers, instead of larger \acp{MMF}. Due to the smaller entrance aperture, or slit, these spectrographs can be reduced in size, reducing stability constraints and are also free of conventional modal noise \cite{Crepp2016}. Several attempts have been made to couple \acp{SMF} to these large telescopes, but the coupling efficiency is strongly affected by the quality of initial fiber alignment, as well as beam drifts and higher-frequency tip-tilt motions due to telescope or instrument mechanics and vibrations. Conventional tip-tilt sensing solutions include imaging of the \ac{PSF} on a quad-cell detector\cite{Esposito1997}, imaging a pinhole mirror, using the telescopes \ac{AO} system, or accelerometer based disturbance feed-forward control\cite{Gluck2017}. Yet, these approaches can suffer from fundamental limitations that limit their accuracy. These include \ac{NCP} vibrations, limited dynamical range, low response speed and additional throughput losses. In this work we introduce a tip-tilt sensor consisting of a \ac{MLA} printed on top of a fiber bundle that overcomes many of these limitations. The design is optimized to perform both tip-tilt sensing using \acp{MMF} and simultaneously couple light into a \ac{SMF} to feed the spectrograph. This concept is based on a prototype device introduced by Ref.~\citenum{Dietrich2017} but uses \acp{MMF} for sensing to improve sensitivity. The \ac{MLA} will be printed on top of the fiber bundle by in-situ two-proton lithography to produce these free-form lenses and achieve high alignment precision\cite{Dietrich2018}. The design is optimized to be installed in the front-end prototype of the iLocater spectrograph at the \ac{LBT} to increase \ac{SMF} coupling efficiency. In Sec.~\ref{sec:design_considerations} we introduce the iLocater spectrograph, its optical properties and a short analysis of its tip-tilt vibration challenges. Sec.~\ref{sec:final_design} describes the preliminary design of the tip-tilt sensor taking into account the requirements of the telescope and instrument including its modeled performance and manufacturing plans. Sec.~\ref{sec:KOOL} introduces the \ac{AO} testbed, \ac{KOOL} where we are performing tests for \ac{SMF} coupling and tip-tilt sensing. This is followed by Sec.~\ref{sec:discussion}, which outlines advantages and a comparison to a first prototype described by Ref.~\citenum{Dietrich2017}, and Sec.~\ref{sec:conclusion} summarizes and highlights future work. | \label{sec:conclusion} \acresetall In this work we have introduced a preliminary design for a tip-tilt sensor with integrated \ac{SMF} coupling that is optimized to be used with the prototype front-end of the \ac{SM} spectrograph iLocater. This design can be integrated into the existing fiber coupling optics without any modifications. When sensing tip-tilt motion of the incoming beam, modeled performance yields a linear response, which simplifies signal processing correction algorithms. The device can be modified to fit system requirements and performance goals. We have discussed the advantages of this sensor when compared to conventional tip-tilt sensing options. This includes the compactness of the device, the capability to integrate it into existing optical systems easily, the sensing at the focal plane to avoid \ac{NCP} vibrations and a higher sensitivity and sampling frequency as detectors can be chosen much more freely. Due to its wide dynamical range, this design can be used for initial fiber and target alignment. Furthermore, it can be used to feed \ac{NCP} aberration optimization algorithms. We have also introduced the adaptive optics (AO) testbed KOOL which can be used to introduce and correct \ac{LBT} vibrations and higher order aberrations. We will use this testbed to test, characterize and optimize this device. We are currently in the final design stages and will be manufacturing the final device soon. This will be tested at \ac{KOOL} and then be integrated and tested at the \ac{LBT}. | 18 | 8 | 1808.03190 |
1808 | 1808.04315_arXiv.txt | We investigate the possibility of inflation with models of antisymmetric tensor field having minimal and nonminimal couplings to gravity. Although the minimal model does not support inflation, the nonminimal models, through the introduction of a nonminimal coupling to gravity, can give rise to stable de-Sitter solutions with a bound on the coupling parameters. The values of field and coupling parameters are sub-planckian. Slow roll analysis is performed and slow-roll parameters are defined which can give the required number of e-folds for sufficient inflation. Stability analysis has been performed for perturbations to antisymmetric field while keeping the metric unperturbed, and it is found that only the sub-horizon modes are free of ghost instability for de-Sitter space. | Inflation as a theory, has been successfull in describing the structure and evolution of our universe \cite{guth1981,starobinsky1980}. As ordinary matter or radiation can not source inflation, several models have been built to describe inflation where a hypothetical field may it be scalar, vector or tensor drives the inflation \cite{martin2014}. Many theories have considered the scalar field called ``inflaton" as the source for inflation and are able to describe the cosmology of universe \cite{gottlober1990,roberts1994,parsons1995,barrow1995,linde1986,linde1993}. Most of the scalar field models having simple form of potential are ruled out as they are not compatible with the Planck's observational data for the cosmic microwave background \cite{martin2014,yuan2011,gomes2018}. Another class of models considers a vector field as an alternative to the inflaton \cite{ford1989,burd1991,golovnev2008,darabi2014,bertolami2015}. But almost all of these models suffer from instabilities like ghost instability \cite{peloso2009} and gradient instability \cite{ryonamba2017} which leads to an unstable vacuum. As the quantum corrections in cosmology and their possible phenomenological implications are becoming relevant \cite{fabris2012}, models with connections to high energy theories like the string theories provide an interesting alternative to traditional inflation model building. A particular theory of interest is that of a rank-2 antisymmetric tensor field, which appears in all superstring models \cite{rohm1986,ghezelbash2009}. Antisymmetric tensors have been studied before in several aspects, including phase transitions, strong-weak coupling duality \cite{quevedo1996,olive1995,polchinski1995,siegel1980,hata1981,buchbinder1988,duff1980,bastianelli2005a,*bastianelli2005b} and even some astrophysical aspects \cite{damour1994}. More recently, quantum aspects of antisymmetric fields in different settings have been studied \cite{altschul2010,buchbinder2008,shapiro2016,aashish2018,aashish2018b}. However, efforts for cosmological studies with antisymmetric tensors were rare until the past decade. A few pertinent works with regard to inflation scenarios with $N$-form fields in anisotropic spacetime was carried out in Refs. \cite{koivisto2009a,koivisto2009b} and near a Schwarzschild metric in Ref. \cite{prokopec2006}. More recently, two-form fields have been studied in the context of anisotropic inflation \cite{asuka2015} and gravitational waves \cite{obata2018}. In this paper, we study the possibility of inflation with antisymmetric tensor field by considering minimal and nonminimal models originally considered in Altschul \etal \cite{altschul2010}. We find that the minimal model does not support inflation. However, introducing a new parameter in the form of nonminimal coupling to gravity helps to achieve inflation. The nonminimal coupling terms we incorporate here are part of a general action constructed in \cite{altschul2010} and are inspired by spontaneous Lorentz violation theories. The most general nonminimal nonderivative couplings upto quadratic order in antisymmetric tensor $B_{\mu\nu}$ (restricted to parity-even terms) are written as \cite{altschul2010} \begin{eqnarray} \mathcal{L}_{NM}=\frac{1}{2\kappa}\xi B^{\mu \nu} B_{\mu \nu} R + \frac{1}{2 \kappa} \zeta B^{\lambda \nu} B^{\mu}_{\ \nu} R_{\lambda \mu} + \frac{1}{2 \kappa}\gamma B^{\kappa\lambda}B^{\mu\nu}R_{\kappa\lambda\mu\nu} \end{eqnarray} Demanding a stable Schwarzschild solution, we do not consider the coupling with $R_{\kappa\lambda\mu\nu}$, but we will consider the remaining couplings ($\xi$ and $\zeta$ term) because our model does not contain the cosmological constant ($\Lambda$) \cite{prokopec2006}. We also set up a perfect slow roll scenario for this inflationary model, prior to developing a full perturbation theory for antisymmetric tensor in future works. However, an instability analysis for the perturbations to only the antisymmetric tensor field is performed. Although, in Ref. \cite{koivisto2009a} a similar instability analysis was done for $R$ coupling and possibility of ghosts was found, the present analysis is different in the following ways: (i) the spacetime is isotropic and homogeneous; (ii) background structure of $B_{\mu\nu}$ is specified; and (iii) choice of parameter space takes into account the conditions for slow-rolling inflation. This work is organized as follows. In Sec. \ref{sec2}, we introduce background structures of the metric and the antisymmetric tensor, and establish the general setup of our analysis through a simple model of a massive antisymmetric tensor field minimally coupled to gravity. It is shown that minimal model cannot give rise to inflation. Three cases of nonminimally coupled extensions of this model are considered in Sec. \ref{sec3}. The conditions for inflation and the de-Sitter space solutions have been derived. In Sec. \ref{sec4}, we check the stability of possible de-Sitter space. In Sec. \ref{sec5}, the slow roll parameters for the nonminimal models are constructed and the number of e-folds are calculated. Sec. \ref{sec6} presents stability analysis for perturbations to antisymmetric tensor field, while keeping the metric unperturbed. | 18 | 8 | 1808.04315 |
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1808 | 1808.04123_arXiv.txt | M87 has been identified as a displaced supermassive black hole (SMBH) candidate. We investigated this possibility by a temporal analysis of twelve Adaptive-Optics assisted-VLT and HST images spanning twenty years. We found that the centre of the isophotal fitting to the nuclear region of M87 -assumed to mark the centre of mass of the galaxy- changes location depending on the image and size of the image analysed. In an absolute frame of reference, the change varies from 15 to 130 miliarcseconds (mas) with respect to the active galactic nucleus (AGN), which remains stable within an uncertainty of $\pm$15 mas in both x and y axis. The temporal analysis of the results indicates that the major displacements measured coincide with a powerful outburst that took place between 2003 and 2007, where there was a flux increment in the nucleus and the first knot of the jet. After the outburst, the isophotal centre remains stable and is consistent with the AGN location. This suggests that the displacements are artificially caused by a flux variation in the galaxy and that the SMBH actually resides in the equilibrium position. We caution about the determination of the galaxy photocentre by isophotal fitting in cases of nuclear variability and/or presence of photometric irregularities, and advise a long-term temporal analysis of the results to confirm possible displaced SMBHs. | It is generally assumed that most massive galaxies contain a supermassive black hole (SMBH), which is expected to reside at the large-scale potential minimum of the host galaxy. However, there are several scenarios that would lead to a displacement of the SMBH from its equilibrium position \defcitealias{Batcheldor2010}{Paper~I}(\citealt{Batcheldor2010}, hereafter \citetalias{Batcheldor2010}). The possibilities include orbital motion of a SMBHs binary system, acceleration due to asymmetric radio jets, interaction with massive perturbers and gravitational recoil resulting from the coalescence of a SMBHs binary system. The latter mechanism has attracted the most interest in the past years. In this scenario, the merged SMBH can attain a kick velocity as a result of anisotropic emission of the gravitational waves \citep{Bekenstein1973,Favata2004}. Recent simulations of merging black holes predicted that recoil velocities of a few hundred to a few thousand km s$^{-1}$ can be reached \citep{Campanelli2007,Pretorius2007,Lousto2011,Blecha2016}. Such velocities would result in a variety of observational consequences, such as active galactic nuclei (AGNs) spatial and velocity offsets.\\ To date, several candidates have emerged from the search for recoiling SBHs \citep{Batcheldor2010,Civano2012,Lena2014,Kim2017,Skipper2018}. One of these candidates is the active giant elliptical M87 (NGC4486). This galaxy was studied by \citet{Batcheldor2010} and \citet{Lena2014}\defcitealias{Lena2014}{Paper~II} (hereafter, \citetalias{Lena2014}), who performed a photometric analysis of archival HST images with the aim of directly measuring a spatial offset between the AGN point source and the photocentre position. Their method assumes that the AGN marks the SMBH position and the photocentre, as defined by the galaxy isophotes, marks the minimum of the galactic potential. As a result, \citeauthor{Batcheldor2010} reported that the SMBH in M87 is relatively displaced to the galaxy photocentre position by $6.8 \pm 0.8$ pc in the counter jet direction, based on the analysis of two combined ACS images. They concluded that this displacement could be explained by jet acceleration and gravitational recoil. On the other hand, \citeauthor{Lena2014} analysed the same ACS images as \citetalias{Batcheldor2010} and four images taken with WFPC2 and NICMOS2. They found that the displacements measured in different images differ in amplitude and significance and they reported an error-weighted average displacement of $4.3 \pm 0.2$ pc roughly in the counter jet direction. Nevertheless, the origin of the discrepancies among \citetalias{Lena2014} results was unclear and no satisfactory explanation could be found. \\ Predictably, results relative to M87 have aroused great interest among the astrophysics community. This giant elliptical galaxy hosts one of the nearest and best studied AGNs. Furthermore, a relativistic jet is ejected from the galactic centre, where a SMBH of $(6.6 \pm 0.4) \cdot 10^{9} M_{\odot}$ \citep{Gebhardt2011} resides. The proximity of M87 has made its kiloparsec jet into a point of reference in the study of AGN jets. In addition, as M87 harbours one of the most massive SMBHs known, gravitational waves from the galaxy are expected to be detected with the Laser Interferometer Space Antenna (LISA). With all this, the study of the SMBH position is crucial to understand the evolution of this well known galaxy. \\ Here, we perform a photometric analysis of different HST and VLT images -some of the HST images used also by \citetalias{Batcheldor2010} and \citetalias{Lena2014}- with the aim of clarifying the presence of a photocentre-AGN displacement in M87. The method used is similar to that from \citetalias{Batcheldor2010} and \citetalias{Lena2014} and consists of measuring the relative distance between the AGN and the photocentre position. As a difference, we analyse images taken in different epochs and perform a temporal analysis of the results. Moreover, we add the size of the region analysed in the isophotal fitting as a new variable. These analysis allow us to obtain a more realistic view of the results found. We take the distance to M87 to be 16.7 Mpc, then 1\arcsec$\approx$81 pc \citep{Blakeslee2009}. | The aim of this study is to clarify the existence of a displaced SMBH in M87, which was argued to be the case in previous works in the literature. We have determined the photocentre position of several HST and VLT adaptive optics images taken at different epochs by means of an isophotal fitting of the central 9\arcsec radius of M87. The photocentre- AGN displacement was estimated assuming that the SMBH is at the AGN location, the latter being identified as a point-like source from UV to IR bands. From the analysis of the results we can draw the following points: \begin{enumerate} \item Displacements between the isophotal fitting centre and the AGN location from different images differ in amplitude and position angle. From the same set of images used in previous works \citepalias{Batcheldor2010,Lena2014} our values are consistent with those found by these authors. The maximum displacement found is $\approx$0.1\arcsec (8.1 pc). However, in some images the isophotal centre is consistent with the AGN location. The calculation of the positions in an absolute frame of reference, which is defined by the average position of five globular clusters in the central 12$\arcsec$x12$\arcsec$ of M87, indicates that the origin of the displacements when found is due to an isophotal fitting centre change and that the AGN remains stable within an uncertainty of $\pm$15 mas in both x and y directions. \item The isophotal fitting centre depends on the region analysed in each image. When the isophotal fitting is restricted to an annulus between 1 and 3\arcsec radius from the centre, the displacements are roughly aligned in the jet direction (PA$\approx$290$\degr$). If the isophotal fitting starts instead from $\approx$ 2\arcsec onwards, the displacements decrease and move away from the jet direction. The temporal analysis of the results exhibits a link between the displacements and a powerful outburst that took place from 2003 to 2007 in M87. This event led to an increase of the flux density in the nuclear source and in the first knot of the jet, HST-1, during those years. This flux increase explains the discrepancies of the results from different images and computed in separated regions, i.e. the flux increase correlates with the displacements of the isophotal centres. \item After M87 outburst, the apparent stability of the isophotal fitting centre in a position that is consistent with the AGN favours the idea that the SMBH is actually in the potential minimum of M87. However, we found two images obtained before the outburst that exhibit a significant displacement. The origin of these results is unclear. We speculate that events related to the intrinsic variability of M87 could have caused these displacements. \end{enumerate} We conclude that there is strong evidence that the presence of a displacement between the SMBH and the photocentre in M87 is due to an isophotal fitting centre change caused by M87 nuclear-jet flux variation, and not to displacements of the SMBH as interpreted in previous works \citepalias{Batcheldor2010,Lena2014}. All things considered, we determine that the SMBH location in M87 remains at the photocentre of the galaxy. We caution about the photocentre determination in cases of nuclear variability and/or the presence of additional sources close to the nucleus of a galaxy, like the knots of a jet, and advise a long-term temporal analysis of the data to confirm possible displaced SMBHs or a SBMHs binary in the centre of galaxies. Gaia space mission will find possible candidates of BH motions in centre of quasars. However, high angular resolution will still be needed together with the temporal analysis to disentangle the cause of the nuclear motion, as the presence of emitting blobs in optical jets could be detected as proper motion shifts. | 18 | 8 | 1808.04123 |
1808 | 1808.08543_arXiv.txt | We begin with a review of the predictions for cycle~24 before its onset. After summarizing the basics of the flux transport dynamo model, we discuss how this model had been used to make a successful prediction of cycle~24, on the assumption that the irregularities of the solar cycle arise due to the fluctuations in the Babcock--Leighton mechanism. We point out that fluctuations in the meridional circulation can be another cause of irregularities in the cycle. | Let us begin with a disclaimer. This review will focus on the physics of predicting solar cycles from dynamo models and will refrain from presenting any detailed prediction for the upcoming cycle~25, which is nowadays becoming a hot topic of research. \begin{figure} \begin{center} \includegraphics[width=5.1in]{fig1.eps} \caption{Different predictions of the strength of cycle~24, adopted from Pesnell (2008). The two predictions based on theoretical dynamo models (Dikpati et al.\ 2006; Choudhuri et al.\ 2007) are indicated by arrows. The horizontal line added by us indicates the actual peak strength of cycle~24 reached around April 2014.} \label{fig1} \end{center} \end{figure} Now that we know what the present cycle~24 has been like, let us take a look at the many predictions of cycle~24 before its onset. Pesnell (2008) produced a plot combining all the different predictions of the peak sunspot number of cycle~24. Figure~1 is adopted from this plot, indicating the two predictions based on theoretical dynamo models. The first theoretical prediction by Dikpati et al.\ (2006) was that cycle~24 would be a very strong cycle, whereas the other prediction by Choudhuri et al.\ (2007) was that it would be a fairly weak cycle. All the other predictions shown in Figure~1 were based on various precursors and empirical projections. We can see that the predictions covered almost the entire range of possible values of the peak sunspot number from $\approx$40 to $\approx$190. The horizontal line indicates the actual peak sunspot number of cycle~24 and was added by us while preparing this presentation. It is clear that Choudhuri et al.\ (2007) predicted the cycle~24 peak almost correctly. If several people make several predictions covering the entire possible range, then somebody's prediction has got to come out right! Were Choudhuri et al.\ (2007) simply the lucky persons whose prediction accidentally turned out to be correct? Or did they get it correct because they figured out the correct physics for making such predictions? We would like to argue that they figured out the correct physics partially, but not fully. Their success in predicting cycle~24 was due to a combination of intuition and luck. In a classic work, Parker (1955) envisaged that the solar cycle is produced by an oscillation between the toroidal and poloidal magnetic fields of the Sun. Sunspots form out of the toroidal field due to magnetic buoyancy and provide an indication of the strength of the toroidal field. On the other hand, the magnetic fields in the polar regions of the Sun are a manifestation of the poloidal field. We now know that there is truly an oscillation between these two field components. The polar (i.e.\ poloidal) field becomes strongest around the time when the sunspot number (i.e.\ toroidal field) has its lowest value and vice versa. Svalgaard et al.\ (2005) and Schatten (2005), whose predictions for cycle~24 are included in Figure~1, suggested that the polar field at the beginning of a cycle is a good precursor for the strength of the cycle and used the weak polar field at the beginning of cycle~24 to predict essentially the same low value of the cycle peak that was predicted from the theoretical dynamo calculations of Choudhuri et al.\ (2007). While discussing the physics of cycle prediction, we need to address the question of why the polar field at the beginning of a cycle acts as such a good precursor of the cycle strength. | Within the last few years, the flux transport dynamo model has emerged as an attractive model for explaining the solar cycle and there is increasing evidence that other solar-like stars also may have similar dynamos working inside them (Karak et al.\ 2014b; Choudhuri 2017). It is important that we understand how the irregularities in the cycle arise, since such an understanding may enable us to predict a future cycle before its onset. It appears that fluctuations in the BL mechanism and fluctuations in MC are the two main sources of irregularities in the solar cycle. Before the beginning of cycle~24, the role of MC fluctuations was not generally appreciated. The successful theoretical prediction of Choudhuri et al.\ (2007) was based on the assumption that irregularities in the solar cycle are caused only by fluctuations in the BL mechanism. With the realization that MC fluctuations also can introduce additional irregularities, it is necessary to develop prediction methods taking this into account. {\it Acknowledgments.} I thank Gopal Hazra for help in preparing the manuscript. My research is supported by DST through a J.C.\ Bose Fellowship. \def\apj{{\it ApJ}} \def\mnras{{\it MNRAS}} \def\sol{{\it Solar Phys.}} \def\aa{{\it A\&A}} \def\gafd{{\it Geophys.\ Astrophys.\ Fluid Dyn.}} | 18 | 8 | 1808.08543 |
1808 | 1808.03467_arXiv.txt | {At the end of their lives AGB stars are prolific producers of dust and gas. The details of this mass-loss process are still not understood very well. {\it Herschel} PACS and SPIRE spectra which cover the wavelength range from $\sim$~55 to 670 $\mu$m almost continuously, offer a unique way of investigating properties of AGB stars in general and the mass-loss process in particular as this is the wavelength region where dust emission is prominent and molecules have many emission lines. } {We present the community with a catalogue of AGB stars and red supergiants (RSGs) with PACS and/or SPIRE spectra reduced according to the current state of the art. } {The {\it Herschel Interactive Processing Environment} (HIPE) software with the latest calibration is used to process the available PACS and SPIRE spectra of 40 evolved stars. The SPIRE spectra of some objects close to the Galactic plane require special treatment because of the weaker fluxes in combination with the strong and complex background emission at those wavelengths. The spectra are convolved with the response curves of the PACS and SPIRE bolometers and compared to the fluxes measured in imaging data of these sources. Custom software is used to identify lines in the spectra, and to determine the central wavelengths and line intensities. Standard molecular line databases are used to associate the observed lines. Because of the limited spectral resolution of the PACS and SPIRE spectrometers ($\sim$~1500), several known lines are typically potential counterparts to any observed line. To help identifications in follow-up studies the relative contributions in line intensity of the potential counterpart lines are listed for three characteristic temperatures based on local thermodynamic equilibrium (LTE) calculations and assuming optically thin emission. } {The following data products are released: the reduced spectra, the lines that are measured in the spectra with wavelength, intensity, potential identifications, and the continuum spectra, i.e. the full spectra with all identified lines removed. As simple examples of how this data can be used in future studies we have fitted the continuum spectra with three power laws (two wavelength regimes covering PACS, and one covering SPIRE) and find that the few OH/IR stars seem to have significantly steeper slopes than the other oxygen- and carbon-rich objects in the sample, possibly related to a recent increase in mass-loss rate. As another example we constructed rotational diagrams for CO (and HCN for the carbon stars) and fitted a two-component model to derive rotational temperatures. } {} | \label{intro} The initial mass of a star determines its evolution and therefore also the final stages of its life. After leaving the main sequence, stars with an initial mass between $\mathrm{\sim 0.8~M_{\odot}}$ and $\mathrm{\sim 8~M_{\odot}}$ will climb the red giant and asymptotic giant branches (RGB and AGB), while more massive stars will go through a supergiant phase. During the AGB and supergiant phases, mass loss dominates the evolution and a star will expel a significant part of its initial mass via a stellar wind. The ejection of stellar material creates a cool and extended circumstellar envelope (CSE) containing dust grains and molecular gas-phase species. In this way, AGB and supergiant stars contribute significantly to the return of gas and dust to the interstellar medium (ISM) from which new generations of stars are born. From a qualitative point of view it is known that the mass-loss processes are closely related to the intrinsic characteristics of the star, like mass, luminosity, variability and chemical composition \citep{Habing_and_Olofsson_2003, HO2018}. Despite extensive research efforts, stellar evolution models are not yet able to quantitatively predict the mass-loss history of AGB or supergiant stars from first principles. The details of the physical processes that govern the mass-loss dynamics and its variation in time remain unclear. A fulfilling description of the different key chemical processes that determine the wind's chemical composition is also lacking. Observationally characterising the full dynamical and chemical structure of the CSE from the stellar atmosphere up to the most outer parts of the wind will be helpful in clarifying the underlying mass-loss mechanism by providing models with as many constraints as possible. The Herschel Space observatory (hereafter \textit{Herschel}; \citealt{Pilbratt_etal_2010}) plays a key role in these analyses. \textit{Herschel} collected data at far-infrared and submillimetre wavelengths which cover a large part of the wavelength region where the gas and dust in the extended CSE emit most of their continuum and line radiation. In this way, \textit{Herschel} bridges the gap between ground-based instruments which are only able to obtain data in selected atmospheric windows at shorter (near- and mid-infrared) and those that can obtain data at longer (millimetre and radio) wavelengths. Due to the good spatial and spectral resolution of the instruments on board, \textit{Herschel} revealed new insights in the structure and chemistry of CSEs. This paper presents consistent and carefully reduced data of the PACS and SPIRE instruments on board \textit{Herschel} of all AGB and supergiants stars that were observed by the PACS and SPIRE spectrometers. A large fraction of the data presented here were obtained within the framework of the Mass loss of Evolved StarS (MESS) Guaranteed Time Key Programme \citep{Groenewegen_etal_2011} and were published in part in earlier publications, using the best available data reduction at that time. \citet{Royer_etal_2010} presented PACS and SPIRE data on VY CMa and an initial model for the CSE, while a more elaborate analysis was presented by \cite{Matsuura2014}, using radiative transfer models to fit the $^{12}$CO, $^{13}$CO, SiO and water lines in these spectra and to derive mass-loss rates (MLRs) and the gas temperature profile in the CSE. \citet{Decin_etal_2010_Nature} presented the detection of high-excitation lines of H$_{\rm 2}$O in the carbon star CW Leo (IRC +10 216) and suggested that interstellar UV photons could penetrate deep into the clumpy CSE, an alternative scenario to the one proposed by \citet{Melnick01} of the vaporisation of a collection of orbiting icy bodies based on the detection of a single line with {\it SWAS}. This analysis was later extended by \citet{Lombaert_etal_2016} who studied the water lines in 18 C stars (including six targets from the MESS program). \citet{Danilovich2017} studied the water isotopologues in four M-type stars (R Dor, IK Tau, R Cas, and W Hya) including data from MESS. Other studies analysed molecular line data (based partly on MESS data) for CW Leo \citep{DeBeck2012}, OH~127.8\,+0.0 \citep{Lombaert_etal_2013}, W Hya \citep{Khouri2014a, Khouri2014b}, W Aql \citep{Danilovich2017}, and R Dor \citep{VandeSande2018}, typically using radiative transfer models to derive properties of the CSE, such as abundances or abundance profiles. The present paper also discusses PACS and SPIRE imaging data, but only for the targets which have spectroscopic data. Initial PACS and SPIRE photometry was presented in \citet{Groenewegen_etal_2011}, but not all observations had been completed at that time. An overview of the PACS imaging of all 78 MESS targets can be found in \cite{Cox_etal_2012}, showing and discussing, amongst other things, four different classes of wind-ISM interaction observed in $\sim 40\%$ of the sample. MESS imaging data have been discussed in more detail for individual objects as well. \citet{Ladjal2010} discussed the bow shock around CW Leo seen in SPIRE data (discovered a few months earlier in GALEX UV data by \citealt{Sahai2010}) while \citet{Decin2011} presented the discovery of multiple shells around this object. \cite{Decin2012} discussed the detection of the bow shock around Betelgeuse, while the interesting class of C-rich objects with detached shells have been discussed in \citet{Kerschbaum2010} (AQ And, U Ant, and TT Cyg) and \citet{Mecina2014} (S Sct and RT Cap). The CSE of stars showing binary interaction have been discussed by \citet{Mayer2013} (R Aqr and W Aql) and \citet{Mayer2014} ($\pi$ Gru). The paper is structured as follows. Section~\ref{S-Data} presents the data sample and describes the adopted data reduction and processing strategy. In Section~\ref{SectQual} the flux level of the PACS and SPIRE spectra is compared to that measured independently by the PACS and SPIRE bolometer arrays in order to have an estimate of the flux level consistency and to identify possible problematic stars or wavelength regions. n Section~\ref{LineMeasurement} the strategy to extract the molecular lines is outlined, while Sect.~\ref{S_Cont} describes the determination of the dust continua. Section~\ref{S_DA} discusses the identification of the molecular lines, the construction of rotational diagrams, and the derivation of rotational temperatures for CO (and HCN for the carbon stars), and the slopes of the dust continua. Section~\ref{sect:discussion} summarises this paper. When this paper was submitted we became aware of the article by \citet{THROES} that presents PACS range spectroscopy of 114 evolved stars. The sample they consider also includes planetary nebula and post-AGB stars and is therefor larger than ours. For the reader it is important to know that our effort and theirs were were carried out independently of each other. | \label{sect:discussion} \subsection{Continuum slopes} Figure~\ref{Fig:HistSlopes} shows histograms of the continuum slopes based on the data in Table~\ref{Table:powerlawresults}. The seven stars with the highest MLRs (in excess of $10^{-4}$~\msolyr) are colour-coded explicitly as OH/IR stars (OH\,21.5, OH\,26.5, OH\,30.1, OH\,32.8, and AFGL~5379, IRAS~18448, IRAS~19067, which all three are also known OH maser sources). The source OH~127.8 is not included in this subsample as its MLR is high but nevertheless a factor of two lower than the lowest MLR of the other seven stars. In the 55--100~\mic\ region the continuum slopes for M-, S-, and C-stars; OH/IR stars; and RSGs are very similar, roughly between 1.8 and 2.8. What is interesting is that the spread in the slopes is larger in the 100--190~\mic\ region, and that some of the OH/IR stars stand out as having very steep slopes ($>$3.5), while for the other classes the slope is similar to that at shorter wavelengths. In the SPIRE regime there are fewer stars, and sometimes the slope is only based on the SSW part, but the trend is the same. It is beyond the scope of the current paper to perform the detailed radiative transfer modelling that would be required to discuss this observation in more detail, but qualitatively a steeper slope at longer wavelengths could point to a lower MLR in the past, or, to reverse the timeline, to a recent start of the so-called superwind phase that would indicate the beginning of the end of AGB evolution. This scenario was invoked a long time ago \citep{Heske1990} based on the relative weakness of the lower-J CO lines and has been discussed in the literature since (e.g. \citealt{Justtanont1996,Justtanont2006,Decin2006,Decin2007,Justtanont2013,deVries2014}). \begin{figure*} \centering \includegraphics[width=0.7\hsize]{slopes_3panels.ps} \caption{Histogram of the continuum slopes for the three wavelength regions. } \label{Fig:HistSlopes} \end{figure*} \subsection{Solid-state features} The {\it Infrared Space Observatory} (ISO) has revolutionised our knowledge of dust spectroscopy (see e.g. the review by \citealt{Henning2010}). Based on that success one of the aims of the MESS {\it Herschel} Key Programme \citep{Groenewegen_KP} was to look for (new) dust features. \citet{Posch2005} discuss solid-state features that are potentially observable in the PACS range, and that include some ices and silicate dust species. Figures~\ref{Fig:DustOH}--\ref{Fig:DustS} show the continuum spectra divided by a power law for the M-, C- and S-type targets. This is useful in order to assess the potential presence of solid-state features Previously, \citet{Sylvester1999} discussed the spectra of OH/IR stars based on ISO-SWS (2.4--45~\mic) and LWS (45--197~\mic). In the wavelength region covered by PACS they claim the detection of water ice in emission at 62~\mic\ in OH~127.8, OH~26.5, and AFGL 5379 (all in our sample), and detect forsterite silicate at 69~\mic\ in OH~127.8, OH~26.5, OH~32.8, and AFGL 5379 (all in our sample). Unfortunately, the 61--63~\mic\ region is a difficult one for PACS as the relative spectral response function (RSRF) is not easy to calibrate due to the presence of a spectral feature in one of the filters in the light path, resulting in a spectral feature that depends on the spatial structure of the source and on the pointing error of the corresponding observation. The variation due to the RSRF can be judged from the spectra of the C-stars where no water ice is expected to be present. The typical shape of the spectra of the M-stars in that region is not markedly different. Unfortunately, the problematic PACS data in that wavelength region do not allow us to confirm or deny the claim by \citet{Sylvester1999} regarding the presence of water ice in OH~127.8, OH~26.5, and AFGL 5379. The location and shape of the forsterite feature near 69~\mic\ in PACS spectra of evolved stars has been extensively discussed in \citet{Blommaert2014} and \citet{deVries2014}. No new sources showing forsterite have been found, nor any new possible solid-state features, at least down to a level of $\sim$~5\% of the continuum. \begin{figure*} \centering \includegraphics[width=0.9\hsize]{M1typeDustFeatures_SMALL.ps} \caption{Continuum divided spectra of M-type targets. The problematic region for the RSRF where the water ice feature is located, and the location of the forsterite feature are indicated.} \label{Fig:DustOH} \end{figure*} \setcounter{figure}{6} \begin{figure*} \centering \includegraphics[width=0.9\hsize]{M2typeDustFeatures_SMALL.ps} \caption{Continued.} \end{figure*} \begin{figure*} \centering \includegraphics[width=0.9\hsize]{CtypeDustFeatures.ps} \caption{As Fig.~\ref{Fig:DustOH} for the C-type targets.} \label{Fig:DustC} \end{figure*} \begin{figure*} \centering \includegraphics[width=0.9\hsize]{StypeDustFeatures.ps} \caption{As Fig.~\ref{Fig:DustOH} for the S-type targets.} \label{Fig:DustS} \end{figure*} \subsection{Rotational temperatures} \label{SS-RT} A two-component model (or a single temperature when not enough data were available) to trace the excitation temperature is obviously a very crude approximation to the true gas temperature profile in a CSE, which is determined by several heating and cooling mechanisms whose relative contributions vary throughout the CSE \citep{Goldreich1976}. It is reassuring to see that for the C-stars where we modelled both CO and HCN the temperatures (the single value, or both hot and cool components) agree within the error bar. The temperatures are relatively uniform between 80 and 120~K for the cool, and between 450 and 650~K for the hot component. This justifies a posteriori a maximum temperature of 500~K in the LTE calculations of Sect.~\ref{Sect:Identification}. The cool component is still relatively warm because the lowest CO transitions (J= 1-0 to 3-2), which that trace the outermost parts of the CSE, are outside the SPIRE wavelength range. When no SPIRE spectrum is available, the lowest detectable transition is the J= 14-13 transition. We have compared the rotational temperatures to the gas temperature profiles available in the literature, typically based on detailed studies of individual objects that solved the thermal balance equation in a self-consistent way (\citet{Maercker_2016_2} for R Dor, R Cas, TX Cam, and IK Tau; \citet{Danilovich2014} for W Aql; \citet{Schoier_etal_2011} for $\chi$ Cyg; \citet{Maercker2008} for W Hya, and IK Tau; \citet{Decin2006} for VY CMa; \citet{Schoier2001} for LP And and R For; \citet{Ryde1999} for II Pup; \citet{Groenewegen1998} for CW Leo; \citet{Justtanont1996} for OH 26.5; and \citet{Groenewegen1994} for OH 32.8). Such a comparison is qualitative only as it assumes that the gas temperature equals the rotational temperature of the CO molecule, which is typically not the case, as there is heating and cooling by other molecules and the excitation temperature of the different CO transitions are typically not equal. Nevertheless, we find that the hot component traces the gas temperature at $8-10$ to $20-30$ stellar radii, while the cool component traces the gas temperature in the region $50$ -- $150$ stellar radii. The two exceptions are OH~26.5 and OH~32.8. For OH~26.5 the hot component is typical for the temperature at $\sim 40$ stellar radii and the cool temperature of 90~K is actually never reached. Due to the superwind nature of the MLR profile adopted in \citet{Justtanont1996} (i.e. a lower MLR in the outer part of the wind which leads to higher temperatures, due to the lower density and smaller photo dissociation radius of water, the major coolant) the temperature never drops below $\sim$200~K. This model has been refined by \citet{Justtanont2013} but the corresponding temperature profile has not been published. For OH~32.8 the low temperature of 15~K is only reached at several hundred stellar radii. PACS and SPIRE spectra are presented for a sample of 40 AGB stars and red supergiants, reduced according to the current state of the art reduction strategies. Molecular lines have been identified and line fluxes measured. The full spectra, and the continuum spectra with all identified lines removed are made available to the community. In addition, we derive rotational diagrams and rotation temperatures for CO. In future works this will be extended to line radiative transfer modelling for all detected lines (plus literature data). Finally, we measure the slope of the dust continua. In future works this will be extended to dust radiative transfer modelling of the spectra (and other photometric data) over the entire wavelength region. | 18 | 8 | 1808.03467 |
1808 | 1808.03184_arXiv.txt | GIARPS (GIAno \& haRPS) is a project devoted to have on the same focal station of the Telescopio Nazionale Galileo (TNG) both high resolution spectrographs, HARPS--N (VIS) and GIANO--B (NIR), working simultaneously. This could be considered the first and unique worldwide instrument providing cross-dispersed echelle spectroscopy at a resolution of 50,000 in the NIR range and 115,000 in the VIS and over in a wide spectral range ($0.383 - 2.45\ \mu$m) in a single exposure. The science case is very broad, given the versatility of such an instrument and its large wavelength range. A number of outstanding science cases encompassing mainly extra-solar planet science starting from rocky planets search and hot Jupiters to atmosphere characterization can be considered. Furthermore both instruments can measure high precision radial velocities by means the simultaneous thorium technique (HARPS--N) and absorbing cell technique (GIANO--B) in a single exposure. Other science cases are also possible. GIARPS, as a brand new observing mode of the TNG started after the moving of GIANO--A (fiber fed spectrograph) from Nasmyth--A to Nasmyth--B where it was re--born as GIANO--B (no more fiber feed spectrograph). The official Commissioning finished on March 2017 and then it was offered to the community. Despite the work is not finished yet. In this paper we describe the preliminary scientific results obtained with GIANO--B and GIARPS observing mode with data taken during commissioning and first open time observations. | \label{sec:intro} The discovery of a hot Jupiter orbiting 51Peg\cite{mayorandqueloz1995} triggered the quest for extrasolar planets using the radial velocity technique, by which the presence of a planetary companion is inferred by the wobble it induces on the parent star. In the present, the search for more exotic stellar hosts and the race for the lightest planets point towards M dwarfs. These stars, which are the most abundant in the Universe, are also the smallest ones. Since the RV variation induced by a planet on a star scales with M$_{\star}^{-2/3}$, the amplitude of the effect induced on a M star is significantly larger. As an example, a planet of an identical mass at the same distance from the stellar host produces a RV variation with an amplitude $\sim 3$ times larger on an M5 star than on a G2 star. The drawback is that since they are much colder, M dwarfs are much fainter in optical wavelengths. The RV surveys of low-mass stars points then towards the exploration of a new wavelength domain, the IR, where the luminosity of these objects peaks. This type of stars can show surface inhomogeneities like stellar spots, being young and active, which can mimic or hide Doppler signal due to a planet. Observing in the NIR, as opposed to VIS, the contrast between these surface inhomogeneities and the stellar disk is strongly reduced\cite{reinersetal2010}, helping to discriminate between colored signal (activities, pulsations etc.) and planetary signal. This highlights another advantage of measuring radial velocities in NIR. In this framework GIARPS (GIAno \& haRPS--N) \cite{claudietal2016}, the new common feeding for both the high resolution spectrographs, HARPS-N\cite{cosentino2014} in the visible and GIANO\cite{oliva2006} in the NIR, represents a good chance to investigate this class of objects in the next future. GIARPS allows to have the two instruments on the same focal station of the Telescopio Nazionale Galileo (TNG) working simultaneously. The science case is very broad, given the versatility of such an instrument and the large wavelength range, encompassing mainly extra-solar planet science starting from rocky planet search and hot Jupiters atmosphere characterization can be considered. But not only, also young stars and proto--planetary disks, cool stars and stellar populations, moving minor bodies in the solar system, bursting young stellar objects, cataclysmic variables and X-ray binary transients in our Galaxy, supernovae up to gamma--ray bursts in the very distant and young Universe, can take advantage of the unicity of this facility both in terms of contemporaneous wide wavelength range and high resolution spectroscopy. | Since Fall 2017, GIARPS works routinely at the TNG that has a high-resolution spectroscopy station. Thank to its wide wavelength range (up to 2.5 $\mu$m) it is unique in the northern hemisphere and up to the commissioning of NIRPS (the NIR counterpart of HARPS) at the 3.6m ESO Telescope, the unique in this world. The flexibility of the three observing modes: HARPS-N alone, GIANO--B alone and GIARPS itself will allow users to select the best wavelength range useful for their preferred science case. From small bodies of the Solar System to the search for extrasolar planets will be the major science cases. Moreover it allows to reach a RV precision of 8m/s in the short-term, and 14m/s in the long-term in the NIR range. Furthermore, we presented the great contribution of the simultaneous observations to retract or confirm exoplantes orbiting active stars. | 18 | 8 | 1808.03184 |
1808 | 1808.01654_arXiv.txt | {The astro-photometric parameters of the open star cluster Dolidze 41, which located in the constellation of Cygnus, have been investigated using the {\em Gaia-ESO} DR2 large Survey merging with the near Infrared Two Micron All Sky Survey {\em 2MASS} database. The radial density distribution (limited, core and tidal radii), color-magnitude diagrams, the galactocentric coordinates, distances, color excess, and age of Dolidze 41 are presented. Thanks to {\em Gaia} DR2 astrometry, which help us to define the membership of the cluster stars easily. The luminosity \& mass functions, the entire luminosity \& mass, and the repose time of the cluster have been estimated as well. | One of the foremost necessary constituents in studying the structure and evolution of the Milky Way system are star clusters. Known stellar clusters comprise about 160 globular clusters and about 3000 open clusters \cite{2013yCat..35580053K}. If we extrapolate the solar vicinity to the whole disc, we may reach about 100,000 open clusters \cite{2018IAUS..330..119B}. Many of such objects are newly discovered and need photometric investigations, as well as confirmation of their physical nature. We believe that most of them will be detected and investigated in the {\em Gaia} DR2 era. The name {\em GAIA} was originally derived as an acronym for Global Astrometric Interferometer for Astrophysics. This reflected the optical technique of interferometry that was originally planned for use on the spacecraft. While the working method evolved during studies and the acronym is no longer applicable; however, the name {\em GAIA} remained to provide continuity with the project. It is the backbone in the science program of the European Space Operations (ESO), which launched on 19 December 2013, and situated 1.5 million km from Earth. The spacecraft {\em GAIA} monitored each of its target objects about 70 times to a magnitude of G = 20 over a period of five years to study the precise position and motion of each one. The {\em Gaia} DR2 was released on 25 April 2018 for 1.7 billion exquisite precision sources in astrometric five-parameter solutions of coordinates, proper motions in right ascension and declination, and parallaxes ($\alpha$, $\delta$, $\mu_\alpha \cos{\delta}$, $\mu_\delta$, $\pi$). In addition, magnitudes in three photometric filters ($G, G_{BP}, G_{RP}$) for more than 1.3 billion sources \cite{2018arXiv180409365G}. {\em Gaia} Archive is available through the web page {\em (http://www.cosmos.esa.int/gaia)}. \\ \\ With the help of the Virtual Observatory tool {\em TOPCAT} and {\em ALADIN} we could use the cross-matched data of {\em Gaia} DR2 and the {\em 2MASS} surveys to compile a useful photometrical data to investigate Dolidze 41 \cite{2000A&AS..143...33B, 2005ASPC..347...29T}. The current work could be a part of our continued series whose goal is to get the most astrophysical properties of newly, antecedently unstudied and/or poorly studied open clusters utilizing the foremost newly databases \cite{2008NewA...13..370T, 2008MNRAS.389..285T, 2009NewA...14..200T, 2009Ap&SS.323..383T, 2011JKAS...44....1T}. The most important aspect of using {\em Gaia} DR2 with the {\em 2MASS} surveys lies in the positions, parallax and proper motions for the cluster' stars, which makes the member candidates can be easily determined. \\ \\ J2000.0 coordinates of Dolidze 41 are $\alpha=20^{h}\ 18^{m}\ 49^{s},\ \delta=+37^{\circ}\ 45^{'}\ 00^{''},\ \ell= 75.707^{\circ} ~\& \ b= 0.9925^{\circ}$ in the Cygnus constellation with a roughly angular size of 12 arcmin as shown in Fig.~\ref{Fig-1}. Kazlauskas et al. \cite{2013NewA...19...34K} and Dias et al. \cite{2014A&A...564A..79D} mentioned some few astrometric information about the cluster. No real photometric data has been published for Dolidze 41 to date except the study of Tadross \& Nasser \cite{2010arXiv1011.2934T}, hereafter TN (2010). In the present work, we re-study Dolidze 41 in the light of {\em Gaia} DR2 database. The near-infrared {\em JHK} photometry of the 2MASS catalog of \cite{2006AJ....131.1163S} is also used to estimate and confirm the most main properties of the cluster. \\ This paper is arranged as follows: The target data is presented in Section 2. The radial density profile is described in Section 3. Color-Magnitude Diagrams are presented in Section 4. Luminosity - mass functions and the dynamical status of the cluster are discussed in Section 5. Finally, the conclusion of the present study is given in Section 6. | The open cluster Dolidze 41 is poorly studied objet, the only photometric study, which found in the literature was carried out by TN \cite{2010arXiv1011.2934T} using 2MASS database. According our analysis for refining the fundamental parameters of Dolidze 41 in the {\em GAIA} era, we presented a real astro-photometric study here, which is somewhat different from the previous TN \cite{2010arXiv1011.2934T} one. The present and previous results are summarized and compared in table 1. \begin{table} \caption{Comparisons between the previous and present results.} \begin{tabular}{lll} \hline\noalign{\smallskip}Parameter& TN (2010)& Present work \\\hline\noalign{\smallskip} pm $\alpha$ cos $\delta$ &--~-- & -2.95 $\pm$ 0.14 mas/yr.\\ pm $\delta$ &--~-- & -4.68 $\pm$ 0.14 mas/yr.\\ Parallax & --~--& 0.23 $\pm$ 0.06 mas\\ Age& 400 Myr.& 200 Myr.\\ Metal abundance& 0.019&0.0152\\ $E(B-V)$& 0.53 mag.& 0.54 mag.\\ $R_{v}$& 3.25& 3.1\\ Intrinsic Modulus& 12.20 mag.& 13.30 $\pm$ 0.10 mag.\\ Distance& 1763 pc.& 4625 $\pm$ 210 pc.\\ Limited radius& 5.0$^{'}$ & 6.6$^{'}$ (8.8 pc.)\\ Core radius& --~--& 1.8$^{'}$ (2.4 pc)\\ Tidal radius& --~--& 12.6 pc.\\ Membership& --~--& 480 stars\\ $R_g$& 8.2 kpc.& 8.5 kpc.\\ X$_{\odot}$& --435 pc.& 1140 pc.\\ Y$_{\odot}$& 1708 pc.& 4480 pc.\\ Z$_{\odot}$& 31 pc.& 80 pc.\\ Total luminosity& --~--& --4.3 mag. \\ {\it IMF} slope& --~--&$\Gamma = -2.3 \pm 0.24$\\ Total mass&--~--& $\approx$ 640 $\mathcal{M}_{\odot}$ {\it (minimum)}\\ Repose time& --~--& $<$ 40 Myr.\\ $c$ & --~--& $\approx$ 0.6 \\ $\tau$ & --~--& $\approx$ 6.0 \\ \hline \end{tabular} \end{table} | 18 | 8 | 1808.01654 |
1808 | 1808.00652_arXiv.txt | {Studying molecular species in protoplanetary disks is very useful to characterize the properties of these objects, which are the site of planet formation.} {We attempt to constrain the chemistry of S-bearing molecules in the cold parts of circumstellar disk of GG Tau A.} {We searched for H$_2$S, CS, SO, and SO$_2$ in the dense disk around GG Tau A with the NOrthem Extended Millimeter Array (NOEMA) interferometer. We analyzed our data using the radiative transfer code DiskFit and the three-phase chemical model Nautilus.} {We detected H$_2$S emission from the dense and cold ring orbiting around GG Tau A. This is the first detection of H$_2$S in a protoplanetary disk. We also detected HCO$^+$, H$^{13}$CO$^+$, and DCO$^+$ in the disk. Upper limits for other molecules, CCS, SO$_2$, SO, HC$_3$N, and $c$-C$_3$H$_2$ are also obtained. The observed DCO$^+$/HCO$^+$ ratio is similar to those in other disks. The observed column densities, derived using our radiative transfer code DiskFit, are then compared with those from our chemical code Nautilus. The column densities are in reasonable agreement for DCO$^{+}$, CS, CCS, and SO$_2$. For H$_2$S and SO, our predicted vertical integrated column densities are more than a factor of 10 higher than the measured values.} {Our results reinforce the hypothesis that only a strong sulfur depletion may explain the low observed H$_2$S column density in the disk. The H$_2$S detection in GG Tau A is most likely linked to the much larger mass of this disk compared to that in other T Tauri systems.} | Understanding the physical and chemical structure of protoplanetary disks is needed to determine the initial conditions of planet formation. Studies of protoplanetary disks have led to a global picture in which disks are flared and layered with important vertical, radial density, and temperature gradients. The uppermost layer is directly illuminated by stellar UV and dominated by photodissociation reactions, while molecules stick to dust grains in the very cold midplane. In between there is a rich molecular layer \citep{Kenyon+Hartmann_1987, vanZadelhoff+vanDishoeck+Thi+etal_2001}. Studies of the gas content rely on trace molecules because H$_2$ is not detectable at the temperatures of disks. So far, the molecules that have been detected in T Tauri disks are CO, $^{13}$CO, C$^{18}$O, C$^{17}$O, CN, CS, H$_2$CO, CCH, DCN, HCO$^+$, H$^{13}$CO$^+$, DCO$^+$, N$_2$H$^+$, HC$_3$N, CH$_3$CN, HD, C$_3$H$_2$, C$_2$H$_2$, OH, SO, CH$^+$, N$_2$D$^+$, NH$_3$, CH$_3$OH, H$^{13}$CN, HC$^{15}$N, C$^{15}$N, and HCOOH \citep{Dutrey+Guilloteau+Guelin_1997, Thi+vanDishoeck+Blake+etal_2001, Qi+Wilner+Aikawa_2008, Dutrey+Wakelam+Boehler+etal_2011, Chapillon+Dutrey+Guilloteau+etal_2012, Bergin+Cleeves+Gorti_2013, Qi+Oberg+Wilner+etal_2013, Huang+Oberg_2015, Oberg+Furuya+Loomis+etal_2015, Walsh+Loomis+Oberg_2016, Guilloteau+Reboussin+Dutrey+etal_2016, Salinas+Hogerheijde+Bergin+etal_2016, Guzman+Oberg+Loomis_2015, Hily-Blant+Magalhaes+Kastner_2017, Favre+Fedele+Semenov_2018}. More than a dozen S-bearing species have been observed in dense cloud cores; they are chemically active and often used as chemical clocks in low-mass star forming regions \citep{Buckle+Fuller_2003, Wakelam+Caselli+Ceccarelli_2004, Wakelam+Castets+Ceccarelli_2004}. Some S-bearing species, CS, SO, SO$_2$, and H$_2$S, are observed in Class 0 and Class I sources \citep{Dutrey+Wakelam+Boehler+etal_2011, Guilloteau+DiFolco+Dutrey+etal_2013, Guilloteau+Reboussin+Dutrey+etal_2016} while CS, the second main reservoir of sulfur in the gas phase \citep{Vidal+Loison+Jaziri+etal_2017} is the only S-bearing molecule detected in disks around T Tauri stars. We report the first detection of H$_2$S in a disk around a T Tauri star, GG Tau A. GG Tau, located at 150 pc in Taurus-Auriga star forming region \citep{GAIA_2016,GAIA_2018}, is a hierarchical quintuple system with the GG Tau A triple star \citep[separation $\sim5$ and 38\,au;][]{DiFolco+Dutrey+LeBouquin_2014} surrounded by a dense ring located between 180 and 260\,au and a large disk extending out to 800\,au \citep[see][and references therein]{Dutrey+DiFolco+Beck_2016}. The disk is massive (0.15 M$_{\odot}$) and cold; it has a dust temperature of 14\,K at 200\,au, a kinetic temperature derived from CO analysis of $\sim$20\,K at the same radius \citep{Dutrey+DiFolco+Guilloteau_2014, Guilloteau+Dutrey+Simon_1999}, and little or no vertical temperature gradient in the molecular layer \citep{Tang+Dutrey+Guilloteau+etal_2016}. The large size, low temperature, and large mass make GG Tau A disk an ideal laboratory to search for cold molecular chemistry. Besides the H$_2$S detection, we also report detections of HCO$^+$, DCO$^+$, and H$^{13}$CO$^+$ and discuss the upper limits of CCS, SO$_2$, SO, c-C$_3$H$_2$, and HC$_3$N. | \subsection{Comparison with other sources} The measured H$_2$S column density is a factor of three greater than the upper limits quoted by \citet{Dutrey+Wakelam+Boehler+etal_2011} for DM\,Tau, LkCa\,15, MWC\,480, and GO\,Tau, probably reflecting the larger disk mass of GG Tau A. However, the CS to H$_2$S abundance ratio of $\sim$20 in GG Tau A may still be similar in all sources. The upper limit on HC$_3$N is about two times lower than the detections reported in LkCa\,15, MWC\,480, and GO\,Tau by \citet{Chapillon+Dutrey+Guilloteau+etal_2012}. To make relevant abundances comparisons, we use $^{13}$CO as a reference since H$_2$ column densities are difficult to accurately determine. The results for the disks of GG Tau A and LkCa15 and the dark cloud TMC-1 are given in Table \ref{table:abun}. LkCa15 is a T Tauri star similar to GG Tau A: its disk exhibits a central cavity of radius 50 au \citep{Pietu+Dutrey+Guilloteau_2006} and has a mass on the order of $\sim 0.028$ M$_{\odot}$ \citep{Guilloteau+Dutrey+Pietu_2011}. Determining the uncertainties is difficult because the abundances were obtained from different studies. Therefore, we assume errors of 30\% in the cases of LkCa 15 and TMC-1. For GG Tau A, we take a $^{13}$CO column density, derived from observations, at 250\,au of $\Sigma_{250}$=1.13 10$^{16}$ cm$^{-2}$ (Phuong et al., in prep). For LkCa 15, \citet{Punzi+Hily-Blant+Kastner+etal_2015} found HCO$^+$ abundance relative to $^{13}$CO of 15\,10$^{-4}$, \citet{Huang+Oberg+Qi+etal_2017} gave abundance ratios of DCO$^+$/HCO$^+$ and DCO$^+$/H$^{13}$CO$^+$ of 0.024 and 1.1, respectively, and \citet{Dutrey+Wakelam+Boehler+etal_2011} gave an upper limit of H$_2$S relative to CO of 10$^{-6}$, which we convert to $^{13}$CO using an isotopic ratio $^{12}$C/$^{13}$C\,$\sim60$ \citep{Lucas+Liszt_1998}. In the TMC-1 dark cloud, \citet{Ohishi+Irvine+Kaifu_1992} determined $^{12}$CO abundance relative to H$_2$ of 8\,10$^{-5}$ or 1.3\,10$^{-6}$ for $^{13}$CO. The abundance relative to H$_2$ of HCO$^+$, H$_2$S (upper limit) \citep{Omont_2007}, H$^{13}$CO$^+$, and DCO$^+$ \citep{Butner+Lada+Loren_1995} are then used to get the abundances relative to $^{13}$CO. In L134N, the abundances of these species are similar, but H$_2$S has been detected with an abundance ratio of 60\,10$^{-5}$ \citep{Ohishi+Irvine+Kaifu_1992}, similar to the upper limit obtained in TMC-1. Thus, the disks appear to have very similar relative abundances, suggesting similar chemical processes at play, while the dense core differs significantly. \begin{table} \caption{Molecular abundance relative to $^{13}$CO (X$_{[mol]}$/X$_{[^{13}CO]}\times10^5$)} \label{table:abun} \centering \begin{tabular}{|c|c|c|c|c|} \hline & TMC-1$^\star$ & LkCa 15 & GG Tau \\ \hline HCO$^+$ & $600\pm180$$^{(1)}$ & $150\pm35$$^{(3)}$ &$130\pm12$ \\ \hline H$_2$S& $<45^{(1)}$& $<7^{(4)}$ &$11\pm3$\\ \hline H$^{13}$CO$^+$& $15\pm4$ $^{(2)}$ & $5\pm1.5$ $^{(4)}$ & $4.7\pm0.3$\\ \hline DCO$^+$ & $30\pm9$ $^{(2)}$ & $4.5\pm1.4$ $^{(4)}$ & $3.5\pm0.15$ \\ \hline \end{tabular} \tablefoot{$^\star$ $^{13}$CO abundance is derived from CO abundance in \citet{Ohishi+Irvine+Kaifu_1992}, $^{(1)}$ \citet{Omont_2007}, $^{(2)}$ \citet{Butner+Lada+Loren_1995}, $^{(3)}$ \citet{Punzi+Hily-Blant+Kastner+etal_2015}, $^{(4)}$ \citet{Dutrey+Wakelam+Boehler+etal_2011}, $^{(5)}$ \citet{Huang+Oberg+Qi+etal_2017}.} \end{table} \subsection{Sulfur-bearing species} In the chemical modeling, we found that H$_2$S peaks around three scale heights. The main reason behind this is rapid formation of H$_2$S on the grain surface via the hydrogenation reaction of HS, i.e., grain-H + grain-HS$\rightarrow$grain-H$_2$S. Once H$_2$S is formed on the surface, it is then chemically desorbed to the gas phase. Almost 80\% of the H$_2$S comes from the surface reactions, whereas the contribution of the gas-phase reaction H$_3$S$^+$+e$^-$$\rightarrow$ H + H$_2$S is about 20\%. Below three scale heights, H$_2$S depletes rapidly on the grains because of the increase in density and decrease in temperature. At the same altitude, CS is formed in the gas phase via the dissociative recombination reactions of HCS$^+$, H$_2$CS$^+$, H$_3$CS$^+$, and HOCS$^+$. The modeled CCS and SO$_2$ column densities (shown in Table \ref{table:dens} and in Appendix \ref{apen:chem}) are low, explaining their non-detection but the SO column density is overpredicted. The CCS molecule shows its peak above $z/H$=3 and is due to the gas phase formation via \mbox{S + CCH$\rightarrow$ H + CCS} and \mbox{HC$_2$S$^+$ + e$^-$ $\rightarrow$ H + CCS} reactions. SO$_2$ is made from the OH + SO reaction around this location, whereas SO comes from the S + OH reaction. We found that the UV field has a negligible impact on the H$_2$S desorption and mildly affects the SO/H$_2$S ratio. The key parameter in the model is the initial S abundance. Even with the low value, $8\,10^{-8}$, the chemical model overpredicts H$_2$S and SO by about an order of magnitude, but is compatible with CS and the current limits on SO$_2$ and CCS. In our models, the molecular layer is very thin and situated high above the disk plane at three scale heights. This is at odds with the observations of CS in the Flying Saucer \citep{Dutrey+Guilloteau+Pietu_2017}, where CS appears closer to one scale height. The difference may be due to the larger mass of the GG Tau disk (0.15 M$_{\odot}$). On one side, the high densities limit the UV radiation penetration (which drives the active chemistry) to the uppermost layers, while closer to the midplane, the even higher densities lead to more efficient depletion on dust grains. Our results suggest that chemistry for H$_2$S on the grain surfaces is likely not properly taken into account even with our three-phase model and that a significant amount of H$_2$S should change in some more complex sulfur-bearing species, limiting the overall desorption of S-bearing molecules \citep{Dutrey+Wakelam+Boehler+etal_2011, Wakelam+Ceccarelli+Castets_2005}. Indeed, measurements of S-bearing species in comets 67P performed by ROSETTA indicate a solar value for the S/O elemental ratio within 2$\sigma$ errors \citep{Calmonte+Altwegg+Balsiger_2016}. H$_2$S accounts for about half of the S budget in the comet, suggesting that transformation of H$_2$S to other compounds in ices is limited. The nearly constant H$_2$S/H$_2$O ratio also suggests that H$_2$S does not evaporate alone, but in combination with water \citep{Jim+Caro_2011}. \subsection{Chemistry of DCO$^+$ and other observed species} \paragraph {Chemistry of DCO$^+$:} The measured HCO$^+$/H$^{13}$CO$^+$ ratio is about 30, suggesting partially optically thick emission for HCO$^+(1-0)$ line. The measured DCO$^+$/HCO$^+$ ratio, $\sim0.03$ over the disk, is comparable to the averaged value \citep[$\sim$0.04;][]{vanDishoeck+Thi+vanZadeldoff+etal_2003} derived in the disk of TW Hydra of mass of $\sim$ 0.06 M$_{\odot}$ \citep{Bergin+Cleeves+Gorti_2013}, and in the disk of LkCa\,15 \citep[ratio of $\sim$0.024,][]{Huang+Oberg+Qi+etal_2017}. This shows clear evidence of ongoing deuterium enrichment. HCO$^+$ formation and deuteration is controlled by CO as well as H$_2$D$^+$ and H$_3^+$ ions. These ions are mostly sensitive to the X-ray flux, while UV radiation and cosmic rays play a limited role, and their balance is controlled by the temperature sensitive reaction \mbox{H$_3^+$+HD $\rightleftharpoons$ H$_2$D$^+$+H$_2$}. Because of the temperature dependences, DCO$^+$ is expected to be enhanced around the CO snow-line, as illustrated by the ring structure in HD 163296 \citep{Mathews+Klaasen+Juhasz+etal_2013}. Our model somewhat underpredicts the HCO$^+$ content. At 250 au, HCO$^+$ peaks at three scale heights, where the molecular layer is warm ($\sim$ 30\,K) and forms mainly from the reaction of \mbox{CO + ortho-H$_3$$^+$}. At this altitude, DCO$^+$ forms from the isotope exchange reaction between HCO$^+$ and D because the gas temperature is still high. Closer to the disk midplane, the \mbox{ortho-H$_2$D$^+$ + CO} pathway remains inefficient because of the strong CO depletion that results from high densities. Lower densities just outside the dense ring may lead to lower CO depletion and a more efficient DCO$^+$ formation, explaining the DCO$^+$ peak there. \paragraph{ Other observed species:} We also presented integrated column densities of HC$_3$N and $\it c$-C$_3$H$_2$, in Table \ref{table:dens} and Appendix \ref{apen:chem}. The modeled column densities of HC$_3$N and $\it c-$C$_3$H$_2$ are overpredicted. The high column density of HC$_3$N above three scale heights is due to its rapid formation via \mbox{CN + C$_2$H$_2$$\rightarrow$ H + HC$_3$N} reaction, whereas $\it c-$C$_3$H$_2$ forms from the \mbox{CH + C$_2$H$_2$} reaction, photodissociation of CH$_2$CCH and dissociative recombination of C$_3$H$_5$$^+$. | 18 | 8 | 1808.00652 |
1808 | 1808.10797_arXiv.txt | The cosmic-ray (CR) knee and the compositions contain abundant information for probing the CR's origin, acceleration and propagation mechanisms, as well as the frontier of the fundamental physics. Realizing that major proposals toward the knee's shape can be divided into two categories: the rigidity-dependent (also Z-dependent) knee and the mass-dependent (also A-dependent) knee, where the former one relates to the acceleration or the propagation mechanisms, and the other one is often associated with the new physics, it is essential to precisely measure the individual compositions. Benefit from the high altitude and hybrid detection methods, the LHAASO experiment has the ability in determining the individual component and brings us an opportunity in discriminating these two models. We test this expected ability of LHAASO from 100 TeV to 10 PeV with 3-year observation. And find the dominant component at the knee is essential to this issue, while much heavier nuclei occupying the knee leads to higher significance. In the analysis, the He-dominant knee under the A-dependent case can be recognized at the significance about 6.6 $\sigma$, while the P-dominant knee under the Z-dependent case will be classified with 2 $\sigma$ significance. | The CRs' spectrum, and their compositions, contains abundant information for probing their origin, acceleration and propagation mechanisms. The spectral break of the CR called "knee" was first discovered in 1959 \cite{1959JETP...35....8K}, but its origin still remains a puzzle over nearly 60 years. Although current measurements about the knee are different among the ground-based experiments under different conditions, they can come into an agreement assuming the 15\% uncertainty of the energy reconstruction \cite{2003JPhG...29..809H}. And it is summarized that the knee is described by a resembled double-power law, with the spectral index -2.7 below 4 PeV, and steepening to -3.1 above this energy \cite{2013AIPC.1516..185H}. \par In order to explore the origin of the knee, precise measurement about the spectra of individual compositions is important. Due to the low-flux level of the CR knee about only 1 $m^{-2} yr^{-1}$, current measurements are settled ground-based to obtain the larger detecting aperture. And they are all performed through the indirect method, i.e. the Monte Carlo simulation about the extensive air shower (EAS) induced by the CRs impinging on the atmosphere. In lack of the knee energy data from the accelerator, the adopted hadronic interaction model extrapolated from the lower energy may bring unknown systematic errors in determining the spectrum and the compositions. There have been much efforts spent on the component measurement, but results make large discrepancy. Measurement from KASCADE favours that the knee of the all-particle spectrum is contributed by the steepening of the light species \cite{2005APh....24....1A}, and a heavy knee (mainly by the iron) is observed around 80 PeV \cite{2011PhRvL.107q1104A}. But the ARGO+WFCT finds the spectral break of the light species appears at $\sim$ 700 TeV \cite{2015PhRvD..92i2005B}. Meanwhile, in the analysis with various experiments, it is found the dominant specie at the knee is likely to be the Helium \cite{2006JPhG...32....1E}, and measurement from GAMMA attribute the knee to the light species (P and He) \cite{2014PhRvD..89l3003T}. Different from those, the $AS\gamma$ experiment indicates the knee originates from the nuclei higher than the Helium \cite{2006PhLB..632...58T}. \par With the poor information about the individual components, various explanations about the the knee's origin have been proposed (see \cite{2004APh....21..241H} and references therein). From the point of view about the astrophysical origin, many proposals attribute it to the change of the acceleration mechanism \cite{2007ApJ...661L.175B, 2002PhRvD..66h3004K, 2004A&A...417..807V}, the single source contribution \cite{2009arXiv0906.3949E, 2014PhRvD..89l3003T}, or the propagation effect in the Galaxy \cite{1995ICRC....2..697S, 1993A&A...268..726P, 2001ICRC....5.1889L, 2014PhRvD..90d1302G, 2015PhRvD..91h3009G}. In the consideration of the interaction effect, diverse models are proposed including the new channel of the hadronic interaction model \cite{2010EPJC...68..573D, 2001ICRC....5.1760K} and collision with exotic particles \cite{2003GReGr..35.1117K, 2008JCAP...12..003M, 2009JCAP...06..027B, 2003APh....19..379W, 2016arXiv161108384J} (relic neutrinos, the Dark Matter, SUSY, graviton, etc.). Besides, the regular process such as pair-production and disintegration of photons \cite{2009ApJ...700L.170H, 2001ICRC....5.1979T, 2002APh....17...23C} also belong to such kind of model. By investigating these proposals, it is found that most of them can be divided into two categories, the Z-dependent knee and the A-dependent knee (A, Z correspond to the CR nuclei's atomic number and charge respectively). The former one mainly relates to the CR's acceleration and propagation mechanisms, while the other one indicates many of them associate with the new physics processes. Thus, distinguishing between these two model is essential for exploring both the fundamental problems of CR and the new physics. \par The LHAASO experiment \cite{LHAASO_review} is the next generation of the ground-based experiment located at high altitude of 4410 m, at which the EAS induced by nuclei around the knee develops to the maximum and is expected to have less dependence on the hadronic interaction model. On the other hand, the LHAASO experiment combines the hybrid detection method, including detecting the charged particles, muons, as well as the Chrenkov/fluorescence photons. The charged particles construct the major part of the EAS' lateral distribution which is useful in determining the arrival directions, core locations, and primary energies, while the collected Cherenkov image is a good estimator about the CR energy and also sensitive to the CR component. The muons, generated by the decay of the charged pions, depend on the primary mass of CRs and have the ability in recognition of the primary CR species as well. \par Benefit from those advantages, the forthcoming LHAASO experiment will bring us an opportunity on the precise measurement about individual CR compositions. In this work, we investigate the capability of LHAASO in distinguishing the Z-dependent and A-dependent knee models. The contents are organized as follows: the section II contains the brief information about LHAASO, the section III contains the detail procedure of the analysis and the results of both the Z-dependent and the A-dependent knee models. In the last section, a conclusion and discussion is delivered. | Realizing that different interpretations about the origin of the knee correspond to different spectral shapes, where the acceleration or the propagation origin result in the Z-dependent knee, and many of the interaction models with new physics results in the A-dependent knee, precise measurement for the individual component is important. Benefit from the merit of the high altitude and the hybrid detection method, the forthcoming LHAASO experiment is expected sensitive to the individual components. In this work, we investigate the capability of LHAASO in distinguishing these knee models. In the consideration of the energy range $10^5 \ \sim \ 10^7$ GeV with 3-year observation statistics, we find the the Z-dependent hypothesis can be excluded at the significance of 6.6 $\sigma$ under the A-dependent knee (He-dominant), while the A-dependent hypothesis mixes with the Z-dependent knee and is harder to be excluded with the significance only 2 $\sigma$. \par The influence of the systematic uncertainty can be addressed if we attribute the major part to the energy calibration and the detecting efficiency. These concerned systematic uncertainties lead to the integral shift of the spectrum along the axes. Due to the only acting factor in this analysis is the the ratio of the knee energy of the He and P, which is split into 4 and 2 with respect to different knee models, the integral shift of the spectrum is not expected to influence the ratio significantly. Thus, this test is insensitive to the variance of the systematic uncertainties, which is a unique advantage. \par The lack of significant in recognition of the Z-dependent knee in this analysis is mainly due to the relative narrow energy band around the knee and the P-dominant assumption. Other modes of LHAASO that focus the higher energies is required to further determine the spectral index above the He's knee. Meanwhile latest measurement from ARGO \cite{2014arXiv1408.6739D, 2016NPPP..279....7M}, ARGO+WFCT \cite{2015PhRvD..92i2005B} and $AS\gamma$ \cite{2006PhLB..632...58T} also find some hints that the knee of the light components occurs at hundreds TeV, which corresponds to higher flux at the knees of the P and He. If this observation is confirmed by further LHAASO experiment, the analysis will fall into the concerned energy range naturally and can be performed much more conveniently with higher statistics. | 18 | 8 | 1808.10797 |
1808 | 1808.00464_arXiv.txt | {We present a novel method to constrain the mass of ultra-light bosons as the dark matter using stellar streams formed by disrupting Globular Clusters in the Milky Way. The turbulent density field of Fuzzy Dark Matter (FDM) haloes results in perturbations and dynamical heating of thin streams. Using numerical simulations based on an effective model, we explore the magnitude of this phenomenon and show that this is observable for the range of axion masses $m_a$ that is interesting for alleviating the `small-scale problems' of $\Lambda$CDM. We derive an analytical model for the thickening of thin stellar streams and obtain an early conservative lower limit for the boson mass of $m_a > 1.5 \times 10^{-22}~$eV, using pre-Gaia literature data for six Milky Way streams and after marginalizing over physical parameters. This demonstrates the great promise for using this novel dynamical method as a fully independent probe of FDM, to complement results based on Lyman-$\alpha$ forest data.} \begin{document} | \label{sec:intro} Ultra-light bosons \citep{Peccei,Wilczek,WeinbergE} have been proposed as a viable alternative to cold dark matter (CDM) \cite{Spergel,HuBarkana,Matos,Peebles,Goodman}. A bosonic scalar field is compatible with current cosmological constraints \cite{Li,Hlozek,Hlozek2}, as it behaves like CDM on large scales. However, the two models differ at non-linear scales for axion masses $m_a\lesssim 10^{-21}~$eV, corresponding to a de Broglie wavelength of $\sim1~$kpc. Differences include a suppression in the small-scale power spectrum \cite{Frieman,Amendola,Marsh} and the formation of centrally cored dark matter haloes \cite{Lee,Guzman}, both of which could ease some of the small-scale challenges of galaxy formation in CDM \cite{MarshR,Hui}. For instance, an axion mass of $m_a\lesssim 2\times10^{-22}~$eV would reproduce the observed kinematics of multiple stellar sub-populations in the Milky Way dwarf Spheroidal satellites \cite{Pop,Chen,GonzalezMorales}, without the need to invoke the strong stellar feedback required in CDM \cite{Weinberg,Bullock}. As such, Fuzzy Dark Matter (FDM) has recently attracted attention as a viable alternative to CDM. A number of analytic studies have addressed the properties of dark matter haloes in FDM, predicting the formation of a stable self-gravitating Bose-Einstein condensate characterized by a central constant-density core \cite{Gleiser,Sin,Chavanis}. Recent numerical studies \cite{Schive,moczTB} have confirmed this prediction and showed that, as in CDM, the density profile of FDM haloes is well fit by a Navarro-Frenk-White profile \cite[NFW,][]{NFW}, in which the most central region of the cusp is replaced by the soliton core. The size of the latter varies with both halo mass $M_{vir}$ and $m_a$ \cite{Schive,moczTB}: for a Milky Way sized halo with $M_{vir}=10^{12} M_\odot$, it is $r_{sc}\sim 0.2/m_{22}~$kpc, where $m_a=m_{22}\times10^{-22}~$eV. Both analytic work and numerical simulations have shown that FDM haloes are characterized by a sustained turbulent behavior, with order unity oscillations in the density field \cite{moczSP,Schive,moczTB}. These fluctuations, in the form of soliton-sized clumps \cite{Schive}, are caused by the reconnection of quantum vortex lines \cite{moczTB}, a phenomenon that is shared by Bose-Einstein condensates in absence of self-gravity \cite{Kobayashi,Baggaley}. Turbulence may cause observable dynamical heating, offering means to constrain $m_a$. However, Ref. \cite{Hui} finds that the timescales for {\it(i)} the disruption of binary stars, {\it(ii)} the thickening of the Galactic disk and {\it(iii)} the heating of open and globular clusters (GCs), are exceedingly long for these processes to provide useful constraints\footnote{Except possibly for the case of GCs close to the Galactic center.}. Here, we explore the dynamical heating of stellar streams of disrupting GCs, using both analytic and numerical methods. We set out to investigate what is the magnitude of the effect of quantum turbulence on the dynamics of GC streams in the interesting range of axion masses. We find that thin, kinematically cold stellar streams in the Milky Way represent a promising dynamical probe of the axion mass, providing means to set local dynamical constraints to this alternative dark matter model. In Section 2 we derive an analytical model for the dynamical heating of stellar streams, which we test using effective numerical simulations in Section 3. In Section 4 we use pre-Gaia literature data for a selection of Milky Way GC to illustrate the promise of this novel method and to derive a first constraint on the axion mass. | We have shown that the turbulent behavior of FDM haloes produces observable dynamical heating of the kinematically cold stellar streams of GCs. Using a phenomenological description of the fluctuating dark matter density field, we have shown that this effect can be important for thin stellar streams in the Milky Way and that it is potentially observable for the range of axion masses that is interesting for cosmology. This shows the promise for obtaining independent local dynamical constraints to FDM. Currently available constraints to the axion mass are all similarly derived from modelling the power spectrum of the Lyman-$\alpha$ forest \cite{Irsic,Armengaud,KobayashiL}. These studies consistently appear to disfavour the range of axion masses that would result in significant astrophysical implications. Taken at face value, these studies rule out FDM models with $m_{22}\lesssim20$, entirely excluding the window of boson masses required to alleviate the small-scale tensions with the CMD model. Since these results all rely on the same physics and on similar modelling, it is important to identify different, independent probes of the axion mass. It has been claimed, in fact, that a number of astrophysical effects may cause the constraints above to be substantially relaxed, perhaps by an order of magnitude \cite{Hui}. Based on entirely different physics, the perturbation and heating of GC streams in the Milky Way can provide us with such a fully independent test of the FDM model. For this purpose, we have constructed an analytic model for the effects of quantum turbulence on the stream thickness. This is based on a quasiparticle effective model of the turbulent density field of FDM haloes. We find that streams experience a process of diffusive heating, causing their internal velocity dispersion and vertical angular width to grow as $t^{1/ 2}$. Because of the secular dynamics of the stream, the evolution of the angular width in the orbital plane is faster. We have tested these scalings using a suite of tailored numerical simulations and found that our analytic model well reproduces the process of diffusive dynamical heating caused by the random encounters with the FDM clumps. \begin{figure} \centering \includegraphics[width=.5\columnwidth]{fig3.pdf} \caption{\label{in} The minimum angular width in degrees for young streams (age $t=2$~Gyr) with progenitors of mass $\log \left(M_{GC}/M_\odot\right)=4$ and orbital circularity $j=0.85$. The widths in the direction perpendicular and parallel to the orbital plane are displayed by black full lines and red dashed lines. } \end{figure} Using conservative assumptions on the properties of six Milky Way streams and pre-Gaia literature data, we have derived a first lower limit of $m_{a}>1.5\times10^{-22}$~eV. Improved data for the same streams and tighter constraints on the gravitational potential of the Galaxy will likely allow for a correspondingly tighter limit. Furthermore, the detection of previously unknown GC streams in the central regions of the Galaxy may also allow new stringent limits. Figure~\ref{in} shows the minimum angular width (in degrees) predicted by Eqs.~(\ref{mthz}) and~(\ref{mthp}) for young streams (age $t=2$~Gyr) with progenitors of mass $\log \left(M_{GC}/M_\odot\right)=4$ and almost circular orbits ($j=0.85$), for a range of orbital energies and axion masses. The detection of any single stream that is thinner than predicted here would essentially rule out a significant range of the parameter space. In this paper, we have focussed on the readily observable mean angular width and velocity dispersion of streams. However, future observations of the detailed properties of the stream's density and kinematic fields will provide additional constraining power, as the heating process by the quantum fluctuations is not fully diffusive, as shown by Figures~\ref{puppets} and~\ref{puppetsecc}. In fact, the perturbations caused by FDM turbulence and by low-mass CDM subhaloes may be degenerate in some regimes, especially within analyses that concentrate on a single stream. Analyses based on a set of streams with different orbital energies will be able to disentangle the two effects, based on the markedly different dependence on galactocentric distance of the heating caused by FDM turbulence. Though guided by the results of current self-consistent FDM simulations, the current model remains however quite simplified. Future self-consistent simulations of FDM haloes will be able to better resolve the dynamics of individual oscillations in the dark matter density field. The analytic framework we have presented can easily be improved to incorporate a more faithful description of the properties of real quantum clumps. For instance, a better statistical description of the spectrum of effective masses and of their motion would be beneficial, together with a characterization of any secondary dependences between the two quantities. We anticipate that the analytic model presented here can be used in combination with future numerical results from self-consistent simulations to more accurately predict the effect of dynamical heating. Thanks to the Gaia mission and upcoming Galactic surveys \cite{Gaia,DESI,LSST,4MOST,WEAVE}, dynamical heating of stellar streams in the inner Milky Way provides a promising novel method for setting independent constraints on FDM models, with the potential for improved and competitive limits. | 18 | 8 | 1808.00464 |
1808 | 1808.02168.txt | Many asteroids are rubble piles with irregular shapes. While the irregular shapes of large asteroids may be attributed to collisional events, those of small asteroids may result from not only impact events but also rotationally induced failure, a long-term consequence of small torques caused by, for example, solar radiation pressure. A better understanding of shape deformation induced by such small torques will allow us to give constraints on the evolution process of an asteroid and its structure. However, no quantitative study has been reported to provide the relationship between an asteroid's shape and its failure mode due to its fast rotation. Here, we use a finite element model (FEM) technique to analyze the failure modes and conditions of 24 asteroids with diameters less than 30 - 40 km, which were observed at high resolution by ground radar or asteroid exploration missions. Assuming that the material distribution is uniform, we investigate how these asteroids fail structurally at different spin rates. Our FEM simulations describe the detailed deformation mode of each irregularly shaped asteroid at fast spin. The failed regions depend on the original shape. Spheroidal objects structurally fail from the interior, while elongated objects experience structural failure on planes perpendicular to the minimum moment of inertia axes in the middle of their structure. Contact binary objects have structural failure across their most sensitive cross sections. We further investigate if our FEM analysis is consistent with earlier works that theoretically explored a uniformly rotating triaxial ellipsoid. The results show that global shape variations may significantly change the failure condition of an asteroid. Our work suggests that it is critical to take into account the actual shapes of asteroids to explore their failure modes in detail. | \label{Sec:Intro} Over the last few decades, spacecraft explorations and ground observations have revealed that many small asteroids are loosely packed aggregates, so-called rubble piles, and have irregular shapes. These asteroids are subject to many different kinds of external forces that could change their spin states. Some forces may be small but act continuously, generating significant effects on their rotational and orbital evolution over the lifetime. Such forces include solar radiation pressure. Solar radiation pressure generates small but continuous forces on sunlit surfaces of planetary objects. If asteroids are small enough to be affected by solar radiation pressure-driven forces, their asymmetric bodies experience torques and change their spin states \citep{Rubincam2000}. Ground observations have detected rotational acceleration/deceleration of small asteroids due to solar radiation pressure \citep[e.g.][]{Lowry2007, Taylor2007, Durech2008}. The breakup of the active asteroid P2013/R3 has been interpreted as the result of fast rotation caused by solar radiation pressure \citep{Jewitt2014R3, Jewitt2017}. Spin-state variations due to solar radiation pressure, called the YORP effect, depend on an asteroid's orientation of an asteroid towards the Sun \citep{Nesvorny2007} and its shape \citep{Scheeres2007B}, causing complex rotational dynamics \citep{Scheeres2008Rotational}. Topographic sensitivity of the YORP effect plays a significant role in rotational dynamics of small asteroids significantly \citep{Statler2009}. The YORP effect is also responsible for the formation of binary, triples, and pairs \cite[e.g.][]{Cuk2005, Goldreich2009, Jacobson2011}. The YORP effect is considered to have caused small asteroids to reach their spin limits. The behavior of these asteroids at the spin limits is key to answering their evolution. Friction affects shape equilibrium \citep{Holsapple2001} while cohesion may help asteroids keep asteroids' original shapes at fast rotation \citep{Holsapple2007}. Surface deformation processes also contribute to material shedding \citep{Scheeres2015Land}. A hard-sphere discrete element model showed that the equatorial ridge of a top-shaped object might result from the movement of materials toward the equator due to fast rotation \citep{Walsh2008, Walsh2012}. Shape deformation changes the YORP-driven torque, causing stochastic variations in the rotational state of an asteroid \citep{Cotto2015}. Soft-sphere discrete element methods have shown that a randomly packed sphere might have internal deformation at fast spin \citep[e.g.][]{Sanchez2012, Sanchez2016} although a heterogeneity in the internal structure would control the failure modes and conditions \citep{Zhang2017}. Substantial progress has been made in theoretical modeling of the internal deformation processes of asteroids. A key trend is that models assumed an asteroid to be a triaxial ellipsoid. This assumption allows for deriving the internal stress analytically, making problems clear and reasonably solvable \citep[e.g.][]{Love1927, Dobrovolskis1982, Holsapple2001}. While we have seen many pioneering works that shed light on the deformation mechanism of asteroids \citep[e.g.][]{Dobrovolskis1982, Davidsson2001, Holsapple2001, Holsapple2004, Holsapple2007, Holsapple2010, Sharma2009}, we assert that the shape evolution due to rotationally induced deformation is still an open question. The main reason is that asteroids do not have ideal shapes; in other words, they are neither spheres nor ellipsoids. The purpose of this study is to use a finite element model (FEM) analysis \citep{Hirabayashi2014DA, Hirabayashi2016Itokawa, Scheeres2016Bennu, Hirabayashi2016Nature} to quantify the failure modes and conditions of 24 observed asteroids of which high-resolution polyhedron shape models were generated. We choose asteroids smaller than 30 - 40 km in diameter because asteroids in this size range could be spun up/down by the YORP effect \citep{Vokrouhlicky2015}. The plastic FEM technique developed by the authors is based on the work done by \cite{Holsapple2008A}. Here, we will investigate the stress conditions of these asteroids at different spin rates and evaluate their failure modes. We outline the contents employed in this paper. First, we will discuss the strength model used. Second, we will categorize 24 asteroid into four shape types: contact binary objects, elongated objects, spheroidal objects, and non-classified objects. Although this classification is subjective, it will help us quantify the failure modes and conditions of these asteroids. Third, we will review our plastic FEM technique. Fourth, using the FEM technique, we will compute the failure mode and condition of each asteroid. Finally, we will compare the results from our FEM technique and those from earlier works that used a volume-averaging technique to explore a uniformly rotating triaxial ellipsoid \citep[e.g.][]{Holsapple2004, Holsapple2007, Holsapple2010, Sharma2009, Rozitis2014, Hirabayashi2014Biaxial}. We finally note that we distinguish ``failure mode" and ``failure condition." The failure mode means what regions in an asteroid would structurally fail while the failure condition describes when the asteroid experiences such a failure mode. | In this study, we discussed how the shape of an irregularly shaped body would evolve due to the YORP effect, using a finite element model technique. Assuming that materials in objects were homogeneously distributed, we investigated the YORP-driven failure conditions and modes of 24 asteroids ($<$ 40km in diameter) observed at high resolution. Our results showed that the irregular shape of an asteroid is a critical factor that controls the failure mode and condition, pointing out a limited capability of the well-accepted averaging technique. We used a subjective shape classification that divided asteroids into four shape classes: spheroidal objects, elongated objects, contact binary objects, and non-classified objects. We found distinctive trends of the failure mode for each shape type, shedding light on the shape formation processes of asteroids. Structural failure of the spheroidal objects always started from the interior, while the elongated objects had structural failure in the middle of their structure. The contact binary objects, on the other hand, experienced structural failure around their neck region. Also, the failure conditions were highly controlled by these shape features. Further investigations will give constraints on the formation processes of asteroids. | 18 | 8 | 1808.02168 |
1808 | 1808.09918_arXiv.txt | {The most metal-poor stars are the relics of the early chemical evolution of the Galaxy. Their chemical composition is an important tool to constrain the nucleosynthesis in the first generation of stars. The aim is to observe a sample of extremely metal-poor star (EMP stars) candidates selected from the Sloan Digital Sky Survey Data Release 12 (SDSS DR12) and determine their chemical composition.} {We obtain medium resolution spectra of a sample of six stars using the X-Shooter spectrograph at the Very Large Telescope (VLT) and we used ATLAS models to compute the abundances. } {Five stars of the sample have a metallicity [Fe/H] between -2.5~dex and -3.2~dex. We confirm the recent discovery of SDSS~J002314.00+030758.0. As a star with an extremely low [Fe/H] ratio. Assuming the $\alpha$-enhancement [Ca/Fe] = +0.4 dex, we obtain [Fe/H] = -6.1 dex. } { We could also determine its magnesium abundance and found that this star exhibits a very high ratio [Mg/Fe] $\ge$ +3.60~dex assuming [Fe/H] = -6.13 dex. We determined the carbon abundance and found A(C) = 6.4 dex. From this carbon abundance, this stars belongs to the lower band of the A(C) - [Fe/H] diagram. } {} | The lambda Cold Dark Matter ( $\Lambda$-CDM) cosmological model has received an impressive confirmation from the Wilkinson Microwave Anisotropy Probe (WMAP) and PLANCK satellites \citep[and references therein]{planck2016} over in recent years. The average redshift at which reionization occurs is found to lie between z = 7.8 and 8.8, depending on the model of reionization adopted. It may be that all the first stars were massive or exceedingly massive, with a very short lifetime \citep{bromm2009}, although other more recent numerical simulations suggest that the distribution of possible masses of the first stars may be much broader than previously believed \citep{hirano2015}, and may even extend down to solar mass or below \citep{greif2011} leading to stars which are still alive and observable today. Until recently, the deepest survey searching for metal-poor stars was the Hamburg-ESO survey (HES), which reached V=16 \citep{christlieb2008}, although the first aim of this survey was the search for distant quasars. Thanks to the Hamburg-ESO survey, several stars of extremely low iron content have been discovered. However, these stars are extremely rich in C and O, so that their overall metal content is in fact comparable to the metal content of Globular Cluster stars, with [M/H] $\simeq$-2.3 dex. Major progress with respect to HES can be found in an exploration of the data of the Sloan Digital Sky Survey (SDSS). \citet{ludwig2008} have developed an analysis tool that allows us to estimate the metallicity of Turn-off (TO) stars from the low resolution SDSS spectra. This tool can be used to derive the metallicity and to select extremely metal-poor candidates using the strongest lines (Calcium H\&K) in the spectrum used as a proxy for the metallicity. It is, however, not yet possible to firmly evaluate the metallicity precisely below [Fe/H] = -3.0 dex, because at very low metallicity the metallic lines become almost impossible to detect at the resolution of the SDSS spectra \citep{aoki2013}. The only solution to alleviate this degeneracy is to observe these candidates at a higher resolution to confirm their metallicities. The SDSS-based metallicities found by the method of \citet{ludwig2008} are essentially confirmed by the analysis at higher resolution. This method has been very successful and has led to a series of papers that have already been published \citep{caffau2011a, caffau2011b, caffau2012, caffau2014, bonifacio2015, caffau2016, bonifacio2018}. The most metal-poor stars are formed out of gas that has been very likely enriched by the ejecta of a single or a few supernovae. From the determination of the chemical composition of these stars, we can derive important constraints on the nucleosynthesis in the first generation of stars who enrich the primordial gas and on the chemical inhomogeneites during the early evolution of our Galaxy. These abundance determinations can also be used to constrain the scenario of formation of the first low mass stars \citep{caffau2011a}. From the recent analysis of SDSS DR12 data, we have detected new extremely metal-poor candidates that have never been observed at high resolution. In this article, we report the detailed analysis of six new extremely metal-poor candidates observed with the X-Shooter spectrograph installed on Kueyen at the ESO Very Large Telescope (VLT) on Cerro Paranal in Chile. Similar observations have been conducted for a different set of stars from the northern hemisphere at the Subaru telescope using the High Dispersion spectrograph (HDS) in the framework of a French-Japanese collaboration. | In this article, we reported the chemical analysis of six new extremely metal-poor candidates observed with the X-Shooter spectrograph. We determined the abundances of some elements (C, Mg, Ca, Si, Sr, and Ba) in the majority of these stars. Five stars of the sample show abundance ratios that are typical of metal-poor stars in the metallicity range -3.5 dex $\le$ [Fe/H] $\le$ -2.0 dex. The dwarf star SDSS~J002314.00+030758.0 appears to be extremely iron-poor. We found an upper limit of [Fe/H] $<$ -4.00 dex, a value which is in agreement with the even stronger upper limit computed by \citet{aguado2018} with [Fe/H] $<$ -6.60 dex. Our low S/N ratio does not allow us to give a more constraining [Fe/H] determination. Assuming a [Ca/Fe] representative of the halo population ([Ca/Fe] = +0.4 dex), we obtain [Fe/H] = -6.1 dex. We could determine the carbon abundance using the G band. We obtained A(C) = 6.4 dex (6.05 in 3D), which places this star in the lower band of the A(C)-[Fe/H] diagram \citep{spite2013, bonifacio2015} confirming the existence of the lower carbon band in the most metal-poor stars. Assuming the lower limit [Fe/H] = -6.6 dex from \citet{aguado2018} as a conservative estimate of its iron content, the [C/Fe] ratio would give $\simeq$ +4.6 dex. Assuming [Fe/H] =-5 dex, the [C/Fe] remains extremely high with a value of [C/Fe] = +3.0 dex. Given the large amount of carbon present in this star, its total metallicity lies in the range of the metallicities found in Galactic globular clusters. Adopting the same lower limit [Fe/H] = -6.6 dex from \citet{aguado2018}, we found that SDSS~J0023+0307 has remarkably high magnesium and calcium abundances, sharing this peculiarity with CS 22949-037 \citep{depagne2002}, SMSS J031300-670839.3 \citep{keller2014}, and HE~1327-2326 \citep{aoki2006}. These four stars have also a high [Mg/Ca] ratio ($>$ 0.90 dex) in contrast with the other extremely iron-poor stars HE~0107-5240 \citep{christlieb2004} and SDSS~J1313-0019 \citep{frebel2015}, suggesting different channels for the enrichment of the gas that formed most metal-poor stars we observe today. | 18 | 8 | 1808.09918 |
1808 | 1808.05131_arXiv.txt | New telescopes are being built to measure the Cosmic Microwave Background (CMB) with unprecedented sensitivity, including Simons Observatory (SO), CCAT-prime, the BICEP Array, SPT-3G, and CMB Stage-4. We present observing strategies for telescopes located in Chile that are informed by the tools used to develop recent Atacama Cosmology Telescope (ACT) and \textsc{Polarbear} surveys. As with ACT and Polarbear, these strategies are composed of scans that sweep in azimuth at constant elevation. We explore observing strategies for both small (0.42 m) aperture telescopes (SAT) and a large (6 m) aperture telescope (LAT). We study strategies focused on small sky areas to search for inflationary gravitational waves as well as strategies spanning roughly half the low-foreground sky to constrain the effective number of relativistic species and measure the sum of neutrino masses via the gravitational lensing signal due to large scale structure. We present these strategies specifically considering the telescope hardware and science goals of the SO, located at $23\degree$ South latitude, $67.8\degree$ West longitude. Observations close to the Sun and the Moon can introduce additional systematics by applying additional power to the instrument through telescope sidelobes. Significant side lobe contamination in the data can occur even at tens of degrees or more from bright sources. Therefore, we present several strategies that implement Sun and Moon avoidance constraints into the telescope scheduling. Scan strategies can also be a powerful tool to diagnose and mitigate instrumental systematics either by using multiple scans to average down systematics or by providing null tests to diagnose problems. We discuss methods for quantifying the ability of an observation strategy to achieve this. Strategies for resolving conflicts between simultaneously visible fields are discussed. We focus on maximizing telescope time spent on science observations. It will also be necessary to schedule calibration measurements, however that is beyond the scope of this work. The outputs of this study are algorithms that can generate specific schedule commands for the Simons Observatory instruments. | \label{sec:intro} % Precision measurements of the anisotropy of the cosmic microwave background (CMB) have become a cornerstone of modern cosmology. These anisotropies have both temperature and polarization components. The polarization anisotropies can be decomposed into both even-parity ($E$-mode) and odd parity ($B$-mode) signals. Primordial scalar (density) fluctuations can only source $E$-mode polarization. $B$-mode polarization in the CMB can be created by the gravitational lensing of the primordial $E$-mode polarization or by primordial tensor perturbations (gravitational waves). The lensing $B$-mode signal is expected to peak at sub-degree scales while the gravitational wave signal is expected to peak at degree scales. A detection of the gravitational wave signal would provide powerful evidence for the theory of cosmic inflation. Precision measurements of the lensing signal through the $B$-mode channel or four point statistics could detect a non-zero sum of neutrino masses or additional relativistic species in the universe beyond what is predicted by the standard model.\cite{2016arXiv161002743A} Current and next generation CMB telescopes are seeking to make increasingly sensitive maps of the the CMB polarization. The observation strategy used by an instrument can have a major impact on the ability to resolve the polarization anisotropies. The Simons Observatory (SO) will deploy a 6 meter crossed Dragone telescope\cite{Parshley2018} designed to measure the lensing $B$-mode signal and three 0.42 m refractive telescopes to search for the primordial gravitational wave signal\cite{Galitzki2018}. Each class of telescope will observe the sky following a strategy optimized for its portion of the SO science goals. The angular scale or multipole $\ell \approx 360 / \theta ^\circ$ range targeted by an instrument is a primary consideration. \begin{figure} \begin{center} \begin{tabular}{l r} \includegraphics[height=5cm]{lat_fields_inferno.png} & \includegraphics[height=5cm]{sat_fields_inferno.png} \end{tabular} \end{center} \caption{ \label{fig:lat3} Left: Fields for a proposed large aperture, large area survey over the Planck dust intensity map. The fields inside the boxes cover 17{,}095 square degrees and represent the region targeted by the telescope boresight. These are the fields used by the Classical strategy. Right: Fields for a proposed small aperture survey. The boresight targets 4{,}920 square degrees, but in practice the observed area is much larger due to the $35\degree$ field of view of the telescopes.\cite{Galitzki2018} These fields are used by both the Classical and Opportunistic strategies.} \end{figure} Measuring the reionization bump at very low multipoles ($\ell \lesssim 10$) requires a dedicated scan strategy \cite{2014SPIE.9153E..1IE,2014JLTP..176..733M,2016JLTP..184..786O} and will not be a target of the SO. The bulk of the inflationary $B$-mode signal will be at the recombination bump around $50 < \ell < 200$ and will be the primary target of the SO small aperture telescopes. The lensing $B$-mode signals will be a primary target of the SO large aperture telescope and peaks around $500 < \ell < 2000$ Typically, scan strategies for instruments searching for the inflationary $B$-mode feature have observed a small area ($f_{\rm sky}\sim 1\%$) in the lowest foreground regions of the sky.\cite{2015PhRvL.114j1301B,2010SPIE.7741E..1NF,2010arXiv1008.3915E,2013ApJ...768....9B} Since the $B$-mode polarization due to tensor perturbations is expected to be small, prior to a B-mode detection it can be advantageous to scan a small portion of the sky to concentrate the instrumental sensitivity into a small area and reduce noise variance in the final map as much as possible. Targeting large angular scale polarization also necessitates having a strategy composed of scans with large throws at the same location in ground coordinates, which is necessary due to the statistical properties of detector noise. These scans should also have a long duration to allow the sky to move enough relative to the ground to orthogonalize signals fixed in the sky and signals fixed on the ground. There is an essential trade off between these geometric considerations and the statistical benefits of scanning a smaller fraction of sky. Telescopes designed to measure small scale modes in the CMB are driven by a separate set of considerations. The need for large throws and long scans are mitigated at smaller feature sizes. Small scale temperature, E-mode, and B-mode anisotropies are expected due to primordial effects, gravitational lensing, galaxy clusters, and other sources, so cross correlation (and therefore patch overlap) with optical surveys such as LSST and DESI are valuable. These science cases generally favor a larger sky area to reduce the sample variance due to measuring a finite number of modes on the sky. Since there is no similar tension between sky area and scan geometry, these scan strategies have a significantly more open parameter space. At the map depths projected for CMB Stage-4 \cite{2016arXiv161002743A} the lensing signal will become a significant foreground for inflation measurements. This signal can be removed given a measurement of the lensing deflection field. This template can be measured using an external tracer of large scale structure \cite{2015PhRvD..92d3005S} or using the lensing signal measured from a high-resolution CMB instrument. For this reason it is advantageous to have significant area overlap between small and large aperture scanning strategies. A significant systematic in CMB measurements is galactic foregrounds. These can be subtracted using information from multiple frequencies, but the subtraction becomes difficult if the foreground contamination is large. CMB surveys therefore target low-foreground regions off the galactic plane, as in Figure \ref{fig:lat3}. A scan strategy must additionally ensure good overlap between the regions scanned at different frequencies. Thus, for the LAT, it is preferred to cover fields significantly larger than the field of view, which would minimize the area that different frequencies do not overlap compared to the total coverage. Additionally, there is a natural complementarity in the observation strategies between experiments in the Atacama and the South Pole. Instruments in the Atacama have access to a larger sky area and can achieve closer to isotropic mode coverage on the sky by observing the same point on the sky at multiple elevations. However, Chilean instruments must contend with a less stable and uniform atmosphere, less stable ground pickup due to lack of a featureless horizon, as well as scheduling constraints due to patches setting below the horizon. A detailed comparison of the trade offs between the two sites is beyond the scope of this paper. % We explore techniques for developing observing strategies for Chilean telescopes with a particular emphasis on application to SO. Section \ref{sec:fieldsandstrategies} describes two different approaches to generating observing strategies: ``classical'' and ``opportunistic;'' Section \ref{sec:sunandmoonavoidance} describes how these strategies avoid observing close to the Sun and the Moon; Section \ref{sec:proposedstrategies} describes examples of both classical and opportunistic observing strategies for the Simons Observatory's Large Aperture Telescope and Small Aperture Telescopes; Section \ref{sec:comparison} provides a few direct comparisons between the two styles of strategies and proposes some figures of merit for evaluating them. | 18 | 8 | 1808.05131 |
|
1808 | 1808.00008_arXiv.txt | The advanced stages of several high-mass stars are characterized by episodic mass loss shed during phases of instability. Key for assigning these stars a proper evolutionary state is to assess the composition and geometry of their ejecta alongside the stellar properties. We selected five hot LBV candidates in M33 to refine their classification, investigate their circumstellar environments and explore their evolutionary properties. Being accessible targets in the near-infrared, we conducted medium-resolution spectroscopy with GNIRS/GEMINI in the $K-$band to investigate their molecular circumstellar environments. Two stars were found to display CO emission, which was modeled to emerge from a circumstellar or circumbinary Keplerian disk/ring. The identification of the carbon isotope $^{13}$C and, for one of the two stars, a significantly low $^{12}$CO/$^{13}$CO ratio, implies an evolved stellar state. As both CO emission stars are highly luminous and hence do not undergo a red supergiant phase, we suggest that stripping processes and equatorial high-density ejecta due to fast rotation are responsible for the enrichment of the stellar surface with processed material from the core. A candidate B[e]SG displays an absorption CO profile, which may be attributed to a jet or stellar pulsations. The featureless infrared spectra of two stars suggest a low-density gas shell or dissipation of the molecule due to the ionizing temperature of the star. We propose spectroscopic monitoring of our targets to evaluate the stability of the CO molecule and assess the time-dependent dynamics of the circumstellar gas structures. | Mass loss constitutes the most substantial property that drives massive stars throughout the diverse evolutionary channels \citep{2014ARA&A..52..487S}. The latest stellar models incorporate the updated knowledge on mass loss and provide a way to assign observed stars a current evolutionary state and further predict their fate \citep{2012A&A...537A.146E}. Observational evidence of massive stars exhibiting eruptive events, however, clearly indicate deviations from the conventional and well determined line-driven mass losses that are typically shown during the early phases of the stellar life \citep{2001A&A...369..574V}. Instead, the proximity to the Eddington luminosity limit, the high rotational velocities, encounters with companions, and variability in the outer stellar layers due to pulsations, and energetic shocks are shown to trigger or enhance large amount of expelled gas \citep{2012ARA&A..50..107L,2012A&A...538A..40G}. Episodic and/or eruptive mass-loss events introduce and characterize the peculiar transient states of Luminous Blue Variables \citep[LBVs,][]{1994PASP..106.1025H}, B[e] supergiant stars \citep[B{[}e{]}SGs,][]{1985A&A...143..421Z}, and Yellow Hypergiants \citep[YHGs,][]{1998A&ARv...8..145D}, which are found to occupy distinct regions in the upper Hertzsprung-Russell (HR) evolutionary diagram. From the evolutionary point of view, B[e]SGs are classified as evolved, post main-sequence hot stars with log$\,L/$L$_{\odot}>4$. They were originally suggested as fast rotators possessing a two-component wind; a fast and low-density polar component and a slow, dense equatorial outflow \citep{1985A&A...143..421Z, 1986A&A...163..119Z, 2005A&A...437..929C}. Observational studies on the geometry of the ejecta confirm the presence of circumstellar disks and rings, which however, are found to be in Keplerian rotation \citep{2012A&A...548A..72C, 2012MNRAS.423..284A, 2016MNRAS.456.1424A, 2017ASPC..508..219K, 2017AJ....154..186K, 2012A&A...543A..77W, 2014AdAst2014E..10D, 2017ASPC..508..213M}. Compared to B[e]SGs, both classes of LBVs and YHGs on average display higher luminosities (log$\,L/$L$_{\odot}\gtrsim 5.4$) with the latter class being constrained by the Humphreys$-$Davidson limit \citep{1994PASP..106.1025H} at log$\,L/$L$_{\odot}\sim 5.8$. \cite{2018MNRAS.478.3138D} recently refined this limit at log$\,L/$L$_{\odot}\sim 5.5$, although their study focused exclusively on the luminous cool population of the Magellanic Clouds. The LBVs in quiescence span an extended region in the HR diagram that is believed to host two separate populations associated with different evolutionary paths; a high-luminosity population formed by LBVs that may still be at their core-hydrogen burning phase, and a low-luminosity population that comprises evolved stars, which have already passed through the red supergiant (RSG) phase and undergo a blueward loop \citep{2013A&A...558A.131G, 2017ApJ...844...40H}. YHGs are also regarded as post-RSG stars \citep{2001ApJ...560..934S} although it is not clear whether they constitute ancestors of LBVs or an end-point state of intermediate-mass massive stars \citep{2014A&A...561A..15C}. The evolutionary link between B[e]SGs and the other two evolved classes has been discussed in many studies \citep[e.g.][]{1996ApJ...468..842S, 1996ApJ...470..597M, 2008A&A...477..193M} but is still under investigation. From the spectroscopic point of view, these three types of peculiar stars display signatures of gas ejecta indicated by the broad hydrogen emission, which either stands out from H$\alpha$ survey catalogs \citep[e.g.][]{2007AJ....134.2474M} or systematically or occasionally appear during spectroscopic monitoring \citep[e.g.][]{2003ApJ...583..923L}. Low-excitation lines of singly ionized metals, such as \ion{Fe}{II} and [\ion{Fe}{II}] typically make their appearance in the spectra of the three types \citep{1998A&A...340..117L, 2014A&A...561A..15C, 2017ApJ...836...64H}. Emission of [\ion{O}{I}] and [\ion{Ca}{II}] further characterizes the classes of B[e]SGs and YHGs \citep[e.g.][]{2012MNRAS.423..284A,2017ASPC..508..239A,2017ApJ...836...64H}, but is absent or not prominent in LBVs. With respect to their spectral energy distribution (SED), B[e]SGs show a remarkable near-infrared excess arising from warm/hot circumstellar dust \citep{1986A&A...163..119Z,2009AJ....138.1003B, 2010AJ....140..416B}. In contrast, LBVs lack hot dust and typically display free-free emission in the near infrared due to winds. Excess toward longer wavelengths is frequently observed, and implies extended volumes of cool dust formed due to past eruptive events \citep{1986A&A...164..435W,1988ApJ...324.1071M}. YHGs are often enshrouded by warm dust (although not as prominently as B[e]SGs are), and differentiate from the other hot two classes showing a red continuum in the optical that is consistent with A$-$G type stars. Characterizing the stellar properties of the three extreme types is undermined in the sense that, in outbursts, actual photospheres are usually veiled under optically thick material that is formed during severe mass-loss events \citep{1987ApJ...317..760D, 2009ASPC..412...17O}. Alternatively, the properties of the ejected gas e.g. temperature, composition, and density, allow insight on stellar properties such as the intensity of the radiation field and surface abundances. In turn, assessing the internal mixing processes and the geometry of the ejecta could help to infer on the rotational status of the star \citep{2007A&A...464.1029D}. An optimal tracer for the distribution of the expelled gas is found to be the CO molecule observed in the $K-$band \citep{2000A&A...362..158K,2013A&A...549A..28K}. Given its dissociation temperature at 5\,000 K, the CO traces the warm circumstellar gas, which survives the strong radiation field from the hot star \citep{1988ApJ...334..639M}. The study of the $^{13}$C isotope as a product of core-helium burning processes is a step further to shed light on the evolutionary state of the star \citep{2010MNRAS.408L...6L, 2009A&A...494..253K, 2013A&A...549A..28K}. Observations of $^{13}$CO indicate enrichment of the stellar surface via internal mixing or/along with stripping processes induced by enhanced mass losses. Measuring the ratio $^{12}$CO/$^{13}$CO serves as a robust method to evaluate the abundance of processed material \citep[e.g.][]{2009A&A...494..253K,2014ApJ...790...48H,2015AJ....149...13M} with a low ratio even promoting a post-RSG classification \citep[e.g.][]{2013A&A...558A..17O}. Nearby galaxies with well established distances offer the opportunity to acquire reliable luminosities, which are essential for discriminating among the various subclasses of B[e] stars \citep{1998A&A...340..117L} and for selecting potential YHGs against their lower mass counterparts and the foreground contaminants \citep{2016ApJ...825...50G}. Moreover and as previously mentioned, luminosity is the main property to tell about different evolutionary origin of LBV stars. Studies in the Magellanic Clouds, M31 and M33 galaxies exploit the accessibility of such targets to undertake optical and infrared studies at resolutions sufficient for assigning a proper classification and identifying molecular enrichment \citep{2013A&A...558A..17O,2014ApJ...780L..10K, 2016A&A...593A.112K, 2014ApJ...790...48H, 2017ApJ...836...64H, 2015MNRAS.447.2459S, 2018A&A...612A.113T}. In this frame, we discuss on the evolutionary state of five hot candidate LBVs in M33 by combining newly presented near-infrared spectroscopy with multi-band photometry. We refine the classification of these stars and suggest scenarios that could be subjects of follow-up studies towards a comprehensive picture of the mass-loss mechanisms. The paper is structured as follows: in Sec. \ref{sample} we describe the input data and discuss on the infrared colors of the targets, in Sec. \ref{infspec} we describe the spectroscopic observations and model the CO first-overtone band heads of two stars showing emission, in Sec. \ref{sed} we model the SEDs of the stars to infer on the stellar and surrounding properties, and in Sec. \ref{disc}, we discuss our results alongside the literature studies. Concluding remarks are given in Sec. \ref{concl}. | \label{concl} We undertook $K-$band spectroscopy of five luminous stars in M33 characterized by high mass-loss rates, to gain insight into their gaseous surroundings and to better understand the advanced stages in the lifetime of extreme massive stars. The objects are bright targets in the infrared with M$_{[3.6]}<-8$ mag, occupying regions where B[e]SGs, LBVs, and YHGs reside. By modeling the SED of our stars with photospheric, wind and dust components, we estimated effective temperatures and derived bolometric luminosities. With accurate distance measurements in hand, these extragalactic targets well confine their location on the evolutionary diagram thus allowing a direct comparison to theoretical models. We report two stars, J013248.26+303950.4 and J013442.14+303216.0, showing absence of the CO molecule. Of these, the spectrum of the luminous LBV candidate J013248.26+303950.4 indicates prevention of molecular formation due to the hot temperature of the star. For the less luminous warm supergiant, insufficient gas densities or a non-recent RSG phase could justify observations. The hot B[e]SG candidate J013333.22+303343.4 displays a remarkable absorption CO profile, which is attributed to gas interfering between the observer and the star e.g. a bipolar stream. Alternatively, stellar pulsations may be responsible for a variant CO profile depending on the compression or expansion phase of the circumstellar gas. The remaining two stars in our sample show emission in the CO bandheads. Of these, we suggest the hotter, J013235.25+303017.6, as an Oe star where emission emerges from an early-formed circumstellar disk. The most luminous star of our sample, J013406.63+304147.8, leaves room for a faint cool companion, which could support both the striking luminosity of the main star owing to mass transfer and the formation of a circumbinary disk/ring, which consists of processed material ejected from the system. The current near-infrared data can serve as milestone to follow-up studies, which will assess the evolutionary picture of the five extreme stars and their analogues. Spectroscopic monitoring of the objects is needed to evaluate the stability of the CO molecule. A variable profile could be investigated in the frame of pulsation cycles, interaction of the molecular circumstellar gas with shockwaves, or the physical expansion, and thus dilution of the gas volume. In addition, optical high-resolution spectroscopy on [\ion{Ca}{II}] and [\ion{O}{I}] lines will allow insight into the kinematics and disk/ring structures \citep[e.g.][]{2012MNRAS.423..284A,2016MNRAS.456.1424A,2017ASPC..508..239A,2016A&A...593A.112K,2017ASPC..508..213M,2018A&A...612A.113T}. Profile variability as a consequence of orbital motion within a binary could reveal the dominant role of stellar encounters to the rotating status of the discussed stars, the chemical enrichment through deposit or stripping processes and the formation of circumbinary rings. Along with time-series photometry obtained by long-term monitoring surveys such as Pan-STARRS \citep{2016arXiv161205560C} and Gaia \citep{2016A&A...595A...1G}, future studies could associate phases of enhanced mass loss to stellar and environmental modulations. | 18 | 8 | 1808.00008 |
1808 | 1808.06238_arXiv.txt | The recent multi-messenger discovery of binary neutron star (BNS) merger GW170817 showed that $\gamma$-ray emission in short GRBs is wider than the central energetic narrow cone, and weakly expands out to tens of degrees. Here we explore some of the observational consequences of this structured emission, taking the reconstructed angular emission profile of gamma-ray burst GRB170817A to be typical. We calculate the expected fraction of gravitational-wave observations from BNS mergers that will have an observed GRB counterpart to be $\sim30\%$, implying that joint gravitational-wave and GRB observations will be common. Further, we find that $\sim10\%$ of observed short GRBs occur within 200\,Mpc. Finally, we estimate a BNS merger rate of $\sim500$\,Gpc$^{-3}$yr$^{-1}$. | The observation of binary neutron star (BNS) merger GW170817 demonstrated the impact of combining information from multiple astrophysical messengers \cite{2017PhRvL.119p1101A,2017ApJ...848L..12A,2017ApJ...848L..13A,2017ApJ...850L..35A}. Multi-messenger information from this one event, among others, firmly established the connection between neutron star mergers and short gamma-ray bursts (GRBs), gave us information on the structure of the relativistic outflow \cite{2018arXiv180502870A,2017arXiv171203237L,2018Natur.554..207M,2018PTEP.2018d3E02I,2017arXiv171005896G,2017ApJ...848L..25H,2018arXiv180502870A,2018arXiv180207328V} and composition of the dynamical ejecta \cite{2017ApJ...851L..21V,2017Sci...358.1565E,2017arXiv171109638W}, constrained the maximum mass and equation of state of neutron stars \cite{2017ApJ...850L..19M,2018ApJ...852L..29R}, and gave an independent estimate of the Hubble constant \cite{2017Natur.551...85A}. The detection of a GRB from the first BNS merger observed through GWs was unexpected \cite{2012ApJ...746...48M,2015MNRAS.448.3026W}. Gamma-ray emission from short GRBs is beamed, reducing the rate of events observable from Earth. The level of beaming can be estimated using the sudden steepening of GRB afterglow light curves (the so-called jet break) recorded for some GRBs. This yields a typical beaming half angle of $\sim10^\circ$ \cite{2014ARA&A..52...43B}. This means that only one in every $\sim100$ GRBs should be observable \cite{2012ApJ...746...48M}. Similar beaming can be inferred from comparing the observed rate of short GRBs to the expected rate of BNS mergers, the latter of which can be estimated using the observed Galactic binary population and population synthesis models \cite{2004ApJ...601L.179K,2010CQGra..27q3001A,2014ARA&A..52...43B}. GWs, on the other hand, are emitted by BNS mergers in all directions. The difference between the GW amplitude along the direction of strongest emission--the orbital axis, and the direction-averaged emission is only a factor of $1.5$ \cite{2011CQGra..28l5023S}. This means that GWs can be observed with limited dependence on the source orientation. Therefore, for highly beamed gamma emission we expect a high fraction of GW detections not to be accompanied by a detected GRB. Assuming a GRB beaming factor of 100, accounting for the fact that gamma-rays will be emitted along the orbital axis of the binary after merger, and assuming that all GRBs facing towards Earth can be detected within the distance range of GW detectors, about $1.5^3/100\approx3\%$ of GW detections should have an observable GRB counterpart. This paradigm changed with the discovery of GWs from the BNS merger GW170817 by the LIGO and Virgo GW detectors, which was accompanied by a GRB, GRB170817A, discovered by the Fermi Gamma-ray Burst Monitor (Fermi-GBM) and the Anti-Coincidence Shield for the Spectrometer for the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) \cite{2017ApJ...848L..13A}. The coincidence for the first discovery was previously deemed unlikely. Further, from the identification of the host galaxy together with the GW signal one could estimate the binary's inclination, found to be $32^{+10}_{-13}$\,deg \cite{2018arXiv180404179F}. Short GRBs as a group cannot be detected up to this inclination. A jet opening half angle $\theta_{\rm j} = 30^\circ$ would imply a beaming factor of $f_{\rm b}=(1-\cos(\theta_{\rm j})^{-1} \approx 7$. This is inconsistent with the (non-collapsar) short-GRB rate density of $\sim10$\,Gpc$^{-3}$yr$^{-1}$ \cite{2006ApJ...650..281N} and the binary neutron star merger rate density of $\sim 10^{3}$\,Gpc${-3}$yr$^{-1}$ \cite{2010CQGra..27q3001A,2017PhRvL.119p1101A}. We do not typically detect GRBs with these inclinations at cosmological distances. Other than the possibility that GRB170817A is unusual, this can be explained if we assume that gamma-ray emission weakens at large observing angles $\theta_{\rm obs}$ measured from the GRB jet axis. At low $\theta_{\rm obs}$ gamma-ray luminosity is high, and GRBs can be detected from large distances. At greater $\theta_{\rm obs}$, gamma-ray luminosity diminishes and only the closest events can be detected. In this scenario the rate density of observed GRBs is determined by the narrow cone of high-luminosity GRB which has a large effective beaming factor. This picture is further corroborated by the measured isotropic-equivalent energy of GRB170817A, $E_{\rm iso}\approx 3\times10^{46}$\,erg \cite{2017ApJ...848L..13A}. This is about a factor of a 1000 below the isotropic-equivalent energy of the weakest GRB previously observed with known redshift \cite{2014ARA&A..52...43B}. Observations of the GRB afterglow provided a wealth of additional information on the structure of the outflow that produce gamma rays. The delayed onset of the X-ray afterglow is consistent with $\theta_{\rm obs}$ being greater than the opening half angle, i.e. that the GRB was observed off-axis \cite{2017Natur.551...71T,2017ApJ...848L..25H}. The afterglow's temporal and spectral evolution can also be used to reconstruct the properties of the relativistic outflow from the source. Here, an interesting result is that the relativistic outflow interacts with the lower-velocity ejecta from the merger, affecting both the afterglow and the gamma-ray emission. Numerical simulations of the process show that the observed afterglow is inconsistent with the simple {\it top-hat} jet, in which the relativistic outflow produces uniform emission within the opening angle and zero emission outside of it. Observations are, however, consistent with structured relativistic outflow, more specifically the off-axis observation of a narrow cone of ultra-relativistic material surrounded by a slower outflow that extends to greater angles \cite{2017arXiv171203237L,2018ApJ...856L..18M}. Another possible explanation is a quasi-spherical, mildly relativistic ejecta produced by the energy injection of a narrow relativistic jet into slow-moving ejecta \cite{2017Sci...358.1559K,2018Natur.554..207M,2018ApJ...858L..15D,2018arXiv180609693M}. Observations cannot yet differentiate between these latter scenarios. For either of these possibilities, gamma-ray emission, at least for GRB170817A, is more directionally extended than previous estimates of GRB beaming suggested. This is good news for the joint observability of GWs and GRBs from BNS mergers, and possibly neutron star and black hole mergers. While the weak extended emission does not affect the detection rate of GRBs at cosmological distances, within the limited distance range of GW observations it can play an important role. In this paper we investigate the observability and inferred rate of BNS mergers in light of structured gamma-ray emission. For this we assume that GRB170817A is a typical short GRB, and adopt the reconstructed angular gamma-ray emission profile of a structured outflow computed by Margutti {\it et al.} \cite{2018ApJ...856L..18M}. Using this profile, we examine: what fraction of BNS mergers detected via GWs that will be also detected via GRBs; whether there could be a significant population of nearby detectable GRBs, within $\sim200$\,Mpc; and the inferred rate of BNS mergers. | We investigated the observational consequences of structured $\gamma$-ray emission in short GRBs produced by BNS mergers. Structured jets lead to increased detectability of short GRBs in the local Universe ($\lesssim 200$\,Mpc) than for more distant sources due to weak emission out to larger viewing angles. We find the following observational consequences of this effect, using GRB170817 as a fiducial short GRB: \begin{itemize} \item A short GRB will be observed from more than 30\% of BNS mergers discovered via GWs, making such multi-messenger detections common. \item About 10\% of observed short GRBs occurred within 200\,Mpc from Earth. This means that a significant fraction of short GRBs with no reconstructed distance are nearby. \item The local rate density of BNS mergers is about $500$\,Gpc$^{-3}$yr$^{^-1}$. \end{itemize} These results assume that all short GRBs from BNS mergers are like GRB170817A, which is not necessarily the case. Future multi-messenger observations of BNS mergers will help improve these estimates. In addition, our results rely on the simulations of Margutti et al. \cite{2018ApJ...856L..18M}, which may be improved as more observations of the afterglow of GRB170817A, and more detailed numerical studies of the outflow, become available. Taking into account the changing $\gamma$-ray spectrum as a function of viewing angle will further improve the accuracy of the results. The structured jet profile in Margutti et al. \cite{2018ApJ...856L..18M} is provided only out to $40^\circ$. Presumably nearby GRBs may be detectable at even larger viewing angles, which makes our results, in this regard, conservative. Finally, there are also alternative emission models to consider, in particular the presence of a mildly relativistic, wide-angle outflow \cite{2017Sci...358.1559K,2018Natur.554..207M,2018ApJ...858L..15D,2018arXiv180609693M}, which will likely yield different predictions to the parameters obtained in this work. Near future observational constraints on these parameters will also provide strong constraints on structured emission. \newline The authors thank Szabolcs Marka and Daichi Tsuna for their useful suggestions. IB is thankful for the generous support of the University of Florida. This paper was approved for publication by the LIGO Scientific Collaboration and Virgo Collaboration under document number LIGO-P1800252. | 18 | 8 | 1808.06238 |
1808 | 1808.03301_arXiv.txt | The Arizona Lenslets for Exoplanet Spectroscopy (ALES) is the world’s first AO-fed thermal infrared integral field spectrograph, mounted inside the Large Binocular Telescope Interferometer (LBTI) on the LBT. An initial mode of ALES allows 3-4 $\mu$m spectra at R$\sim$20 with 0.026” spaxels over a 1”x1” field-of-view. We are in the process of upgrading ALES with additional wavelength ranges, spectral resolutions, and plate scales allowing a broad suite of science that takes advantage of ALES’s unique ability to work at wavelengths $> $2 microns, and at the diffraction limit of the LBT’s full 23.8 meter aperture. | Integral field spectrographs (IFS’s) have become ubiquitous on large adaptive optics (AO) telescopes, in particular for their ability to obtain spectra of directly-imaged exoplanets. Most of these IFS's operate in the near-infrared (1-2$\mu$m). ALES is the world's first adaptive optics IFS that operates in the thermal infrared (3-5$\mu$m), where gas-giant planets peak in brightness, and various molecular features of exoplanets, circumstellar disks and Solar System bodies become accessible. The original implementation of ALES\cite{2015SPIE.9605E..1DS} comprised a small field-of-view lenslet array, and a single plate-scale and disperser setting. We are in the process of upgrading ALES to (1) increase its field-of-view, (2) add plate scales appropriate for seeing-limited, adaptive optics and interferometric observations, and (3) add new disperser modes with R$\sim$10-200 resolution across various bandpasses from 1.5-5 $\mu$m. The original implementation of ALES is described in Section 2. The upgraded ALES, which will be installed in Summer 2018, is described in Section 3. | 18 | 8 | 1808.03301 |
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1808 | 1808.08964_arXiv.txt | We present the discovery of a slowly-evolving, extragalactic radio transient, FIRST J141918.9+394036, identified by comparing a catalog of radio sources in nearby galaxies against new observations from the Very Large Array Sky Survey. Analysis of other archival data shows that FIRST J141918.9+394036 faded by a factor of $\sim$50 over 23 years, from a flux of $\sim$26 mJy at 1.4 GHz in 1993 to an upper limit of 0.4 mJy at 3 GHz in 2017. FIRST J141918.9+394036 is likely associated with the small star-forming galaxy SDSS J141918.81+394035.8 at a redshift $z=0.01957$\ ($d=87$ Mpc), which implies a peak luminosity $\nu L_\nu \gtrsim 3\times10^{38}$ erg s$^{-1}$. If interpreted as an isotropic synchrotron blast wave, the source requires an explosion of kinetic energy $\sim10^{51}$ erg some time prior to our first detection in late 1993. This explosion could plausibly be associated with a long gamma-ray burst (GRB) or the merger of two neutron stars. Alternatively, FIRST J141918.9+394036 could be the nebula of a newly-born magnetar. The radio discovery of any of these phenomena would be unprecedented. Joint consideration of the event light curve, host galaxy, lack of a counterpart gamma-ray burst, and volumetric rate suggests that FIRST J141918.9+394036 is the afterglow of an off-axis (``orphan'') long GRB. The long time baseline of this event offers the best available constraint in afterglow evolution as the bulk of shock-accelerated electrons become non-relativistic. The proximity, age, and precise localization of FIRST J141918.9+394036 make it a key object for understanding the aftermath of rare classes of stellar explosion. | \label{sec:intro} Multiple large radio interferometric surveys were conducted in the 1990s with sufficiently high spatial resolution to facilitate comparison to optical surveys. These first robust statistical samples enabled novel tests of the physics of high-redshift quasars \citep{2000ApJ...538...72B}, star-forming galaxies \citep{2004ApJS..154..147A}, Galactic pulsars \citep{2000ApJ...529..859K}, constraints on the beaming factors of gamma-ray bursts (GRBs) \citep{2006ApJ...639..331G}, and more. Today, a new generation of high-energy, optical, and radio sky surveys are being conducted with a focus on sensitivity to time-domain phenomena like supernovae and GRBs \citep{2004ApJ...611.1005G, 2009PASP..121.1395L, 2016ApJ...818..105M}. The Karl G.~Jansky Very Large Array (VLA) is in the midst of a sky survey whose design explicitly supports transient science \citep[``VLASS'';][]{vlass}. Transient surveys at optical and high-energies have revolutionized our understanding of relativistic transients like GRBs \citep{2006ARA&A..44..507W} and tidal disruption events \citep[TDEs;][]{2011Sci...333..203B,2012Natur.485..217G}, but they are less sensitive to the vast majority of events not beamed in our direction \citep{2001ApJ...562L..55F}. In contrast, radio emission traces the total kinetic energy ({\it calorimetry}) of the interaction of ejecta with the interstellar medium. This makes radio observations valuable for transient discovery and unbiased rate estimates \citep{2005ApJ...619..994F}. Even non-relativistic explosions, such as the hypothesized magnetar-powered supernovae \citep{2010ApJ...717..245K}, may produce radio emission at late times, once the supernova ejecta become transparent to free-free absorption and the birth nebula of the inner engine is revealed \citep{2016MNRAS.461.1498M,2017ApJ...841...14M,2017ApJ...850...55N,2018MNRAS.474..573O}. The recent association of a fast radio burst \citep[FRB;][]{2017Natur.541...58C} with a luminous, persistent radio source has suggested that such slowly-evolving radio transients may provide signposts for the discovery of past energetic events giving birth to FRB-sources \citep{2017ApJ...839L...3K, 2018arXiv180605690M}. Tremendous effort has been invested in blind radio transient surveys \citep{2010ApJ...719...45C, 2010ApJ...725.1792B, 2011MNRAS.415....2B, 2011ApJ...740...65O, 2013ApJ...768..165M, 2016MNRAS.458.3506R, 2017MNRAS.466.1944M}, but the requirements are severe. For example, orphan-GRB afterglows require a search at mJy-sensitivity, over $10^4$~deg$^2$, with multiple epochs over many years \citep{2002ApJ...576..923L, 2015ApJ...806..224M}. This class of radio survey is only becoming available today by comparison of VLA surveys from the 1990s \citep{1995ApJ...450..559B, 1998AJ....115.1693C} to the VLASS. \citet{2017ApJ...846...44O} collected a sample of galaxies with luminosity distance smaller than 108\,Mpc ($z=0.025$) and compared them to point sources in the VLA FIRST survey \citep{1995ApJ...450..559B}. He identified a set of radio sources with potential association to nearby galaxies and relatively high luminosities ($\nu L_\nu > 3\times 10^{37}$~erg~s$^{-1}$). Here, we describe the analysis of new data from VLASS and archival radio data that reveals that one of these sources, \srcfull\ (hereafter \src), is a luminous radio transient. \S\ref{sec:dis} describes how we discovered the transient in VLASS and \S\ref{sec:cha} presents a radio light curve compiled from multiple telescopes with detections spanning more than two decades. We also describe the properties of the host galaxy of \src, constraints on gamma-ray emission, and an estimate of the volumetric rate for \src-like transients. \S\ref{sec:ori} presents calculations and modeling of the radio data that suggests that \src\ is either the afterglow of an orphan GRB, or is the wind nebula produced in the aftermath of a magnetar powered supernova. We summarize the results and present future tests of this model in \S \ref{sec:con}. | \label{sec:con} We have discovered a decades-long, luminous radio transient, \srcfull, likely hosted by the dwarf galaxy SDSS~J141918.81+394035.8 at a redshift $z=0.01957$. The energy, timescale, host galaxy, and other properties of \src\ suggest that this source is most likely an off-axis long duration GRB. If so, the slow ($\sim t^{-1}$) decay of the radio afterglow confirms predictions for shock physics at late times \citep{ 2013ApJ...778..107S}. The energetics of the transient are also consistent with the ejecta from a binary neutron star merger, though the small host galaxy would be in tension with the hosts seen for short duration GRBs, which are typically $\sim 100$ times more massive. We also discuss the more speculative possibility that \src\ is a wind nebula produced in the aftermath of a magnetar-powered supernova. While the energy output of a newborn magnetar is poorly constrained by previous observations, such a scenario is also consistent with the luminosity, duration, and radio flux decay for \src. The detection of the radio afterglow of an orphan GRB would be the first of its kind \citep[see also][]{2013ApJ...769..130C, 2015ApJ...803L..24C} and improves the prospects for radio discovery of extragalactic transients. We note that \src\ was not identified as a transient or variable in comparisons of the FIRST and NVSS radio sky surveys \citep{2002ApJ...576..923L,2011ApJ...737...45O} because the two surveys happened to observe this slowly-evolving source within a few months of each other. This implies that published limits on radio transient event rates may be weaker than claimed for decades-long transients like \src. Furthermore, the brightness and proximity of \src\ implies that it has a relatively high volumetric rate, potentially in tension with predictions for orphan-GRB afterglows. We estimate that \src\ could have been detected if its peak brightness was as low as 4~mJy, equivalent to a 2.5$\times$\ larger distance. This implies that there may be an order of magnitude more such ``anti-transients'' to be found as FIRST sources that disappear in the VLASS and a comparable number of transients turning on in new observations. The chance of discovery will be improved with larger spectroscopic galaxy samples, better identification of non-nuclear radio sources, and integration of archival radio surveys with the search process. New searches for \src-like radio transients will improve the rate estimate and may reveal that it is inconsistent with that of orphan GRBs. Independent of the progenitor model, \src\ is the oldest and best-localized luminous radio transient. That makes it a good place to search for remnants, since the ejecta should be transparent to free-free radiation. An FRB search may find bursts regardless of whether it is an GRB afterglow or magnetar wind nebula. Milliarcsecond-resolution imaging could distinguish GRB from magnetar models by measuring the size of the late-time radio emission. New measurements of the late-time radio flux of \src\ are needed to better measure its late-time radio decay and to properly search for scintillation. The presence of refractive scintillation at GHz frequencies is sensitive to spatial structure on a size scale of 10 to 100 $\mu$as and would help distinguish between the afterglow and magnetar wind nebula models. | 18 | 8 | 1808.08964 |
1808 | 1808.01242_arXiv.txt | We present chemical abundances of 15 stars in the $\gamma$ Leo moving group based on high-resolution spectra with the Subaru High Dispersion Spectrograph. The sample was picked up by applying wavelet transform to UVW velocity components of stars in the solar neighbourhood. Both photometric and spectroscopic method have been used to determine the stellar parameters of stars. Abundances of 11 elements including Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Y and Ba are measured. Our results show that the member stars display a wide metallicity distribution with abundance ratios similar to Milky way disk stars. We presume that the $\gamma$ Leo moving group is originated from dynamical effects probably related to the Galactic spiral arms. | More and more complex substructures have been discovered in the Milky Way by recent digital sky surveys(\citet{ant12}; \citet{kle08}; \citet{zhao09}). It is well-known that the vicinal velocity field is clumpy and most of the observed overdensities are made of spatially unbound groups of stars, called moving groups. \citet{egg58} have defined and investigated many moving groups, supposing that moving groups are from dissolving open clusters. Later, many theoretical models suggest that the overdensities of stars in some regions of the Galactic velocity UV-plane may be a result of global dynamical mechanisms related to the nonaxisymmetry of the Galaxy \citep{fam05}, namely the presence of the bar \citep{kal91,deh00,fux01}, and/or spiral arms \citep{sim04,qui05}. Since the late 90s of last century, the bar has been believed to be short and fast rotating for long time. This was in very good agreement with the explanation of the Hercules moving group as being due to the bar's outer Lindblad resonance \citep{deh00}. However, recent photometric studies of the Galactic center have shown that the bar could be longer, reopening the debate on the bar's pattern speed and the origin of the Hercules moving group \citep{mon17,per17}. Nowadays, the origins of these moving groups are explained by different theories or hypotheses, such as cluster disruption, dynamical effects and accretion events. \citet{fre02} put forward the chemical tagging technique to reassemble the ancient stellar forming aggregates in the Galactic disk. Since then it has become popular to use detailed chemical abundances from high resolution spectroscopy to disentangle the mechanism that has formed a certain stream. For example, \citet{ben07} found a wide spread in the distributions of age and chemical abundances of the stars in the Hercules stream, and concluded that this group is compatible with being a dynamical feature. According to the homogeneity of the HR 1614 group in age and abundance, \citet{sil07} concluded that it is the remnant of a dispersed star-forming event. In the past, it is hard to determine the stellar members of moving groups due to the lack of parallaxes information, which will become available with Gaia survey. Combined with spectroscopic survey, like LAMOST, we can determine accurate velocity coordinates of stars in the solar neighbourhood. The $\gamma$ Leo (Leonis) moving group hasn't been closely analysed before. The $\gamma$ Leonis group were defined by \citet{egg59a,egg59b} by convergent point method. Its existence has been confirmed by \citet{sku97}. \citet{ant12}, using RAVE data, reidentified two peaks of the $\gamma$ Leo moving group in UV plane by wavelet transform which are confirmed by \citet{lia17} using Gaia-TGAS (\citet{pru16,bro16}) cross-matched with LAMOST DR3 \citep{cui12,zhao06,zhao12}. The objective of this paper is to trace the origin of the $\gamma$ Leo moving group by chemical tagging. Section 2 describes our sample and observational information about this sample. In Section 3, we discuss stellar parameters, chemical abundance and error analysis. The main results and discussions are given in Section 4. In the final section, we present conclusion of our work and expectation for the future. | We have observed 18 candidates of the $\gamma$ Leo moving group members selected by the \textit{UVW} criteria from the LAMOST survey. Three stars are spectroscopic binaries and excluded from the sample. For the remaining fifteen stars, a detailed abundance analysis is carried out. The abundance pattern of member stars shows no evident difference from those of comparision stars. The large dispersion of metallicity in member stars suggests that the $\gamma$ Leo moving group is not from some chemically homogeneous origins. We suppose the $\gamma$ Leo moving group is originated from dynamical effects, perhaps related to the effect of the spiral arms. For example, Figure 18 of \citet{ant11} shows that it is possible that spiral arms can generate a structure at this velocity region. However, small variations of the simulation parameter can produce very different velocity structures. In the future, we will do some dynamical simulations to better understand the origin the $\gamma$ Leo moving group. The Gaia's high precision astrometric data brings great convenience to the study of moving groups in the solar neighbourhood. Chemical abundances from high resolution spectra play an important role in disentangling the degeneracy of many causes determining the local velocity structures. | 18 | 8 | 1808.01242 |
1808 | 1808.01768_arXiv.txt | Observations of multiple rotational transitions from a single molecule allow for unparalleled constraints on the physical conditions of the emitting region. We present an analysis of CS in TW~Hya using the $J=7-6$, $5-4$ and $3-2$ transitions imaged at $\sim 0.5\arcsec$ spatial resolution, resulting in a temperature and column density profile of the CS emission region extending out to 230~au, far beyond previous measurements. In addition, the 15~kHz resolution of the observations and the ability to directly estimate the temperature of the CS emitting gas, allow for one of the most sensitive searches for turbulent broadening in a disk to date. Limits of $v_{\rm turb} \lesssim 0.1 c_s$ can be placed across the entire radius of the disk. We are able to place strict limits of the local H$_2$ density due to the collisional excitations of the observed transitions. From these we find that a minimum disk mass of $3 \times 10^{-4}~M_{\rm sun}$ is required to be consistent with the CS excitation conditions and can uniquely constrain the gas surface density profile in the outer disk. | To understand the planet formation process we must first understand the environmental conditions of planetary birthplaces \citep{Mordasini_ea_2012}. Thanks to the unparalleled sensitivity and resolution provided by the Atacama Large (sub-)Millimetre Array (ALMA), we are routinely resolving sub-structures indicative of in-situ planet formation and on-going physical processing \citep{ALMA_ea_2015, Andrews_ea_2016, Perez_ea_2016, Fedele_ea_2017, Dipierro_ea_2018}. Similar features have been observed in high-contrast AO near-infrared imaging tracing the small grain population well coupled to the gas \citep[e.g.][]{vanBoekel_ea_2017, Pohl_ea_2017, Hendler_ea_2018}. In addition, excess UV emission is interpreted as accretion onto the central star from the disk, with more massive disks accreting at a higher rate \citep{Fang_ea_2004, Manara_ea_2016}. These observations point towards active disks which are able to efficiently redistribute material and angular momentum. Despite the observational evidence for the redistribution of angular momentum, identifying the physical mechanisms which enables this remains elusive. Two scenarios are likely. Firstly, a turbulent viscosity would be sufficient to transport angular momentum outwards. This is the assumption in the frequently implemented `$\alpha$-visocity' disk model of \citet{Shakura_ea_1973}. Although agnostic about the main driver of the turbulence, this model links the turbulent motions to the viscosity of the disk. Alternatively, angular momentum can be removed through winds \citep{Bai_2017}, evidence of which has been observed in several young sources although is lacking in more evolved counterparts. The magneto-rotational instability has been a leading contender as the source for turbulence \citep{Balbus_Hawley_1998, Fromang_Nelson_2006, Simon_ea_2013, Simon_ea_2015, Bai_2015, Flock_ea_2015, Flock_ea_2017}. However estimates of the local ionization rate close to the disk midplane have suggested that there would be insufficient coupling between the rotating gas and the magnetic field \citep{Cleeves_ea_2015a}. Additional instabilities have been shown to generate turbulent without the need for ionization such as the vertical shear instability \citep{Nelson_ea_2013, Lin_Youdin_2015}, gravitational instabilities \citep{Gammie_2001, Forgan_ea_2012} baroclinic instabilities \citep{Klahr_Bodenheimer_2003, Lyra_Klahr_2011} and the zombie vortex instability \citep{Marcus_ea_2015, Lesur_Latter_2016}. Distinguishing between these mechanisms requires a direct comparison of the distributions of non-thermal motions observed in a disk and the predicted distribution from simulations \citep[for example:][]{Forgan_ea_2012, Flock_ea_2015, Simon_ea_2015}. There have been several attempts to detect non-thermal motions in disks through the additional broadening in line emission in the disks of TW~Hya and HD~163296 \citep{Hughes_ea_2011, Guilloteau_ea_2012, Flaherty_ea_2015, Flaherty_ea_2017, Flaherty_ea_2018, Teague_ea_2016}. Although a promising avenue of exploration, this approach is hugely sensitive to the temperature assumed as the Doppler broadening of the lines does not distinguish between the sources of the motions, either thermal or non-thermal. One must make assumptions about the physical structure of the disk in order to break these degeneracies \citep{Simon_ea_2015, Flaherty_ea_2015, Flaherty_ea_2017}. \citet{Teague_ea_2016} attempted to minimize the assumptions made about the disk structure, inferring physical properties directly from the observed spectra and allowing the temperature and turbulent structure to vary throughout the disk. Without the leverage of an assumed model, the constraints on $v_{\rm turb}$ were larger than other attempts, finding $v_{\rm turb} \lesssim 0.3~c_s$ across the radius of the disk. These constraints were limited by assumptions made about the thermalisation of the energy levels, in particular CN emission was shown to be in non-LTE across the outer disk, while for the single CS transition, as the line was optically thin, the degeneracy between column density and temperature could not be broken without assuming an underlying physical structure. However, high-resolution observations of the studied disks show substructures traced in mm-continuum, molecular line emission and scattered light \citep{Andrews_ea_2016, Flaherty_ea_2017, Teague_ea_2017, vanBoekel_ea_2017, Monnier_ea_2017}. Such perturbations from a `smooth' disk model could be sufficient to mask any signal from non-thermal motions which require a precise measure of the temperature to a few Kelvin. In this paper we present new ALMA observations of CS $J=7-6$ and $J=3-2$ transitions in TW~Hya, the nearest protoplanetary disk at $d = 60.1$~pc \citep{Bailer-Jones_ea_2018} with a near face-on inclination of $i \approx 6\degr$. Combined with the previously published $J = 5-4$ observations \citep{Teague_ea_2016}, we are able to fit for the excitation conditions of the molecule, namely the temperature, density, column density and non-thermal broadening component. | \label{sec:discussion} \subsection{Disk Physical Structure} \begin{figure} \includegraphics[width=\columnwidth]{figs/column_density_comparison.pdf} \caption{Comparing a power-law profile fit to the CS column density in gray to the derived values in blue. The bottom panel shows the relative residual between the two. \label{fig:column_density_comparison}} \end{figure} Both LTE and non-LTE approaches paint the same picture: CS is present across the entire extent of the disk in a region which slowly cools from $\approx 40$~K in the inner disk to $\lesssim 20$~K at 200~au. One interpretation for this is that the CS layer is bounded by the CS `snow surface' \citep{Schwarz_ea_2016, Loomis_ea_2018}. The binding energy of CS is 1900~K resulting in a desorption temperature of $T_{\rm desorb} \approx 31$~K, although dependent on local gas pressures which are unknown, comparable to the temperatures observed for CS. However, a clear boundary between gaseous and ice forms of CS may not be present due to the chemical reprocessing expected on the grain surfaces. Observations of high inclination disks would be able to identify whether the CS emission has a sharp lower boundary. The column density profiles show two distinct knee features at 90~au and 160~au, both seen in the $T_B$ profiles in Fig.~\ref{fig:radial_profiles}. Figure~\ref{fig:column_density_comparison} compares the column density derived in Section~\ref{sec:NonLTE_Modelling} with a power-law profile fit (shown by the black line). The residuals, shown in the lower panel, shows deviations of up to 20\% in $N({\rm CS})$ at 120 and 160~au. Despite these deviations, models assuming a simple power-law column density, such as those in \citet{Teague_ea_2016}, would be able to adequately model the true column density profile. The dip at 90~au was previously argued by \citet{Teague_ea_2017} to be due to a significant perturbation in the gas surface density needed to account for a gap traced the scattered light \citep{vanBoekel_ea_2017}. While these results confirm that the emission feature is due to a change in column density rather than temperature, they are unable to distinguish between a local change in CS abundance and a total depletion of gas. Similar features have been observed in high resolution $^{12}$CO observations \citep{Huang_ea_2018}. At the outer edge of the disk the density drops to a sufficiently low value that, unlike inwards of $\approx 190$~au, the volume density of H$_2$ can be constrained. The apparent constraints inwards of 90~au are, as discussed before, an artefact of the limited angular resolution. Observations of higher frequency lines \added{with higher critical densities} will allow for this method to be sensitive to \added{the} higher densities \added{of the inner disk and allow us to extend these surface density constraints} \deleted{, extending the region where the density is constrained} to smaller radii. \subsection{Turbulence} \begin{figure} \includegraphics[width=\columnwidth]{figs/turbulence.pdf} \caption{Top: Upper limits on $\Delta V$ compared to the observed values which span the gray shaded region. Bottom: $2\sigma$ upper limits on the non-thermal broadening. The shaded regions show the limits from \citet{Teague_ea_2016} and \citet{Flaherty_ea_2018}. \label{fig:turbulence}} \end{figure} Determination of the non-thermal broadening requires an accurate measure of the local temperature in order to account for the thermal contribution. By constraining the temperature through multiple transitions minimizes assumptions about the thermal structure and provides \deleted{provides} the most accurate measure of the gas temperature to date. \added{As the CS lines are optically thin, this temperature will be the contribution function-weighted average of the emitting column.} With our derived temperature profile we are therefore able to derive spatially resolved limits on the required non-thermal broadening to be consistent with the data. For TW~Hya, multiple studies have been already been undertaken \citep{Hughes_ea_2011, Teague_ea_2016, Flaherty_ea_2018}, finding a range of non-thermal broadening values and upper limits, $\mathcal{M} \lesssim 0.4$. As discussed in \citet{Flaherty_ea_2018}, differences in these limits are primarily driven by the different assumptions about the underlying thermal structure and how this couples to the density structure. \citet{Teague_ea_2016} caution, however, that constraints of $\mathcal{M} \lesssim 0.03$ requires constraining the thermal structure to near Kelvin-precision, a limit which is achieved with the data presented in this manuscript. Under the assumption that the presented three CS lines arise from the same vertical layer in the disk, an assumption which requires observations of edge-on disks to test, we are able to remain agnostic about the thermal and physical structure of the disk. As shown in the bottom panel of Fig.~\ref{fig:turbulence}, we are able to place a $2\sigma$ upper limit out to 230~au. Both approaches (linear and logarithmic fits of $\mathcal{M}$ in red and blue, respectively) yield comparable results. The rise in limits inwards of 100~au is due to the broadening arising due to the beam smearing, while outside 180~au the limits increase due to the lower SNR of the data. The fits yields limits consistent with the values found by \citet{Flaherty_ea_2018}, $\mathcal{M} \leq 0.13$. These are a factor of a few lower than \citet{Teague_ea_2016} due to the warmer temperature derived ($T_{\rm kin} = 28$~K at 100~au compared to 12~K as in \citet{Teague_ea_2016}) as only a single CS transition was available. Two dips are observed in the profiles at $110$~au and 165~au, consistent with the dips in the column density. The resulting linewidths, plotted in the top panel of Fig.~\ref{fig:turbulence} show that they yield comparable widths to the observations, however over-produce the linewidth outside 200~au, likely due to the lower SNR of the data. We leave interpretation of these features for future work. Deriving limits for $\mathcal{M}$ using a parametric modelling approach, where physical properties and chemical abundances are described as analytical functions, as in \citet{Guilloteau_ea_2012}, \citet{Flaherty_ea_2015, Flaherty_ea_2017, Flaherty_ea_2018} and \citet{Teague_ea_2016} requires a well constrained molecular distribution which for CS is not known. Furthermore, there is mounting evidence that spatially varying abundances of C and O within the disk can radially alter the local chemistry \citep{Bergin_ea_2016}, further limiting the accuracy of a simple analytical prescription. \subsection{Minimum Disk Mass} \begin{figure} \includegraphics[width=\columnwidth]{minimum_disk_mass.pdf} \caption{Inferred minimum gas surface density and resulting minimum disk masses using the derived $n({\rm H_2})$ values. The blue line shows the results when fitting for $\log_{10}\mathcal{M}$ and the red when fitting for $\mathcal{M}$. The gray shaded region shows where $n({\rm H_2})$ has been constrained. The dotted and dot-dash lines show the surface density profiles used to model the HD emission in \citet{Bergin_ea_2013} \citep[using the surface density from][]{Gorti_ea_2011} and \citet{Trapman_ea_2017}. The dashed line shows the surface density used by \citet{vanBoekel_ea_2017} to model the scattered light emission profile. \label{fig:minimum_disk_mass}} \end{figure} \added{Many studies have attempted to measure the mass of the TW~Hya disk through observations of both the mm~continuum and gas emission lines. With differing assumptions these have resulted in a range of masses spanning $5 \times 10^{-4}~M_{\rm sun}$ to $6\times10^{-2}~M_{\rm sun}$ \citep{Weintraub_ea_1989, Calvet_ea_2002, Thi_ea_2010, Gorti_ea_2011, Favre_ea_2013}. Arguably the most accurate approach is to use hydrogen deuteride, HD, as this molecule should be tracing the H$_2$ gas most closely. Modelling of the HD $J=1-0$ transition, \citet{Bergin_ea_2013} concluded that the mass of the disk must be $M_{\rm disk} > 0.05~M_{\rm sun}$. More recently, \citet{Trapman_ea_2017} used additional observations of the $J = 2-1$ transition to find a mass of between $6\times10^{-3}$ and $9\times10^{-3}~M_{\rm sun}$.} This large range is due primarily to the sensitivity of the HD emission to the assumed thermal structure and differences in assumed the cosmic D/H ratio. Models of HD emission show that it is almost entirely confined to the warm inner disk, $r < 100$~au, where the gas is warm enough to sufficient excite the fundamental transition. Although this region accounts for almost all the disk gas mass, the HD flux is insensitive to the cold gas reservoir at smaller radii and is thus a minimum disk mass. As we have limits on the requied H$_2$ density as a function of radius in Section~\ref{sec:NonLTE_Modelling}, we are able to place a limit on the minimum gas surface density and thus disk mass needed to recover the inferred excitation conditions. It is important to note that this technique does not require the assumption of a molecular abundance, such as those using HD, but rather constrains the H$_2$ gas directly through collisional excitation. It therefore provides an excellent comparison for techniques which aim to reproduce emission profiles and provides a unique constraint for surface densities at large radii in the disk. To scale a midplane density to a column density we assume a Gaussian vertical density structure, \begin{equation} \rho_{\rm gas} = \frac{\Sigma_{\rm gas}}{\sqrt{2 \pi} H_{\rm mid}} \times \exp \left( - \frac{z^2}{H_{\rm mid}^2} \right) \end{equation} \noindent where we take the pressure scale height, \begin{equation} H_{\rm mid} = \sqrt{\frac{k T_{\rm mid} r^3}{\mu m_H G M_{\star}}}, \end{equation} \noindent which is dependent on the assumed midplane temperature, $T_{\rm mid}$. Observations of the edge-on Flying Saucer have shown CS emission to arise from $\lesssim 1 H_{\rm mid}$ \citep{Dutrey_ea_2017}. The angular resolution of these observations do not allow for distinction between the case of two elevated, thin molecular layers at $\pm 1 H_{\rm mid}$ or a continuous distribution below $H_{\rm mid}$. Measurements of the midplane temperature estimate this to be 5 -- 7~K for the mm dust \citep{Guilloteau_ea_2016} and $\approx 12$~K for the gas \citep{Dutrey_ea_2017}. As we find $T_{\rm kin} = 20$ -- 35~K, this suggests that CS is not tracing the midplane, but rather a slightly elevated region, so that we would overestimate the pressure scale height and underestimate the midplane density. In combination, these uncertainties should mitigate one another allowing for a first-order estimation of the minimum surface density. For this estimate we use both fits from Section~\ref{sec:NonLTE_Modelling} for the minimum $n({\rm H_2})$ to infer a minimum $\Sigma_{\rm gas}$, shown in Figure~\ref{fig:minimum_disk_mass}. Integrating these minimum surface densities we find an average minimum disk mass of $3\times10^{-4}~M_{\rm sun}$, fully consistent with the estimates from HD emission. Observations of transitions which thermalise at higher densities would extend the sensitivity of this approach such that it can distinguish between models predicting different disk masses. The shaded region at $r > 190$~au highlights the region where $n({\rm H_2})$ was measured rather than a lower limit constrained. In this region we expect the plotted minimum surface densities to be close to the $\Sigma_{\rm gas}$ value rather than just a minimum value, however the accuracy will be limited by the assumptions made above the vertical structure of the disk. Nonetheless, these profiles provide unique constraints on the gas surface density in the outer regions and are highly complimentary to studies using optically thin CO isotopologues which trace $\Sigma_{\rm gas}$ within the CO snowline \citep{Schwarz_ea_2016, Zhang_ea_2017}. In Figure~\ref{fig:minimum_disk_mass} we additionally plot surface density profiles from \citet{Gorti_ea_2011}, used in \citet{Bergin_ea_2013} to model the HD flux, the best-fit profile from \citet{Trapman_ea_2017}, also used to model the HD flux and finally the profile from \citet{vanBoekel_ea_2017} used to model scattered light emission. From our lower limit we are able to rule out the model from \citet{Gorti_ea_2011} which contains insufficient material in the outer disk to recover the excitation conditions required by the CS transitions. The profile from \citet{Trapman_ea_2017} is broadly consistent with the lower limits, however would likely not suffice if the H$_2$ densities inwards of 190~au were constrained and would likely become inconsistent when the height of the CS emission surface was taken into account. Therefore the model from \citet{vanBoekel_ea_2017} provides the most consistent profile in the outer disk. This is not surprising as this profile was found by fitting the radial profile of scattered light out to $\sim 200$~au, while the previous two surface density profiles were inferred from integrated flux values. Although uncertainties in $M_{\star}$, $H_{\rm mid}$, $H_2$ ortho to para ratios and the location of the emission will propagate into the minimum mass, these are negligible compared to the lack in sensitivity of this approach to $n_{\rm H_2} \gtrsim 10^7~{\rm cm^{-3}}$ due to the thermalisation of the $J=7-6$ transition. For commonly assumed power-law surface density profiles, 95\% of the disk mass is within 80~au for TW~Hya, far interior to where this technique is sensitive. Observations of higher density tracers, such as the $J = 9-7$ transition of CS would enable tighter constraints of $\Sigma_{\rm gas}$ at a larger range of radii. | 18 | 8 | 1808.01768 |
1808 | 1808.10381_arXiv.txt | We present an analysis of the kinematics of a sample of 14 galaxy clusters via velocity dispersion profiles (VDPs), compiled using cluster parameters defined within the X-Ray Galaxy Clusters Database (BAX) cross-matched with data from the Sloan Digital Sky Survey (SDSS). We determine the presence of substructure in the clusters from the sample as a proxy for recent core mergers, resulting in 4 merging and 10 non-merging clusters to allow for comparison between their respective dynamical states. We create VDPs for our samples and divide them by mass, colour and morphology to assess how their kinematics respond to the environment. To improve the signal-to-noise ratio our galaxy clusters are normalised and co-added to a projected cluster radius at $0.0 - 2.5$ $r_{200}$. We find merging cluster environments possess an abundance of a kinematically-active (rising VDP) mix of red and blue elliptical galaxies, which is indicative of infalling substructures responsible for pre-processing galaxies. Comparatively, in non-merging cluster environments galaxies generally decline in kinematic activity as a function of increasing radius, with bluer galaxies possessing the highest velocities, likely as a result of fast infalling field galaxies. However, the variance in kinematic activity between blue and red cluster galaxies across merging and non-merging cluster environments suggests galaxies exhibit differing modes of galaxy accretion onto a cluster potential as a function of the presence of a core merger. | \label{sec:intro} Galaxies are known to follow a morphology-density relation, which is pronounced in clusters of galaxies (\citealt{Oemler1974}; \citealt{Dressler1980}; \citealt{Smith2005}). Late-type galaxies are found to dominate at large radii from a galaxy cluster centre, predominantly within the field population. Conversely, early-type galaxies are found to pervade the denser regions at smaller radii, well within galaxy clusters. There are further observable environmental side-effects that follow similar patterns, such as the apparent bimodality of the colour-density relation \citep{Hogg2003,Hogg2004}, where denser regions are populated with quenched, red and elliptical galaxies. Contrarily star-forming, blue and spiral morphologies are found out towards the field population (e.g. \citealt{Lewis2002}; \citealt{Gomez2003}; \citealt{Bamford2009}). Galaxy clusters are consequently an epicentre for environmental interactions. The comparative accretion histories of cluster galaxies between galaxy clusters and the field population can be determined as a function of their environment, indicated by their membership's morphology, colour and star-formation assuming a fixed stellar mass (e.g. \citealt{Postman1984}; \citealt{Hogg2004}; \citealt{Linden2010}). The evolutionary transformation of cluster galaxies could have transpired prior to a galaxy's accretion onto a cluster's potential, since the field population's morphologies, colours and rate of star-formation are mixed (e.g. \citealt{Kauffmann2004}; \citealt{Blanton2005}). Or, it is possible that the harassment and accretion of a galaxy by a cluster leads onto a transformation of blue to red; star-forming to non-star-forming; spiral to elliptical \citep{Moore1996}. This metamorphosis during the infall of a galaxy into a cluster is considered to be the result of an increased probability of tidal galaxy-galaxy interaction mechanisms, or, even the tidal field of the cluster itself. The former being more likely to give rise to the stripping of material, and distortion of a galaxy's structure \citep{Moore1999}. Further observations ostensibly show the shifting of morphologies from late-type to early-type are chiefly to be the result of mergers between two galaxies (e.g. \citealt{Owers2012}). The volume between cluster galaxies contains a sea of hot diffuse gas that represents an intracluster medium (ICM), another form of environmental interaction. An infalling galaxy approaching a cluster centre at higher velocities relative to the ICM will experience ram pressure stripping (\citealt{Gunn1972}; \citealt{Abadi1999}; \citealt{Quilis2000}; \citealt{Sheen2017}). The disc of cold gas surrounding an infalling galaxy will be stripped away over small timescales, however, as the ICM density increases during infall so do the time scales of this process \citep{Roediger2007}. The result of this process retards rates of star-formation to where the infalling galaxy will be quenched completely. The diffuse nature of any hot gas haloes surrounding infalling galaxies lends to their increased likelihood of being ejected from the galaxy's potential. Therefore, the removal of any surrounding haloes of hot gas around an infalling galaxy will inhibit the replenishment of their cold gas reservoirs through radiative cooling, slowly strangling galaxy star-formation, with any remaining cold gas being exhausted \citep{Larson1980}. Ram pressure stripping has been found to be prevalent in the dense cores of clusters through observations of tails with \ion{H}{I} and H$\alpha$ emission lines that are associated with a parent galaxy (e.g. \citealt{Gavazzi2001}; \citealt{Cortese2007}). With galaxy cluster environments hosting extended ICM haloes that interact significantly with field and infalling galaxies, consideration of a cluster's size is therefore needed in order to understand where the boundary between these environments lie. One common definition of a cluster's size is the virial radius, commonly approximated as $R_{vir}\sim r_{200}$. $r_{200}$ represents the radial point at which the average density is $\sim200$ times the critical density (e.g. \citealt{Linden2010}; \citealt{Pimbblet2012}; \citealt{Bahe2013}; \citealt{Pimbblet2014}). However, the proposed splashback radius may represent a more physical boundary, extending farther than $r_{200}$ (e.g. \citealt{More2015}; \citealt{More2016}; \citealt{Baxter2017}). The splashback radius represents the first apoapsis of an observed accreted galaxy that has already passed through its first periapsis or turnaround (\citealt{Sanchis2004}; \citealt{Pimbblet2006}). Despite both of these definitions for a potential cluster boundary, they do not extend to the radii observed with harassed galaxies infalling to the cluster centre; colour-densities and effects on star-formation can continue beyond these defined boundaries (e.g. \citealt{Balogh1999}; \citealt{Haines2009}; \citealt{Linden2010}; \citealt{Haines2015}). A plethora of observations and simulations appear to indicate that there is a natural fluidity between the local cluster environment and the field population of late-type star-forming galaxies. Such simulations have shown the entire cluster boundary to expand even grander scales with ICM haloes extending out to radii of $\sim10$ Mpc from the cluster centre \citep{Frenk1999}. The existence of these large-scale structures therefore indicates the presence of smaller scale clumping of galaxies; more layers of substructure within galaxy clusters are expected (\citealt{Dressler1988}). It is more likely that any accreted galaxies from the field population will undergo \textquoteleft pre-processing' into smaller galaxy groups that help form the substructure within a cluster (e.g. \citealt{Berrier2009}; \citealt{Bahe2013}), inducing evolutionary changes prior to traditional cluster galaxy infall and accretion. In the simulation work of \cite{Haines2015} it is found that star-forming galaxies are unexpectedly quenched at large radii from the cluster centre, models can only account for this if the galaxies have undergone pre-processing into a substructure prior to any further interaction. There is an alternative variant of pre-processing in rarer cluster-cluster merger events, the most famous example of such an event is the Bullet Cluster \citep{Tucker1998}. X-ray observations of the Bullet Cluster show a smaller sub-cluster of galaxies colliding with a larger cluster, thereby ram-pressure stripping causing the removal of the surrounding hot gas \citep{Markevitch2002}. Other \textquoteleft bullet-like' events are shown to effect the local galactic environment in equivalent ways (e.g. \citealt{Owers2011}; \citealt{Owers2012}). This leads on to potential ways to make a comparison between these different environments via their varying dynamical states. We can therefore probe the variation in cluster environments via analysis of the cluster kinematics as a function of radius with Velocity Dispersion Profiles. VDPs represent how the radial velocity dispersions vary from the dense area of accreted early-type galaxies within $r_{200}$, out to sparser star-forming late-types on their infall journey to the centre (see \citealt{Hou2009,Hou2012}). It is therefore possible to test how the shape of a VDP is affected by binning a profile based on different cluster galaxy properties. As an example, \cite{Pimbblet2012} splits the VDP of Abell 1691 into individual high and low mass profiles. It is found that there is a large disparity in the velocities between the high and low mass samples, \cite{Pimbblet2012} argues the large high mass sample velocities could be due to the presence of substructure, or recent arrivals to the system. The shape of the VDP could, however, be affected by any evolutionary change due to the cluster environment. In this work, we aim to test how the average cluster VDP's shape can be altered as a function of radius, parameterised by its member's different evolutionary stages through proxies of varying masses, colours and morphologies, in order to explore the varying dynamics between merging, dynamically active and non-merging, relaxed environments. We therefore present galaxy data taken from the Sloan Digital Sky Survey (SDSS) to form a membership from a defined cluster sample determined from an X-ray catalogue. Details on how the data was acquired can be found in section~\ref{sec:data}. Details on the derivation and production of the VDPs can be found in section~\ref{sec:vdp}. A discussion of the data, results and their consequences are outlined in section~\ref{sec:disc}, followed by a summary of our conclusions in section~\ref{sec:conc}. Throughout the work presented here we assume a $\Lambda$CDM model of cosmology with $\Omega_{M} = 0.3$, $\Omega_{\Lambda} = 0.7$, $H_{0} =$ $100h$ km s$^{-1}$ Mpc$^{-1}$, where $h = 0.7$. | \label{sec:disc} The work presented here shows that across all intrinsic galactic parameter splits, the merging samples possess some form of rising profile. \cite{Hou2009} argues that such a rise indicates an interacting, or merging, system based on a correlation between a sample of non-Gaussian galaxy groups, coinciding with previous work by \cite{Menci1996}. However, these earlier works did not explicitly delineate which class(es) of galaxy are driving this. \subsection{Interpreting the VDPs} \label{sec:ivdp} When analysing the \textquoteleft All Galaxies' profiles for each split of the merging stacks, it can be deduced that these results seemingly back the argument made by \cite{Menci1996} and \cite{Hou2009}. With the non-merging samples generally showing a flat-to-declining series of profile. These results could corroborate recent work by \cite{Mulroy2017} that finds different cluster evolutionary histories must have played a part to explain the prominent colour variation observed in non-merging systems compared to that of merging systems. \cite{Deshev2017} is consistent with this, observing a significant decrease in the fraction of star-forming galaxies in the core of the merging Abell 520 system compared against their non-merging sample, with evidence of a smaller group of galaxies, possessing a higher fraction of star forming galaxies, feeding the merger. One explanation for this observation suggests a non-merging galaxy cluster is formed on long timescales by their haloes inducing the infall, and accretion, through harassment of galaxies from the surrounding field population that leads to the gradual variation from red to blue colours with increasing radius from the centre seen in \cite{Mulroy2017}. Whereas the merging systems are formed primarily from the accretion of pre-processed galaxy groups, meaning the galaxies have undergone heavy interactions leading to evolutionary changes, and are virialised to their local groupings. We find the red populations of the merging stacks are the main contributors to the rising profiles, which illustrates a common and significant amount of interactions occurring at $\sim1.5$ $r_{200}$ radii. Although, consideration should be taken into account that red galaxies could overshadow the total colour distribution of the cluster galaxy sample by numbers alone due to the Malmquist bias \citep{Malmquist1925}, along with the making the sample complete, thereby impeding a true indication on how these two sub-populations behave kinematically. In comparison to the non-merging profiles that clearly illustrate a more relaxed environment with a possible suggestion of infalling blue galaxies, this married with the merging profiles showing the dominant driver of the rising profile shape to be a mix of red and blue elliptical sub-populations. The diverse dynamics between merging and non-merging systems provide further affirmation to the idea of a galaxy infall and accretion bi-modality between merging and non-merging systems. Considering the epochs of differing events that occur in a typical cluster (e.g. infall, accretion, splashback), we can use the timescales between them to try and infer the current physical processes occurring and how they relate to their kinematics. \cite{Haines2015} simulate the accretion paths of multiple galaxies onto a massive cluster from various epochs and classify the infall regions to start $\lesssim 10 h^{-1}Mpc$, or $\lesssim 5 r_{200}$. It is calculated that the timescales from infall to accretion to be $\sim 4$Gyr, a galaxy then becomes accreted once it reaches $r_{200}$ and passes its first pericentre on timescales of $0.5-0.8$Gyr, followed by a significantly slower of $2-3$Gyr for the galaxy to reach its first apocentre (splashback radius). Collectively, the VDPs demonstrate a period of infall in the merging stacks at $\leq r_{200}$, alongside a culmination of interactions occurring as a result of the domination of pre-processed groups. This is corroborated with the merging colour and morphology VDPs, where mixed blue and red populations of galaxies assumed to be undergoing pre-processing are infalling to be accreted onto the cluster, reaffirming the suggestion by \cite{Haines2015} that pre-processing is required to explain star formation being quenched at larger radii from the cluster centre. Furthermore, the VDPs representing spiral morphology could be indicating the galaxies at $\gtrsim1.25$ $r_{200}$ are the start of a $\sim4$Gyr long journey onto the cluster potential, leading to their accretion and possible splashback, thus accounting for the larger surface density of spirals at smaller clustocentric radii (see \citealt{Wetzel2012}; \citealt{Haines2015}; \citealt{Cava2017}). In any case, there are increasingly more observations and simulations that appear to occasionally contradict, where many authors suggests a need for pre-processing (\citealt{Haines2015}; \citealt{Roberts2017}; \citealt{Carvalho2017}). \cite{Mulroy2017} argues for a bi-modality on infall and accretion histories with similar accretion rates, one with pre-processing and one without, in order to explain the variations in colour found in non-merging systems. Further simulations could possibly help to build on this picture for these bi-modal, kinematic outcomes. \subsection{Phase-Space Caustics} \label{sec:psc} In Section \ref{sec:data} we calculate velocity dispersions through a biweight method \citep{Beers1990} and the phase-space surface caustics to determine cluster membership (\citealt{Diaferio1997}; \citealt{Diaferio1999}). The phase-space caustics produced from the chosen methodology follow a trumpet-shape pattern as we move away from the cluster centre, which is a result from galaxies infalling onto the cluster when the potential inundates the Hubble flow \citep{Regos1989}. \cite{Diaferio1997} and \cite{Diaferio1999} both demonstrate the amplitudes of these surface caustics to be a product of random non-radial motions from substructuring, indicating a diverging caustic to be illustrative of a cluster with increasing interactions. Therefore, these caustics represent an escape velocity of the cluster potential. The key benefit, aside from powerfully indicating cluster boundaries, is that these caustics can be produced on redshift data alone. Unlike the rest of the literature, we allow the surface caustics to stretch to a $\Delta$V velocity limit of $\pm 1500$ kms$^{-1}$. This is to allow infallers to be added into the sample of cluster galaxies for each cluster, although, we wish to note that this method involves the risk of adding interloping larger scale structures to the sample. Many of the clusters compiled within this sample have been well studied, with calculated surface caustics and velocity dispersions. Reference values for the latter are presented in both Tables \ref{tab:sub} and \ref{tab:nsub}. The calculated $\sigma_{r_{200}}$ velocity dispersions are fairly consistent with the reference literature, however, there will be differences dependent on which method was used to estimate the velocity dispersions, at what radial point and how many galaxies are available for the membership of the cluster at $\leq r_{200}$ in this work. What follows is a comparison of our phase-space surface caustic analysis with that of the literature. \subsubsection{Abell 85} Abell 85 is a well studied cluster, with multiple calculations of its dispersion of velocities, along with phase-space surface caustics presented within \cite{Rines2006}. The value of $\sigma_{r_{200}}$ from this work is $\sim200$ kms$^{-1}$ offset from the calculated literature values. The primary driver of this offset is due their cluster membership being significantly greater with 497 galaxies within 1.7 $r_{200}$ compared to 234 galaxies within 2.5 $r_{200}$ from the data used here. The vast difference in galaxy membership can induce a slight alternate shape between the resultant surface caustics. \cite{Agulli2016} do not publish the surface caustics on their phase-space diagrams, leaving the surface caustics of \cite{Rines2006}, which indicate a strong constraint in $\Delta V$-space at low radii. Despite the lack of sharp, sudden changes in the surface caustic with increasing R, there are still similarities in the membership from the caustic presented here against that of \cite{Rines2006}. This indicates there is consistency between the two independent calculations of the caustic surface that allows for a more liberal inclusion of galaxies into the membership. \subsubsection{Abell 119} Abell 119 possesses multiple surface caustics in the literature alongside calculations of their velocity dispersions (\citealt{Rines2003}; \citealt{Rines2006}). There is, again, an offset of $\sim100$ kms$^{-1}$ in the calculation of $\sigma_{r_{200}}$, for which similar reasoning is applied from that of our discussion on Abell 85; the radial point at which the velocity dispersion is calculated can push the gaps between the literature further. Additionally, the techniques used for calculating the velocity dispersion from this work varies from that of \cite{Rines2006}, where sigma clipping is used \citep{Zabludoff1990}, this will lead to an underestimating of the velocity dispersion when directly compared to a biweight estimator. The phase-space caustics are the most consistent with the CAIRNS cluster study of \cite{Rines2003}, with very similar profiles. These caustics only deviate where there are discrepancies in the number of galaxies within $\leq10$ Mpc $h^{-1}$. The recalculated caustics presented in \cite{Rines2006} focus on constraining the cluster membership by limiting galaxies in $\Delta V$-space to $\leq 1000$ kms$^{-1}$, creating a surface caustic that is not as smooth, but is effective in the elimination of infallers and the encompassing large scale structure. \subsubsection{Abell 426} Abell 426, commonly known as the \textquoteleft Perseus cluster', does not presently possess any phase-space caustic analysis in the literature. Although, the phase-space surface caustics determined here are relatively simple, and the population of galaxies accumulated does not extend beyond $\sim2$ Mpc$h^{-1}$, providing a smooth distribution with several groupings of member galaxies. The limited and immediate break in the available data, due to the survey's limitations in observing the north galactic cap, lends to an artificial increase in the VDP at larger radii. However, this affect should be reduced when stacked against the other clusters that extend beyond the projected radii of Abell 426. The velocity dispersions of Abell 426 determined within this work are not consistent with those determined within the literature, showing an offset of $\sim500$ kms$^{-1}$ \citep{Struble1999}. The lack of consistency is a result of the significant loss of galaxy members compared to the true scale and size of Abell 426, which contains close to $\sim1000$ galaxy members. \subsubsection{Abell 1650} Abell 1650 is an atypical cluster with a radio quiet cD cluster galaxy at its centre. The surface caustics presented in the literature follow \citep{Rines2006} a similar shape and profile to our surface caustics, with a slight difference to the radial cut used on the sample of galaxies and a wider velocity window to allow for the addition of galaxy infallers. The velocity dispersions produced within this work are consistent with those of \cite{Einasto2012}, within a slight discrepancy of $\sim200$ kms$^{-1}$. Although, the discrepancy in these values is expected due to differing methods used in calculating the dispersion. \subsubsection{Abell 1656} Abell 1656, commonly referred to as \textquoteleft Coma', is a well studied cluster with close to $\sim1000$ members. It has such a strong presence within the literature primarily due to its relatively close proximity (z $\sim0$), which results in a greater sacrifice of cluster galaxies when maintaining completeness. However, this is offset by the extremely high number density of cluster galaxies. The phase-space caustics of the coma cluster presented in this work are the most consistent with \cite{Sohn2017}, this is the result of a more relaxed $\Delta V$-space limit to accommodate the very large nature of the cluster. This consistency is lost at $\sim$ 4 Mpc $h^{-1}$ due to a sudden drop in galaxies present within our MPA-JHU sample. However, an assumption can be made based the consistency is valid due to the trend of the caustic profile following that of \cite{Sohn2017} closely. The same consistency exists for the values of the velocity dispersion with very small offsets when compared to values from the literature (\citealt{Rines2003}; \citealt{Sohn2017}). \subsubsection{Abell 1750} Abell 1750 is a complex triple subcluster system in a pre-merger state, which is briefly discussed in \ref{sec:subs}. The phase-space surface caustics presented here are the most consistent with produced by \cite{Rines2006}, with the exception of allowing infallers at $\sim$2 Mpc $h^{-1}$ to form the cluster membership. The literary values of the velocity dispersion show a discrepancy of $\sim100$ kms$^{-1}$ from the values calculated in this work (\citealt{Rines2006}; \citealt{Einasto2012}). What does remain consistent is the reasoning that alternative, less robust, methods were used to calculate a value for $\sigma$. As well as this, there is a lack of clarity on the exact methodology used to calculate the dispersions of velocities within some of the literature where alternative limits could have been used within their calculations that are otherwise unstated. \subsubsection{Abell 1795} Abell 1795 is a cool core galaxy cluster with an unusually large cavity with no counterpart \citep{Walker2014}. There is currently no phase-space surface caustic analysis within the literature that can be aided to check consistency. However, from our own determined caustics we can see there is a roughly even distribution of member galaxies close to the centre of the cluster, as expected from a typical relaxed cluster. Our calculated velocity dispersion is consistent with those values found in the literature (\citealt{Zhang2011}; \citealt{Einasto2012}). \subsubsection{Abell 2029} Abell 2029 is a massive cluster that possess a powerful cD galaxy at its centre, forming part of a supercluster with complex dynamical interactions within the ICM \citep{Walker2012}. \cite{Sohn2017} has produced surface caustics of Abell 2029 that are inconsistent with our own. There are gaps in the galaxy population size within the phase-space diagram due to the redshift limitations of the MPA-JHU DR8 data. These limitations make our data incomplete for this cluster, whereas \cite{Sohn2017} has used complementary sets of data, and therefore, does not possess the same restrictions as those found in this work. However, the bulk of the galaxies present within the imposed limits of this work match those defined as members within the phase-space surface caustic diagrams of \cite{Sohn2017} that include infallers. The calculated velocity dispersion is calculated in this work is consistent with other determined values within the literature despite the variances in galaxy membership. \subsubsection{Abell 2061} Abell 2061 is a double subcluster system with complex dynamics that is in close proximity to Abell 2067, this is highlighted in more detail in \ref{sec:subs}. The comprehensive CIRS survey by \cite{Rines2006} presents consistent phase-space surface caustics when in consideration for the discrepancy in the range of velocities used. The only discrepancy of note is the the presence of strong foreground substructuring at $\sim3.5$ Mpc $h^{-1}$ inducing the caustic profile to maintain a consistent velocity of $\sim1000$ kms$^{-1}$, which causes the VDP to slight increase beyond the $\sigma_{r_{200}}$ values. The literary values for Abell 2061's velocity dispersion are consistent with our own where \cite{Pearson2014} presents an offset of $\sim100$ kms$^{-1}$, however, this is primarily due to the tighter distribution of galaxies, as well as differing methodologies for calculating the dispersion. \subsubsection{Abell 2065} Abell 2065, at present, does not have any detailed phase-space analysis within the literature for direct comparison. However, from our own analysis, Abell 2065 possesses what appears to be a strong bi-modal distribution, which can be attributed to a complex dynamical system of multiple substructures. This would provide consistency, since Abell 2065 is stated in the literature to possess an unequal core merger, for which the full nature of this is detailed in \ref{sec:subs}. We believe the relatively flat velocity offset at $\sim$ $-2000$ kms$^{-1}$ with increasing R to be the smaller of the two cores. The state of initial merger makes it difficult for the surface caustics to discern where the cluster ends and begins. However, the string of flat galaxies implies something akin to the Kaiser effect \citep{Kaiser1987}, where a flat radial separation against a non-flat separation in the plane of the sky leads to the inference of infallers. \subsubsection{Abell 2142} Abell 2142 is a notorious cluster for its smooth and symmetric X-ray emission, indicative of a post core-merger event, which occurred $\sim1$ billion years ago \citep{Markevitch2000}. The phase-space surface caustics of Abell 2142 vary within the literature, as well as in comparison to the work done here. \cite{Munari2014} presents surface caustics within the confines of $\sim3$ Mpc $h^{-1}$ and appear to be constant with increasing $R$. Again, with \cite{Rines2006} demonstrating a more dynamic and tighter caustic due to differing limits applied in both velocity-space and radial-space alongside data visualisation effects. As usual, the shapes of these caustics are determined by the numbers of galaxies present within the field and how closely, or sparsely, they are distributed as we increase $R$ from the cluster centre. Again, the calculated velocity dispersions from \cite{Munari2014} are inconsistent with our own value, offset by $\sim300$ kms$^{-1}$. This is due to the spread, number and density of the cluster membership determined in the work of \cite{Munari2014} being equally greater. \subsubsection{Abell 2199} Abell 2199 is a relatively local galaxy cluster and provides a good testing-bed for large scale structure formation thanks to its close proximity, this is akin to Abell 1656, another relatively local cluster. The cluster is well studied, possessing several phase-space surface caustics in the literature. The phase-space caustics in this work are the most consistent with \cite{Song2017} and \cite{Rines2003}, where the shape and profile closely matches despite a lower membership. The velocity dispersions calculated here are consistent with those found within the literature \citep{Rines2003}. \subsubsection{Abell 2255} Abell 2255 is a merging galaxy cluster with a complex X-ray distribution, which has yielded a variety of studies to better understand the mechanisms of diffuse radio emission \cite{Akamatsu2017}. The total membership of Abell 2255 in this work is considerably less than that of other literature. However, the surface caustics of this work are still reasonably consistent with the caustics determined by \cite{Rines2006}, if lacking in definition. The velocity dispersion profiles determined here are consistent with those in the literature, despite offsets of $\sim200$ kms$^{-1}$, the drivers are variations in galaxy membership (\citealt{Zhang2011}; \citealt{Akamatsu2017}). \subsubsection{ZWCL1215} The phase-space caustics of galaxy cluster ZWCL1215 determined in this body of work is consistent with those that are produced by \cite{Rines2006}, with only slight variations in the definition of the shape of the surface caustics. The calculated velocity dispersions are also consistent with those determined by \cite{Zhang2011}, with an offset of $\sim200$ kms$^{-1}$, as a result of the reduced membership of galaxies presented within this work. \subsection{Interloping Structures} \label{sec:subs} The clusters that form our sample are not purely isolated potentials, therefore we should take into consideration potential interloping structures as a result of a cluster being a member of supercluster. As an example, during the data accumulation stage of section \ref{sec:data}, the clusters are cross matched against the \cite{Einasto2001} catalogue of superclusters to determine any significant contamination between clusters. Abell 2244 and Abell 2249 are eliminated from the samples due to their strong interloping/overlap in RA-DEC space and z-space within the regions being investigated within this work. Although, their removal from the samples has not altered to shape of the final stacked VDPs to any significant degree. There are also other clusters within the sample that possess unusual substructures. The phase-space diagram of Abell 2065 in Figure \ref{fig:caus} clearly presents two seemingly independent structures. However, Abell 2065 has been documented in the literature to be at the late stage of an ongoing merger \citep{Markevitch1999}. Further X-ray observations with XMM-Newton indicate more specifically the presence of an ongoing compact merger between two subclusters within Abell 2065, where the two cores are at an epoch of initial interaction \citep{Belsole2005}. Higher resolution X-ray observations from Chandra show a surviving cool core from the initial merger, with an upper limit merger velocity of $\lesssim$1900 kms$^{-1}$, adding to the argument that Abell 2065 is an unequal core merger (see \citealt{Chatzikos2006}). This provides an explanation to the slightly off-centre line-of-sight mean velocity distribution of galaxies, with a second, smaller core averaging out to $\sim$ $-1500$ kms$^{-1}$ found in the phase-space diagram of Abell 2065, and naturally will affect the shape of the VDP at larger radii. Abell 1750 is a triple subcluster system with the north subcluster separated from the central subcluster by a velocity offset of -900 kms$^{-1}$ and are all currently in a stage of pre-merger to the point where the plasma between the substructures is significantly perturbed (\citealt{Molnar2013}; \citealt{Bulbul2016}). In contrast Abell 2061, which resides within the gravitationally bound Corona Borealis supercluster with Abell 2065 (see \citealt{Pearson2014}), possesses two optical substructures that will affect the VDP similarly to Abell 2061 \citep{Weeren2011}. It should be noted that Abell 2061 potentially forms a bound system with the smaller cluster/group Abell 2067 (\citealt{Marini2004}; \citealt{Rines2006}), with line-of-sight velocity separation of $\sim725$ kms$^{-1}$ \citep{Abdullah2011}. Observations hint at a likely filament connecting the two systems \citep{Farnsworth2013} aiding to the suggestion of cluster-cluster interloping. There is $\sim30'$ of sky separation and with the prescribed cosmology in section \ref{sec:intro} this provides a rough projected distance of $\sim$2.7 Mpc $h^{-1}$ from the centre of Abell 2061. Yet, this confirms to the cluster-cluster overlapping suggestion with the criteria used to develop cluster membership. Therefore, it is very likely the membership of Abell 2061 is contaminated with the infalling Abell 2067 cluster's member galaxies as we approach $2.5$ $R/r_{200}$. \subsection{The Delta Test} \label{sec:DT} The process of determining whether or not a cluster is merging involved the use of the $\Delta$ test for substructure, devised by \cite{Dressler1988}. Whereby the presence of any substructure to a $\geq99\%$ significance is recorded as a merging cluster environment. The $\Delta$ test, while a powerful and sensitive tool, is limited in its power to test for substructure since it only concerns itself with the sum of the deviations of a local velocity dispersion and mean recession velocity with global cluster values. This could lead to a greater probability of false positives for sub-structuring, along with omissions of those clusters that genuinely possess it. The problem becomes more apparent if an appropriate radial cut-off is not applied when calculating $\Delta$, otherwise the test will classify nearly every cluster to contain substructure. This is a consequence of the varying numbers of cluster galaxies that are added into the calculation of $\Delta$; greater numbers of cluster galaxies help decrease the value of $P(\Delta)$, thereby artificially increasing the significance of subclustering and vice versa. \cite{Pinkney1996} highlights in their comparison of substructure tests how the sensitivity of the $\Delta$ test is affected measurably by the projection angle of the member galaxies, this can lead to a potential loss of genuine merging systems from our sub-sample when their velocities run along $0\degree$ or $90\degree$. One way to potentially alleviate this could be the introduction of more spatial parameters. For example, the \textit{Lee Three-Dimensional Statistic} adapted by \cite{Fitchett1987}, took into consideration angles derived from the projected space and velocity. This test can help to eliminate any potential false positive with its ability to be insensitive to genuine non-merging systems \citep{Pinkney1996}. There are also methods for testing dynamical activity that involve measuring the Gaussianity of the velocity distributions, such as the \textquoteleft Hellinger Distance' measuring the distance between a set of observational and theoretical distributions (see \citealt{Ribeiro2013}; \citealt{Carvalho2017}). Other novel approaches, such as one presented by \cite{Schwinn2018}, test to see whether 2D mass maps can be used to find mass peaks using wavelet transform coefficients. Highlighting discrepancies between definitions of substructure. In contrast, tried and tested methods are evaluated by \cite{Hou2009}, comparing different approaches to analysing the dynamical complexity to groups of galaxies. The authors find a $\chi^{2}$ goodness-of-fit is not best suited for determining a transition away from a Gaussian distribution of velocities. The principles upon which the $\Delta$ test is built upon is a frequentist $\chi^{2}$, which may indicate there is some form of decoupling in the link between sub-structuring and dynamical activity. This apparent decoupling is most likely a result of the limitations of using a singular technique to define if a merging system of cluster galaxies is present, as the $\Delta$ test is only sensitive to average deviations from observed line-of-sight velocities. This is a problem that extends to the VDPs, since they rely on a weighted grouping of objects in velocity-space with a moving Gaussian window function. Therefore, care has to be taken when classifying a galaxy cluster as merging or non-merging based on using the methodology of \cite{Bergond2006} and \cite{Hou2009}. Despite these caveats, the nature of determining substructure with classical statistical testing is simple, sensitive and allows for fast computation on determining our sub-samples. However, there is room to consider how one can accurately define a cluster to be merging or not based solely on limiting velocity-space tests for substructure/grouping of galaxies. For example, there are relic mergers with non-thermal emissions that represent an afterglow of a merging event, or, represent a pre-merging environment as a result from the interactions between intra-cluster media (e.g. \citealt{Giovannini2009}; \citealt{Bulbul2016}). These environments would be insensitive to our traditional statistical testing for substructure due to its constrained application on using the clustering of galaxies as the sole proxy for a merging system. Utilising other parts of the spectrum highlight strong interactions between particles of the ICM, or, of two interacting ICMs from two initially independent systems, and the lack of a comprehensive study can call into question how we best define what is and is not a merging cluster. The VDPs produced here could potentially mask any further variability within the kinematics that would otherwise be visible on a smaller scale \textquoteleft window width'. It is apparent from this work there is some form of sub-layer to the profiles that inhibit a clearer picture being formed in the dynamical nature of galaxies with differing properties. It is a notable possibility that, within some clusters, there is still an inclusion of interloper field galaxies towards $\sim2.5$ $r_{200}$ that distort our final view on the key drivers of these seemingly interacting galaxy sub-populations. The differing merging and non-merging sample sizes present problems of their own that lead to biasing the final stacked VDPs. For example the smoothing kernel, along with the chosen width of the kernel, used will cause a decrease in the sensitivity in how the VDPs respond to substructuring. This problem continues with the stacking procedures, which decrease the sensitivity to the presence of mergers due to each cluster possessing unique environments with different position angles and separations. This problem is further extended when clusters possess limited numbers due to spectroscopic limitations of the survey in the MPA-JHU data. Therefore, unless there is a significant number of galaxies inputted to the calculation of a VDP, the risk of spurious features appearing is still a powerful one. In some cases this is purely a limitation of the data available from marrying the MPA-JHU with DR8 photometry or GZ2 morphologies, in others, an indicator to the limitations in using VDPs as a tool to present the dynamical overview of galaxy clusters. | 18 | 8 | 1808.10381 |
1808 | 1808.02288_arXiv.txt | Radio detection of Ultra High Energetic Cosmic Rays and Neutrinos (UHECRv) which hit the Moon has been investigated in recent years. In preparation for near-future lunar science missions, we discuss technical requirements for radio experiments onboard lunar orbiters or on a lunar lander. We also develop an analysis of UHECRv aperture by including UHECv events occurring in the sub-layers of lunar regolith. It is verified that even using a single antenna onboard lunar orbiters or a few meters above the Moon's surface, dozens of lunar UHECRv events are detectable for one-year of observation at energy levels of $10^{18}$eV to $10^{23}$eV. Furthermore, it is shown that an antenna 3 meters above the Moon's surface could detect lower energy lunar UHECR events at the level of $10^{15}$eV to $10^{18}$eV which might not be detectable from lunar orbiters or ground-based observations. | \label{sec:intro} \nolinenumbers Radio emission from the cascades of energetic particles has been studied intensively in recent years. Particle accelerators such as Stanford Linear Accelerator Center (SLAC) produces radio emission from particle cascades \citep{saltzberg}, \citep{slac}. Along with lab experiments, radio experiments for studying the cosmic energetic particles in air showers and on the lunar surface are being developed. These experiments help to investigate the fundamental questions about the UHECRv such as their origin and the acceleration mechanism. The Moon has been long known as a detector for UHECRv events. The unique properties of Moon regolith such as very low conductivity and low attenuation makes it an ideal environment for detection of coherent radiation based on the Askaryan Effect \citep{ref1}. We refer to this radio emission, also known as Cherenkov-like radiation, as Askaryan radiation through the paper(as stated in \citep{askaref1} and \citep{askaref2}). Askaryan radiation is spread over a broad spectrum covering microwave frequencies (GHz band corresponds to cm wavelengths) in dielectric solids, but it may also reach a peak at lower frequencies within tens of MHz \citep{Scholten}. Most of radio UHECRv experiments at MHz regime operate, however, at frequencies higher than 100 MHz where dispersion in the Earth's ionosphere, and the Galactic background noise, become low. Also the dimension of antennas becomes reasonably smaller at higher frequencies. In our analysis frequencies of 1.5 GHz and 150 MHz represent the GHz and MHz frequency regimes. From theoretical estimates \citep{zas} and the SLAC experiments \citep{saltzberg} can be understood that the dominant mechanism for producing lunar UHECRv radiation is charge excess. This is mainly due to the absence of strong magnetic fields, such as the Earth's magnetic field, which is a key element in radio emission of air showers. The impact of energetic charged particles with lunar regolith generates electromagnetic pulses which develop and propagate as a cascade of electric currents through the layers of lunar regolith and lunar exosphere. Antennas onboard lunar orbiters, a lunar lander or a ground-based array can detect these events by measuring corresponding electric fields (also known as the lunar Askaryan technique \citep{DZ}). In this paper we generalize the analytical methods in the literature for calculating the UHECRv apertures of ground based arrays so it can be used also for lunar orbiter experiments as well as for antennas on the Moon's surface. For the latter we modified the method and included the events occurring in the sub-layers of lunar regolith. The results are used to estimate the number of events that can be detected for various radio experiments for a one-year observation. \begin{itemize} \item Radio Experiments of Lunar UHECRv Emission \item Analysis of Lunar UHECRv Events \item Categorization of Lunar UHECRv Events \item Technical Requirements of Future Lunar Radio Experiments \item Summary and Conclusion \end{itemize} \begin{figure} \includegraphics[width=0.45 \textwidth]{fig1} \caption{Observations of lunar UHECRv events using ground-based, lunar orbiter and Moon's surface radio experiments. Dimensions are not to scale.} \label{fig:i} \end{figure} | We examine the possibility of a radio detection of lunar UHECRv events in two frequency regimes a 150 MHz and a 1.5 GHz. For the selected frequencies, the analytical method is reliable for the whole range of energy (\citep{ref01}, \citep{ref02}). \\ For future lunar missions, we ran a simulation using a single antenna at different distances from the lunar surface and compared the results with ground-based observations. The method can be applied to an array of antennas on the Moon's surface or multiple antennas onboard lunar orbiters. The results show that the size of UHECRv aperture depends roughly on the square of the wavelength of observation. Therefore the chance of detecting UHERCv events significantly increases in the MHz regime. For the system requirements, the size of the antenna depends on the wavelength of the observation so the antenna will be shorter in GHz regime. However, for the digital receiver, the sampling rate of digitizer depends on the frequency bandwidth which makes the digital processing in the GHz band more complicated. Due to the large volume of digitized data in GHz band, data transfer to the Earth could be particularly problematic for ongoing lunar missions. Lunar UHECRv events in the low-frequency regime (kHz to few MHz) are also possible and are likely to be influenced by transition radiation as opposed to charge excess mechanism for the MHz and GHz bands. kHz observation, however, is not practical for lunar missions due to the size of antennas to be deployed in space or on the lunar surface.\\ For selected frequencies, the expected event rates (Fig.\ref{fig:v}, Fig.\ref{fig:viii}) for cosmic rays, GZK neutrinos and TD neutrinos are consistent with the previous numerical and analytical simulations in the literature (\citep{Gusev}, \citep{Stal},\citep{ref01},\citep{ref02}). However, the expected event rate for Z-Burst cosmic neutrinos shows a significant reduction after applying the limits of recent observation (ANITA II, \citep{ANITA2}). This predicts roughly fewer Z-Burst neutrino events by 2 orders of magnitude for both frequencies. For detection of lunar GZK neutrinos, an array of hundreds of antennas on the Moon's surface seems to be the most probable choice, while for higher energy levels (e.g. TD Neutrinos and Z-Burst Neutrinos), lunar orbiter experiments at distances of 500-1000 km would provide the optimum position to observe lunar UHECRv events. \\ We also evaluate the possibility of the detection of neutrinos in the lunar sub-regolith by calculating UHECv apertures. In this paper, we take this into account only for lunar observation at 3 m distance from the lunar surface, and find that an additional contribution of 50\% to 60\% is expected from the sub-regolith. \\ Our preliminary study shows that events occurring in sub-regolith are also detectable as far away as Earth-based observations, since radiation attenuation is very low in the lunar regolith and sub-regolith. This would be an interesting topic for future study to apply electromagnetic properties of the lunar environment in relation to the detection of Askaryan radiation due to the impact of UHECRv. \\ We also investigate the propagation of Askaryan radiation in the lunar regolith. Assuming that the shower cascade generates upward and radial components of the current density, the electric fields will be decaying exponentially while propagating towards the Moon's surface. The radial propagation with a pattern of Bessel's functions. Fig.\ref{fig:xvi} illustrates a summary of lunar UHECRv radio experiments and the effective parameters. \begin{table*}[b] \centering \begin{tabular}{l*{6}{c}r} Radio Emission & Characteristics & Dominant Frequency Spectrum\\ \hline\\ Lunar dust and charged particles& Nearby antenna surface emission& kHz \\ (Micro) Meteorites &Nearby antenna surface emission&MHz-GHz\\ Sky radio sources (Sun, Planets)& Strong point sources& kHz-MHz\\ Terrestrial noise (AKR,RFI)&On Earth visibility& kHz-MHz\\ Galactic background noise&Global emission&MHz\\ UHECR&Lunar surface(horizontal angles) &MHz-GHz\\ UHECv&Lunar surface and regolith&MHz-GHz\\ \end{tabular} \caption{\label{tab:iv} Radio emission in the lunar environment} \end{table*} \begin{table*}[b] \centering Typical parameters of a helical antenna in the presence of a ground plane\\ \begin{tabular}{l|c|c} \hline\\ Frequency(F) & 1.5 GHz & \\ Bandwidth (BW)&1348 MHz-1668 MHz &320 MHz \\ Number of turns(N)&5\\ Spacing between turns(S)&$0.2\lambda$&4cm\\ Circumference of Helix (C)&$\lambda$&0.2m\\ Input Impedance&$140C/\lambda$&140 $\Omega$\\ HPBW (Degrees)&$ 52 \lambda^{1.5}/[C(NS)^{0.5}]$&52\\ Directivity&$15NC^{2}S/\lambda^{3}$&11.76dBi\\ Operation mode& Axial&Circular polarization\\ \end{tabular} \caption{\label{helix} Typical parameters of a helical antenna in the presence of a ground plane calculated \citep{kraus} for the central frequency of 1.5 GHz.} \end{table*} \begin{figure*}[t] \centering \includegraphics[width=0.7\textwidth]{Diagram-revised} \hfill \caption{\label{fig:xv} A block diagram of basic system requirements for a lunar UHECRv radio experiment} \end{figure*} \begin{figure*}[t] \centering \includegraphics[width=0.7\textwidth]{flowchart.pdf} \hfill \caption{\label{fig:xvi} A flowchart of lunar UHECRv radio experiments} \end{figure*} \appendix | 18 | 8 | 1808.02288 |
1808 | 1808.09153_arXiv.txt | We present a comparison between Monte Carlo simulations and a semi-analytical approach that reproduces the theoretical probability distribution functions of the solar neutrino fluxes, stemming from the $pp$, $pep$, $hep$, $^7\mathrm{Be}$, $^8\mathrm{B}$, $^{13}\mathrm{N}$, $^{15}\mathrm{O}$, and $^{17}\mathrm{F}$ source reactions. We obtain good agreement between the two approaches. Thus, the semi-analytical method yields confidence intervals that closely match those found, based on Monte Carlo simulations, and points towards the same general symmetries of the investigated probability distribution functions. Furthermore, the negligible computational cost of this method is a clear advantage over Monte Carlo simulations, making it trivial to take new observational constraints on the input parameters into account. | Over the last century, the progress in particle and nuclear physics has contributed to a thorough understanding of the interior structure of the Sun, and well-constrained solar models, on the other hand, have been used to shed light on the employed input physics. Thus, solar models and observations have led to a better understanding of neutrinos and vice versa \citep[cf.][]{jcd1996, Bahcall1998}. To compare model predictions with observations, it is essential to establish theoretical parameter estimates as well as thorough uncertainties for the investigated parameters. One way to obtain these uncertainties, in the case of the theoretical solar neutrino fluxes from different source reactions, is to map the associated probability distribution functions based on a Monte Carlo simulation. Such an analysis has been performed by \cite{Bahcall2006}, including 10,000 standard solar models and 21 relevant input parameters. Recently, \cite{Vinyoles2017} have published yet another Monte Carlo analysis based on yet another 10,000 standard solar models, including updated solar input physics. While a Monte Carlos analysis is reliable, it is also cumbersome and requires large amounts of computing time. In the present paper, we present a semi-analytical method that is computationally light and capable of reproducing the correct theoretical probability distribution functions. Our work will be presented in the following order: in section~\ref{sec:method}, we elaborate on the method used, and specify both the considered neutrino source reactions and the input physics employed. In section~\ref{sec:results}, we present the results of our analysis and compare these to the results of the Monte Carlo analyses by \cite{Bahcall2006} and \cite{Vinyoles2017}. | We have presented a computationally-light semi-analytical approach to evaluate the theoretical probability distributions of the solar neutrino flux for different source reactions. This approach is based on the linear response of the logarithm of the predicted flux to changes in the logarithm of different input parameters. As pointed out by \cite{Haxton2008} and \cite{Pena2008}, this linear relationship can be expressed in a single parameter, $\alpha(i,j)$. The results obtained from this semi-analytical approach are in good agreement with results obtained from Monte Carlo analyses and reveal the same general symmetries of the PDFs of the neutrino fluxes. Hence, this method reliably provide confidence intervals at any confidence level. Furthermore, the low computational cost of the presented method is a clear advantage over a Monte Carlo analysis. Thus, $\alpha(i,j)$ can be evaluated based on only a handful of solar models. Moreover, as the computational costs of computing the probability distributions is negligible, keeping the uncertainties up to date and including new observational constraints on input parameters is trivial. | 18 | 8 | 1808.09153 |
1808 | 1808.02846_arXiv.txt | Photometric redshifts are necessary for enabling large-scale multicolour galaxy surveys to interpret their data and constrain cosmological parameters. While the increased depth of future surveys such as the Large Synoptic Survey Telescope (LSST) will produce higher precision constraints, it will also increase the fraction of sources that are blended. In this paper, we present a Bayesian photometric redshift method for blended sources with an arbitrary number of intrinsic components. This method generalises \resp{existing template-based Bayesian photometric redshift (BPZ) methods}, and produces joint posterior distributions for the component redshifts that allow uncertainties to be propagated in a principled way. Using Bayesian model comparison, we infer the probability that a source is blended and the number of components that it contains. \resp{We extend our formalism to the case where sources are blended in some bands and resolved in others. Applying this to the combination of LSST- and Euclid-like surveys, we find that the addition of resolved photometry results in a significant improvement in the reduction of outliers over the fully-blended case.} We make available \texttt{blendz}, a Python implementation of our method. | Current photometric surveys such as CFHTLens~\citep{cfhtData}, KiDS~\citep{kidsData} and DES~\citep{desData} image galaxies over large volumes of the Universe to probe the growth of structure and the distribution of matter on large scales. Through techniques such as galaxy clustering and cosmic shear, these surveys are able to constrain cosmological parameters and conduct tests of the standard $\Lambda$CDM cosmological model~\citep[e.g.,][]{cfhtConstraints, desConstraints}. These observational tests require the redshift distribution of the sample to make model predictions for comparison. Additional information can also be obtained by considering the redshift dependence using tomography, where galaxies are placed into one of several redshift bins~\citep[e.g.][]{tomographyHu, tomographyConstraints, tomographyKids}. However, the large number of sources required to constrain cosmological parameters to high precision makes obtaining spectroscopic redshifts for the entire sample unfeasible. As a result, photometric redshifts are a vital part of the cosmological analysis pipeline of galaxy surveys. Photometric redshift methods seek to infer the redshift of galaxies from noisy observations of their flux in several broadband filters. They provide an alternative to spectroscopic redshifts that requires less telescope time, at the expense of a reduction in precision. As a result, photometric redshifts can be applied to galaxies too faint and samples too large for spectroscopic observations. There are two general classifications for photometric redshift methods that utilise flux information; template-based and empirical methods. Template-based methods use a set of galaxy spectra that are assumed to be representative of every galaxy they are applied to. These templates are redshifted and integrated over the response function of each filter to produce predictions of observed fluxes. These predictions are then used to infer the redshift from the observed fluxes. Maximum likelihood methods~\citep[e.g.,][]{hyperz, lePhare} find the best fitting template by minimising $\chi^2$ to estimate the redshift. Bayesian methods, introduced by \cite{bpz}, marginalise over all templates to produce a posterior redshift distribution. This correctly accounts for the uncertainty in the galaxy template that is ignored by maximum likelihood methods. Bayesian methods also include prior distributions that can reduce catastrophic outliers. Empirical methods estimate redshifts by fitting for the mapping between flux and redshift from a set of training data, rather than specifying it \textit{a priori} through a template set. This mapping is typically found using machine learning methods such as neural networks~\citep[e.g., ][]{annz, annz2}, Gaussian processes~\citep[e.g.,][]{firstGPPhotoz, newGPz}, and random forests~\citep[e.g., ][]{randomForest, tpz}. These methods are examples of supervised learning; they require large datasets of fluxes and associated spectroscopic redshifts that are representative of the sample they are applied to. If representative data are available, empirical methods are typically more accurate than template-based methods~\citep{photozAccuracy}. However, redshift estimates of galaxies not represented by the training data are much less reliable~\citep{mlRedshiftBias}. The common case where spectroscopic training data is shallower than the photometry can lead to biases where the redshifts of high redshift galaxies are underestimated~\citep{mlRedshiftShallow}. \resp{ In practice, template-based and empirical methods are not so distinct. The priors of Bayesian methods typically include a set of parameters that are fitted using a set of training data~\citep[e.g.,][also see section~\ref{sec:calibrate-priors}]{bpz, schmidtPriors}. In addition, recent applications of photometric redshifts have used hybrid methods that combine a template-based approach with machine learning methods~\citep[e.g.,][]{somsHybrid, hierarchyHybrid, gpAndHybrid}. } In addition to these methods, clustering redshift methods~\citep[e.g.,][]{clusterFirst, clusterSchmidt, clusterMenard} cross correlate the angular positions of a photometric sample with a spectroscopic sample to estimate the redshift distribution. Clustering redshift methods do not model fluxes as a function of redshift, instead only using the spatial information of photometric data. As such, clustering redshifts are complementary to other photometric redshift methods; \cite{clusterDES} uses clustering redshifts to calibrate biases from other photometric redshift methods, for example. \resp{ Ensuring that photometric redshifts are accurate and precise is necessary for obtaining unbiased constraints on cosmological parameters. \cite{hutererSystematics} found that future tomographic surveys would require the mean of each redshift bin to be known to a precision of $0.003$, though this requirement can be reduced by self-calibration~\citep[e.g., ][]{hutererSystematics, calibrationSun, calibrationSamuroff} and combining weak lensing data with other cosmological probes such as baryonic acoustic oscillations~\citep{lsstsciencebook}. Photometric redshifts are also important in the calibration of other systematics. Multiplicative biases in the measurement of shear can be detected and corrected for, provided that photometric redshifts of galaxies in the sample are unbiased~\citep{shapeBiasStudy}. Weak lensing shape measurement biases can themselves also be redshift dependent; without unbiased redshift estimates to make corrections, these can lead to biases of a few percent in the cosmological parameters $\sigma_8$ and $w_0$~\citep{zDependentShapeBias}. } {Another} key part of precision cosmology is an accurate understanding of uncertainties in parameter constraints. To enable this, uncertainties arising from each step of the analysis should be accounted for and propagated onwards. In cosmological analyses, this is typically accomplished using a Bayesian framework~\citep[e.g.,][]{kidsResults, desYr1CosmicShear}, allowing these uncertainties to be combined and marginalised over for the final constraints. It is therefore essential that photometric redshift methods provide not only point estimates of redshifts, but also a measure of their uncertainties. The uncertainty associated with a redshift estimate can be represented by a single number, i.e., a point estimate with an error bar. However, doing this necessitates making an assumption about how the error is distributed. Uncertainties in photometric redshifts can be highly non-Gaussian, and so are poorly described by a single number such as the variance. Photometric redshift methods that instead characterise their results using a probability distribution function (PDF) can capture all of this information. Photometric redshifts can also suffer from degeneracies that result in high-redshift galaxies having similar colours to those at low redshifts~\citep[e.g.,][]{lymanBalmer}. As a result, several well-separated redshifts are plausible, and an accurate representation of the uncertainty should reflect this. While this can be easily described with a multimodal PDF, a single number can be misleading. Error bars that cover the full range of parameter space between the low- and high-redshift estimates do not show that redshifts between these are disfavoured, inflating uncertainties. Several photometric redshift methods are able to produce PDFs as their result. Bayesian template-based methods \citep[e.g.,][]{bpz} produce a posterior distribution, a PDF of the model parameters conditioned on the data and any model assumptions. In addition to the galaxy redshift, these model parameters can include other quantities of interest such the galaxy template \citep{zebra}. A joint posterior over all of these parameters contains information about the uncertainty of each, including any correlation between them. Machine learning methods can also produce PDFs by utilising ensemble techniques, where the predictions of several models are combined to produce a distribution. Examples of this technique include the combination of decision trees in a random forest \citep{tpz} and committees of neural networks constructed with different network architectures and initialised randomly \citep{neuralNetPDF}. Future galaxy surveys such as the Large Synoptic Survey Telescope~\citep[LSST, ][]{lsstSummary} will obtain extremely high precision constraints on cosmological parameters. By utilising deeper photometry, these surveys will probe greater volumes than previously, resulting in an increased number density of galaxies imaged. While this increased depth drives the high precision these surveys will achieve, it also increases the fraction of objects that overlap with others along the line of sight, known as blending~\citep{lsstDensity}. Most existing deblending methods do not utilise the colour information from photometry, instead using the spatial information contained in an image from a single band. The commonly used SExctrator~\citep{sextractor} searches for adjacent pixels on a flux-thresholded map that separate into disjoint regions as the threshold is increased. Doing this for many thresholds allows each pixel to be assigned to a single object, contributing the entirety of its flux to that object. The SDSS deblender~\citep{sdssDeblend} lifts this restriction, splitting the flux proportionally between objects based on object templates. These templates are constructed by finding peaks in the image and assuming symmetry around them, comparing pairs of pixels and setting them to be equal. Profile fitting methods~\citep[e.g.,][]{galfit, profit} forward model the image using physical profile models, deblending by directly fitting for the galaxy properties. In far-infrared astronomy, blending is common due to the reduced angular resolution of these instruments compared to optical telescopes. As a result, galaxies that are well resolved in optical observations may become blended in the far-infrared. Deblending methods designed for this case such as \cite{xid} can use the unblended observations to place strong priors on the number and position of sources. We refer to this mix of blended and unblended observations as \textit{partial blending} throughout this paper. The ability for most deblending methods to successfully identify blended galaxies depends on their angular separation. Galaxies with too small an angular separation are instead identified as single sources. \cite{lsstBlendReport} estimate that $45-55\%$ of sources in LSST will be blended, with $15-20\%$ of all sources being misidentified as a single source, referred to as ambiguously blended objects. Blending of sources can have an impact that is significant for constraining cosmological parameters. \cite{blendEllipticities} estimate that ambiguously blended objects in LSST will result in an increase in shear noise of $14\%$ for the deepest photometry ($i < 27$) and $7\%$ for the gold standard sample ($i < 25.3$). Since these ambiguous blends are difficult to separate due to their small angular separation, deblending methods that incorporate colour information could be beneficial. Recent deblending methods such as MuSCADeT~\citep{muscadet} and \textsc{scarlet}~\citep{scarlet} incorporate this colour information by using wavelet transforms, enforcing that the representation of components is sparse in this space. Deblending methods that produce a set of component-separated maps are useful for later applying existing analysis techniques designed for individual components to. However, splitting the analysis in this way can lose information, such as the correlation between deblending parameters and the parameters in a subsequent analysis. An analysis method that jointly constrains parameters directly from blended data provides a self-consistent, principled way to characterise and propagate this information. In this paper, we present a method that generalises the \cite{bpz} Bayesian photometric redshift (BPZ) method to the case of blended observations. This is a template-based method where the task of determining the component redshifts is cast as a Bayesian parameter inference problem. The product of such an inference is a joint posterior distribution of the redshift and magnitude of each component in the blended source. This distribution characterises the complete statistical uncertainty in the result in a way that can be propagated through the rest of the cosmological analysis. Determining the number of components in an observed source, i.e., whether or not it is blended, is treated as a model comparison problem. In this way, our method allows the identification of blended sources from aperture photometry alone. Throughout, we use \textit{source} to refer to the (possibly) blended system that is observed, and \textit{component} to refer to the underlying physical objects that make up this source. For parameters defined for each component in a source, we index over component using greek letters and indicate the collection of these using sets, i.e., $\{\theta\} \equiv \{\theta_\alpha, \theta_\beta, \dots \theta_{N}\}$. Vector quantities defined for each filter band are in bold $\vct{q}$, and observed quantities are denoted with a hat $\hat q$. Where necessary, quantities defined for a specific number of components are distinguished by a subscript number in brackets, i.e., $q^{(1)}$ is the definition of $q$ for a single component. A summary of our notation is provided in Table~\ref{tab:notation}. \begin{table*} \caption{A summary of the notation used throughout this paper.} \centering \label{tab:notation} \begin{tabular}{p{0.15\textwidth}p{0.75\textwidth}} \hline Symbol & Description \\ \hline $N$ & Number of components \\ $T$ & Number of templates \\ $B$ & Number of filter bands \\ $z_\alpha$ & Redshift of component $\alpha$ \\ $m_{0, \alpha}$ & Reference band magnitude of component $\alpha$ \\ $t_\alpha$ & Template index of component $\alpha$ \\ $\{z\}$ & Set of redshifts of each component \\ $\{m_0\}$ & Set of reference band magnitudes of each component \\ $\{t\}$ & Set of template indices of each component \\ $b$ & Index over filter bands \\ $b_0$ & Index of reference band filter \\ $\hat F_0$ & Observed flux in reference band \\ $\hat{\vct{F}} $ & Vector of observed fluxes, excluding the reference band \\ $\sigma_{0}$ & Error on the reference band flux\\ $\sigma_{b}$ & Error on the flux in band $b$\\ $F^{(1)}_{ t, b } \big(z, m_0 \big)$ & Model flux for a single component in band $b$, at redshift $z$, with reference band magnitude $m_0$ and templates $t$\\ $F^{(N)}_{ \{t\}, b } \big(\{z\}, \{m_0\} \big)$ & Model flux for $N$-component blended source in band $b$, at redshifts $\{z\}$, with reference band magnitudes $\{m_0\}$ and templates $\{t\}$\\ $\chi$ & Set of cosmological parameters $\Omega_{\textrm{m}}$, $\Omega_\Lambda$ and $H_0$ \\ $\xi^{(N)}_\chi\big(\{z\}\big)$ & Combination of up to $N$-point correlation functions describing the extra probability of $N$ galaxies jointly sitting at redshifts $\{z\}$ due to clustering \\ \hline \end{tabular} \end{table*} This paper is organised as follows. In section~\ref{sec:formalism}, we describe our formalism for estimating redshifts as a parameter inference problem, describing its application to partially blended systems in section~\ref{sec:part-blend}. In section \ref{inference}, we discuss our inference methods, detailing how we use model comparison to identify blended objects in section~\ref{sec:model-select}. In section~\ref{sec:sim-results}, we test our method on simulated observations. Section~\ref{sec:gama-data} describes a test of our method on the Galaxy And Mass Assembly survey~\citep[GAMA, ][]{gamaData} blended sources catalogue~\citep{gamaBlends}, for which spectroscopic redshifts are available. We conclude in section~\ref{sec:conclusions}. | \label{sec:conclusions} Blended sources will become far more common in future galaxy surveys than are found currently due to increases in the depth of photometry and as a result, the number density of galaxies. We present a Bayesian photometric redshift method that generalises the existing BPZ~\citep{bpz} method to the case of blended observations. We derive a posterior for the redshift and magnitude of each component which we sample to obtain estimates of the redshift. We also use this posterior in a model comparison procedure to infer the number of components in a source. By doing this, the method is able to infer both the redshift of each component within a blended source, and identify that a source is blended from its broadband photometry alone. The joint posterior distribution of the redshifts of all components in a blend provides a complete accounting of the correlations in the final result, information that can be lost when separating components and estimating redshifts for each component separately. This uncertainty information is essential for obtaining accurate uncertainties on cosmological parameters that rely on the photometric redshift estimates. A Python implementation of the method, \texttt{blendz}, is available to download. \resp{ By inferring the redshifts of components directly from their blended photometry, the method presented here is directly applicable to ambiguously blended objects that cannot otherwise be deblended. The partial-blending formalism described in section~\ref{sec:part-blend} also enables the catalogue-level joint analysis of sources in space- and ground-based surveys such as Euclid and LSST. The complementarity of these surveys will allow cosmological parameters to be constrained more precisely than either survey could individually, and analysis of blended sources from their aperture photometry will be simpler than a joint pixel-level analysis~\citep{synergy}. } \resp{ The method presented here could also be combined with existing deblending methods that utilise the spatial information of images directly. These methods are complementary; image-based deblending methods are effective provided that components are sufficiently well separated. If this is not the case, there is too little spatial information to be able to separate components, and colour information is necessary. Combining these methods could allow future surveys to identify a greater proportion of blended sources, reducing their effects on cosmological constraints. Deblending methods that also incorporate colour information would need to be combined with this method more carefully however, as the colour information would be used twice and thus the blending probabilities would not be independent. This method could instead be extended to incorporate imaging data by constructing a forward model of the galaxy in each band and constraining both morphology and redshift simultaneously. } | 18 | 8 | 1808.02846 |
1808 | 1808.07464_arXiv.txt | {Do void statistics contain information beyond the tracer 2-point correlation function? Yes! As we vary the sum of the neutrino masses, we find void statistics contain information absent when using just tracer 2-point statistics.} Massive neutrinos uniquely affect cosmic voids. We explore their impact on void clustering using both the \texttt{DEMNUni} and \texttt{MassiveNuS} simulations. For voids, neutrino effects depend on the observed void tracers. As the neutrino mass increases, the number of small voids traced by cold dark matter particles increases and the number of large voids decreases. Surprisingly, when massive, highly biased, halos are used as tracers, we find the opposite effect. The scale at which voids cluster, as well as the void correlation, is similarly sensitive to {the sum of neutrino masses and} the tracers. This scale dependent trend is not due to simulation volume or halo density. The interplay of these signatures in the void abundance and clustering leaves a distinct fingerprint that could be detected with observations and potentially help break degeneracies between different cosmological parameters. This paper paves the way to exploit cosmic voids in future surveys to constrain the mass of neutrinos. | {Can the underdense regions in our universe reveal information inaccessible to the dense regions?} The cosmic web \citep{Bond1996} is a powerful tool to constrain neutrino properties. Cosmic voids are large (typically $10-100\,h^{-1}\mathrm{Mpc}$) underdense regions of the cosmic web that have undergone minimal virialization and are dominated by inward or outward bulk flows \citep{Gregory1978,Shandarin2010,Falck2015,Ramachandra2017}. In contrast to halos, which have undergone non-linear growth that can wash out primordial information, voids offer a pristine environment to study cosmology. As such, voids are a complementary probe to measurements of the cosmic microwave background and galaxy clustering and can help break existing degeneracies between cosmological parameters, thus becoming increasingly popular to study with both simulations and observations \citep[see e.g.][and references therein]{Ryden1995,Goldberg2004,Colberg2008,Viel2008,VanWeygaertErwinPlaten2009,Sheth2003,Chan2014,Hamaus2014,Sutter2014,Sutter2014a,Hamaus2015a,Szapudi2015,Qin2017,Alonso2017,Pollina2018,voidwhitepaper}. The discovery of neutrino oscillations demonstrates that at least two neutrino families must have a nonzero mass \citep{Becker-Szendy1992,Fukuda1998,Ahmed2004}, evidence for beyond the standard model physics. Cosmological observables provide stringent upper bounds on the sum of neutrino masses, $\sum m_\nu$ \citep[see e.g.][]{PlanckCollaboration2018}, and may soon determine the last missing parameter in the standard model. At linear order, neutrinos do not cluster on scales smaller than their free-streaming length, which is a function of the mass $m_\nu$ of the single neutrino species \citep{Lesgourgues2006a}. For example, neutrinos have free-streaming lengths of $130\,h^{-1}\mathrm{Mpc}$ and $39\,h^{-1}\mathrm{Mpc}$ for $\sum m_\nu = 0.06\,\mathrm{eV}$ and $\sum m_\nu = 0.6\,\mathrm{eV}$ (assuming 3 degenerate neutrino species), respectively. Neutrino free-streaming scales for $\sum m_\nu$ of interest thus fall within the range of typical void sizes, making voids an interesting tool for studying neutrinos. Voids are sensitive to a number of effects, such as: redshift space distortions and the relative growth rate of cosmic structure \citep[e.g.][]{Paz2013,Hamaus2016,Achitouv2017,Hamaus2017,Hawken2017}, Alcock-Paczy{\'n}ski distortions \citep[e.g.][]{Alcock1979,Lavaux2012a,Sutter2012a,Sutter2014c,Hamaus2014b,Hamaus2016,Mao2017,Achitouv2018}, weak gravitational lensing \citep[e.g.][]{Melchior2014,Clampitt2015,Sanchez2016,Chantavat2017}, baryon acoustic oscillations \citep{Kitaura2016}, and the integrated Sachs-Wolfe effect \citep[e.g.][]{Granett2008,Ili2013,Kovacs2015,Kovacs2016,Nadathur2016,Naidoo2016,Cai2017,Kovacs2017}. Voids offer an environment with unique sensitivity to signatures of physics beyond the standard model. They are one of the best observables to probe theories of gravity \citep{Odrzywoek2009,Li2012,Clampitt2013,Cai2014,Gibbons2014,Zivick2014,Barreira2015,Hamaus2016,Baldi2016} and dark energy \citep{Lee2009,Bos2012,Lavaux2012a,Sutter2014d,Pisani,Pollina2015}. Since voids are under-dense in matter, they are particularly sensitive to the effects of diffuse components in the universe like radiation and dark energy. For this reason, voids offer an appealing, new avenue to constrain neutrino properties. \citet{Villaescusa-Navarro2013a} studied how massive neutrinos affect voids at high redshifts with Ly$\alpha$ forest analyses using hydrodynamical simulations \citep[see also][]{Krolewski2017}. \citet{Massara2015} focused on how neutrinos affect void abundance, density profiles, ellipticities, the correlation function, and velocity profiles with N-body simulations that included massive neutrinos as an additional collisionless particle component. \citet{Banerjee2016} observed that neutrinos affect the scale-dependent void bias for voids traced by the CDM particle field. They use a spherical void finder and a small volume simulation (700~$h^{-1}$Mpc box length). In recent data analyses voids have been found using finders that do not assume spherical voids \citep[e.g.][]{Hamaus2017,Pollina2017}. It is interesting to analyze the effects of neutrinos on voids with non-spherical shapes, such as in \citet{Massara2015}, which have the advantage of closely following the cosmic web pattern. Work such as \citet{Hamaus2014a} analyzed void power spectra without discussion of neutrinos. Thus far, the effect of neutrinos on voids has not been considered in depth without assuming spherical voids, and their effect on voids traced by halos is especially unexplored. Previous simulations with massive neutrinos did not have the volume and resolution to explore the effect of neutrinos on voids derived from the halo distribution and Halo Occupation Distribution (HOD) mocks \citep[see e.g.][]{Massara2015}. We use N-body simulations with densities and volumes large enough to distinguish the effects neutrinos have on voids derived from the halo distribution and on voids derived from the particle distribution. { Both the void size distribution and clustering respond to $\sum m_\nu$. We uncover and resolve the apparent paradox that voids found in the halo field respond in the opposite manner to $\sum m_\nu$ than voids found in the particle field. The impact of $\sum m_\nu$ on voids changes sign as a function of halo bias. $\sum m_\nu$'s sign dependent impact on void size and clustering does not occur for other cosmological parameters such as $\sigma_8$. The void exclusion scale shifts in response to $\sum m_\nu$, as well, a scale-dependent response unique to voids. The response of voids to $\sum m_\nu$ is thus novel-- neutrinos leave unique fingerprints on voids.} The paper is organized as follows. In \S\ref{sec:sim+nbody} we describe the two sets of massive neutrino simulations used in this work, the Dark Energy and Massive Neutrino Universe Project (\texttt{DEMNUni}) and the Cosmological Massive Neutrino Simulations (\texttt{MassiveNuS}), as well as the void finder used to build our void catalog. We show how neutrinos impact voids in \S\ref{sec:results} and discuss these results in \S\ref{sec:discussion}. We conclude and discuss application to future surveys in \S\ref{sec:conclusions}. | \label{sec:conclusions} We have explored the impact of the sum of neutrino masses $\sum m_\nu$ on void properties with the N-body simulations \texttt{DEMNUni} and \texttt{MassiveNuS}. For the first time we have shown that: \begin{enumerate} \item the effect $\sum m_\nu$ has on void properties depends on the type of tracer the void catalog was built from, \item using voids only derived from the cold dark matter particle field to study neutrinos, as has been assumed in the literature, is not sufficient to capture the effects of neutrinos on voids. Voids are not always smaller and denser in the presence of neutrinos, and tracer properties can actually lead to larger voids, a smaller number of voids, and enhanced void clustering, \item the impact of $\sum m_\nu$ on the void abundance and void-void power spectrum for the \texttt{DEMNUni} {(`low-res')} void catalog derived from the halo distribution is opposite to that for the void catalog derived from the CDM particle field. For voids derived from the cold dark matter field, increasing $\sum m_\nu$ increases the number of small voids, decreases the number of large voids, and damps the void-void power spectrum. The opposite is true for voids derived from the biased halo distribution due to the effects of the effective halo bias, \item the effective halo bias influences how $\sum m_\nu$ affects voids -- this will have interesting impacts on future surveys aiming to constrain the sum of neutrino masses, and \item void power spectra and auto-correlation functions are powerful tools for distinguishing neutrino masses. Neutrinos leave a distinct fingerprint on voids, which can potentially help break the degeneracy between cosmological parameters in halo measurements. We plan to thoroughly explore breaking degeneracies, such as $\sigma_8$, in upcoming work. \end{enumerate} By comparing observations of the number of voids, void abundance, and void clustering to $\Lambda\mathrm{CDM}$ simulations with volume and resolution matching the survey volume and galaxy number density, surveys have a new avenue to place constraints on $\sum m_\nu$. { It is important to note, though, that for a fixed volume, substantially low tracer densities produce large measurement uncertainties due to a small number of voids. Thus, surveys with low tracer densities in combination with smaller volumes relative to those shown in this work may not be able to statistically distinguish the impacts neutrinos have on voids.} However, upcoming surveys like PFS, DESI, { and} Euclid have halo densities {$n_{\rm h}$ of $\approx 6 \times 10^{-4}\,h^3\mathrm{Mpc}^{-3}$ \citep{PFS}, $7 \times 10^{-4}\,h^3\mathrm{Mpc}^{-3}$ \citep{DESI}, and $2 \times 10^{-3}\,h^3\mathrm{Mpc}^{-3}$ \citep{WFIRST}, respectively, for $z\approx 1$} comparable to that of the \texttt{DEMNUni} {(`low-res')} simulation { with $n_{\rm h}\approx 1 \times 10^{-3}\,h^3\mathrm{Mpc}^{-3}$ at $z=1.05$.} {Denser surveys like WFIRST with $n_{\rm h}\approx 9 \times 10^{-3}\,h^3\mathrm{Mpc}^{-3}$ \citep{WFIRST} at the same redshift} can even exceed the \texttt{DEMNUni} {(`low-res')} simulation's density{. Thanks to their high tracer densities and large volumes, these surveys will be capable of measuring the impact $\sum m_\nu$ has on voids}. For these upcoming observations, simulations such as \texttt{DEMNUni} and \texttt{MassiveNuS} are the best tools for evaluating the impact of neutrinos on the observed voids. In the final stages reliable mocks will also be necessary to correctly evaluate the mask and survey boundary effects. The opposite behavior of the \texttt{DEMNUni} {(`low-res')} and \texttt{MassiveNuS} {(`high-res')} simulations to $\sum m_\nu$ indicates there exists a threshold effective halo bias for which the void power spectra, correlation functions, and abundances for voids derived from the halo distribution will be less sensitive to $\sum m_\nu$. It would be interesting to compare surveys with effective halo biases above and below the threshold at which $\sum m_\nu$ induces the inversion effect in the void abundances, number, power spectra, and correlation functions, since lower densities increase the minimum halo mass, and so the effective halo bias, of the survey. In this sense one could imagine an extraordinarily dense low-$z$ survey to be particularly interesting. Within the same survey, it will be interesting to compare void properties for tracers with different luminosity or mass thresholds, i.e. with different biases. The use of multi-tracer techniques is another promising tool for constraining $\sum m_\nu$ and its impact on voids. Utilizing the redshift dependence of these effects and redshift coverage of these surveys could further yield unique constraints on neutrino properties. We explore this interdependence in our upcoming paper. | 18 | 8 | 1808.07464 |
1808 | 1808.08612_arXiv.txt | Reconnection outflows are regions of intense recent scrutiny, from in situ observations and from simulations. These regions are host to a variety of instabilities and intense energy exchanges, often even superior to the main reconnection site. We report here a number of results drawn from investigation of simulations. First, the outflows are observed to become unstable to drift instabilities. Second, these instabilities lead to the formation of secondary reconnection sites. Third, the secondary processes are responsible for large energy exchanges and particle energization. Finally, the particle distribution function are modified to become non-Maxwellian and include multiple interpenetrating populations. | The research of the last two decades has shown that kinetic reconnection is a fast process that develops on \alf time-scales \citep{biskamp}. This result is a spectacular success for kinetic modelling \citep{birnGEM}, now confirmed in situ by the Magnetospheric Multiscale Mission \citep{burch2016electron}. However, fast kinetic reconnection is not the solution to all problems in reconnection: fast kinetic reconnection has thus far been observed and modelled only in localised regions. Instead, in many astrophysical and laboratory systems, large amounts of energy are converted over large domains. How can we bring fast kinetic reconnection to large scales? A possible scenario to reach large energy conversion rates on system scales is to imagine a situation where the initiation of reconnection is followed by a chain reaction of more and more secondary reconnection sites \citep{bulanov1979tearing,loureiro2007instability,lapenta08,tenerani2016ideally}. Under these conditions, reconnection tends to become chaotic with many reconnection sites being spawned by instability and reabsorbed by island coalescence, leading to fast reconnection \citep{bhattacharjee09,skender,pucci2013reconnection,huang2017plasmoid}. Three dimensional reconnection is accompanied by many more instabilities than just the formation of secondary islands in the primary reconnection site seen in two dimensional reconnection: the reconnection inflow\citep{daughton2011role} and the reconnection outflow~\citep{lapenta2015secondary} host instabilities that lead to secondary reconnection. The first mechanism is primarily present in reconnection separatrices in the case of strong guide fields \citep{lapenta2016reconnection}, while the latter is present at all guide fields \citep{lapenta2014separatrices}. Outflows from reconnection are rich in free energy that can drive instabilities. Among the possibilities we consider here: \begin{itemize} \item Velocity shears around the outflow jet that can drive Kelvin-Helmholtz instability \citep{lottermoser1998ion}. \item Density and temperature gradients at the front formed by the outflowing jet interacting with the ambient plasma leads to drift-type instabilities \citep{divin2015evolution}. \item Unfavourable curvature of field lines between the separatrices in the outflow region can lead to interchange (Rayleigh-Taylor-type) instabilities \citep{nakamura2002interchange,guzdar2010simple,lapenta2011self}. \item Flux ropes in the outflows may be kink unstable \citep{kruskal1958instability,shafranov1957equilibrium}. \item Additional instabilities are caused by phase-space features such as anisotropies leading to whistler waves and beams leading to streaming instabilities \citep{goldman2016can}. \end{itemize} All these instabilities can cause strong deformation of the flow, leading possibly to turbulence \citep{pucci2017properties}, energy exchange \citep{lapenta2016energy} and secondary reconnection~\citep{lapenta2015secondary}. The 3D scenario for large scale turbulence is than one where reconnection might lead to a chain-reaction type of sequence of events. Reconnection is initiated at one location but the instabilities associated with the flows and the other sources of free energy induced by reconnection lead to the formation of secondary reconnection sites. While not yet observed in simulation, this scenario on large scales (not yet accessible to simulation) can then progress in successive generation of tertiary and further reconnection sites, filling macroscopic domains. Below, we organize our material as follows. Section 2 reports the type of simulations we use to analyze the reconnection outflows and the instabilities developing there. Section 3 investigate the fluctuation spectrum produced in the outflow. Section 4 discusses how the fluctuations interact with the particles energizing them. Conclusions and future directions are outlined in Sect. 5. | The analysis of reconnection outflows in the present case of a weak guide field (1/10 of the main reconnecting field) show the development of an instability in the lower hybrid regime. In the present case, the instability has at least two components. The first, is due to the presence of density gradients formed in the pileup region where the outflow meets the ambient plasma. The second is the pre-existing velocity shears due to the differential velocity between the Harris plasma and the ambient plasma \citep{karimabadi2003ion,lapenta03,ricci-lhdi,riccietal04b}. The first instability leads to a Rayleigh-Taylor-type interchange instability in the lower hybrid range, while the latter leads to a kinking of the current layer. Both instabilities feed the onset of a turbulent cascade with the presence of coherent structures and intermittency. The outflows becomes host to secondary reconnection sites where the magnetic field topology becomes chaotic \citep{lapenta2015secondary}. We investigate here the effect of these processes on the energization of particles. The ions and the electrons are energized not only in the primary reconnection site but also, and in some cases predominantly, in the reconnection outflows. Particle energization can be linked to the electric fields operating on the particles. Electric fields do not heat particles in the statistical meaning of increasing their thermal spread, rather they coherently energise all particles, creating beams. Beams originating from different regions interact and interpenetrate creating distribution functions with multiple populations. The end result is that the second order moment of the distribution is increased but the process cannot be interpreted as heating proper but rather as the presence of very non Maxwellian distributions with multiple beams. | 18 | 8 | 1808.08612 |
1808 | 1808.06325_arXiv.txt | We report the discovery and characterization of an eclipsing \SecondaryType~dwarf star, orbiting a slightly evolved \PrimaryType~main sequence star. In contrast to previous claims in the literature, we confirm that the system does not belong to the galactic open cluster Ruprecht 147. We determine its fundamental parameters combining K2 time-series data with spectroscopic observations from the McDonald Observatory, FIES@NOT, and HIRES@KECK. The very precise photometric data from the K2 mission allows us to measure variations caused by the beaming effect (relativistic doppler boosting), ellipsoidal variation, reflection, and the secondary eclipse. We determined the radial velocity using spectroscopic observations and compare it to the radial velocity determined from the beaming effect observed in the photometric data. The \SecondaryType~star has a radius of \SecondaryRadius~\Rsun~and a mass of \SecondaryMass~\Msun. The primary star has radius of \PrimaryRadius~\Rsun~and a mass of \PrimaryMass~\Msun. The orbital period is \PERIOD~days. The system is one of the few eclipsing systems with observed beaming effect and spectroscopic radial velocity measurements and it can be used as test case for the modelling of the beaming effect. Current and forthcoming space missions such as TESS and PLATO might benefit of the analysis of the beaming effect to estimate the mass of transiting companions without the need for radial velocity follow up observations, provided that the systematic sources of noise affecting this method are well understood. | To understand the evolution of stars and planetary systems it is fundamental to derive observationally the fundamental parameters of stars in different stages of their evolution and compare those results with stellar evolution models. Although low mass stars with a mass well below one solar mass are most common in our solar neighborhood, they are not yet completely understood, even in regards to their bulk parameters. They show significant discrepancies between theoretical and observed mass radius relation. For very low mass stars (VLMSs) in a mass regime between $0.1~M_{\odot}$ and $0.6~M_{\odot}$ \citet{Mann2015} found that Dartmouth models \citep[][]{Dotter2008} systematically underestimate the radius by $\approx~4.7\%$ and overestimate the effective temperature by $\approx~2.2\%$. \\ One key observational method to determine the mass and radius of low mass stars is the study of detached eclipsing binaries (DEBs). The DEBCat catalog \citep[][]{Southworth2015} of DEBs lists currently 29\footnote{http://www.astro.keele.ac.uk/jkt/debcat; state of June 2018} well characterized VLMSs with a mass below $0.6~M_{\odot}$. DEBcat is limited to DEBs with their bulk parameters determined to a precision better than $2\%$. Many more DEBs not as well characterized are known \citep[e.g.][]{Eigmuller2016, Gillen2017, Chaturvedi2018}. In this paper we present the detailed characterization of a DEB formed by a main sequence star and a M dwarf companion with precise \textit{K2} photometry and ground-based radial velocity follow-up. We deduce the bulk characteristics of an M dwarf companion to a solar like star modeling \textit{K2} light curve and radial velocity follow up measurements. The high precision light curves by the Kepler satellite allow us not only to model the primary eclipse but also to model the occultation as well as reflection, ellipsoidal variation, and the relativistic beaming effect. Due to the high contrast ratio between late and early type stars, secondary eclipse of M dwarfs are only rarely observed in such systems. The observation of the secondary eclipse and reflection allows us to give additional constrains on the luminosity ratio in the binary system, and the albedo of the M-dwarf. The ellipsoidal variation depends mainly on the mass ratio of the two components and the semi major axis of the system, and thus also gives further constrains on the system parameters. \subsection{Relativistic Beaming} The relativistic beaming effect is caused by the reflex motion of the stars introducing photometric flux variations due to the Doppler effect. The theoretical background of the relativistic beaming effect has been discussed for eclipsing binary stars \citep{Zucker2007} as well as for planetary systems \citep{Loeb2003}. Using light curves of CoRoT and the Kepler satellite a few observations of this effect have been reported in the last years \citep[][]{Mazeh2010, Kerkwijk2010, Bloemen2011, Herrero2014, Faigler2015, TalOr2015}. For a few transiting systems \citep{Mazeh2010,Bloemen2011, Faigler2015} and even more non-transiting systems \citep{TalOr2015} spectroscopic radial velocity measurements are available.\\ The measurement of the relativistic beaming effect allows an independent estimate of the radial velocity of the secondary component of the binary system, which can be used to establish the nature of the companion and to determine the mass ratio between primary and secondary object. This effect has been proposed in the literature as a tool to confirm the nature of transiting planetary companions, which otherwise typically require an extensive ground-based follow-up campaign to confirm their nature. The scheduling of ground-based resources is one of the current challenges for space-borne transit surveys like \textit{K2} or, in the future missions, TESS and PLATO. Understanding the limitations of relativistic beaming effect will allow to establish this method as an independent tool to identify low mass stellar companions, one of the main sources of false-positives for transit surveys. Unfortunately, as it will be shown in this paper, the current state-of-the-art approach neglects the influence of stellar variability, which might compromise the retrieval of the radial velocity value from the photometry \citep{Faigler2015, Csizmadia2018}. In case of disagreement between the radial velocity amplitudes between photometry and spectroscopy, the latter value is preferred. | We discovered an eclipsing binary star \snameone, consisting of an \SecondaryType \, orbiting an \PrimaryType\, star. \snameone\, is not a member of the cluster Ruprecht\,147. High precision photometry by the K2 mission and radial velocity data allowed to characterize the system and both components. The \SecondaryType \, star has a mass of $M_2=$\,\,\SecondaryMass \Msun\, and a radius of $R_2=$ \SecondaryRadius \Rsun. The primary star has an mass of $M_1=$ \PrimaryMass \Msun \, and a radius of $R_2=$\PrimaryRadius \Rsun. The high precision photometry allowed us to observe also the photometric beaming effect. Its amplitude is not in agreement with the radial velocity measured spectroscopically. Using the beaming effect to determine the mass of the secondary objects gives $M_{2LC} = 0.13 \pm 0.04$ \Msun\,, which underestimates the mass by $\approx$35\%. However, detailed analysis of the light curve showed that the amplitude of the out of transit variation changes with time which might hint towards the presence of additional variability in our light curve, preventing us from using beaming to estimate the mass of the companion. Especially for short period binaries, where the rotational period is synchronized with the orbital period this might be a common effect. This shows how careful one has to treat the beaming effect when using it to determine the mass of the secondary object. However, for surveys spanning over several orbital periods, a detailed analysis might help to estimate the mass, taking additional stellar variability into account.\\ The upcoming TESS and future PLATO mission are expected to deliver large numbers of planetary candidates, thus resources for spectroscopic follow up will be limited. For cases such as \snameone\, mass estimate of the secondary object will be needed to distinguish between the companion being an highly inflated hot Jupiter, late M-dwarf, and brown dwarf. It therefore has been proposed to classify such systems without spectroscopic radial velocity follow up by using the beaming effect. Our analysis shows that this needs to be done with care. Additional variability needs to be taken into account and might prevent us from proper modelling of the beaming effect. | 18 | 8 | 1808.06325 |
1808 | 1808.01958_arXiv.txt | We present axisymmetric two-temperature general relativistic radiation magnetohydrodynamic (GRRMHD) simulations of the inner region of the accretion flow onto the supermassive black hole M87. We address uncertainties from previous modeling efforts through inclusion of models for (1) self-consistent dissipative and Coulomb electron heating (2) radiation transport (3) frequency-dependent synchrotron emission, self-absorption, and Compton scattering. We adopt a distance $D=16.7$ Mpc, an observer angle $\theta = 20 \degree$, and consider black hole masses $M/M_{\odot} = (3.3\times10^{9}, 6.2\times10^{9})$ and spins $a_{\star} = (0.5, 0.9375)$ in a four-simulation suite. For each $(M, a_{\star})$, we identify the accretion rate that recovers the 230 GHz flux from very long baseline interferometry measurements. We report on disk thermodynamics at these accretion rates ($\dot{M}/\dot{M}_{\mathrm{Edd}} \sim 10^{-5}$). The disk remains geometrically thick; cooling does not lead to a thin disk component. While electron heating is dominated by Coulomb rather than dissipation for $r \gtrsim 10 GM/c^2$, the accretion disk remains two-temperature. Radiative cooling of electrons is not negligible, especially for $r \lesssim 10 GM/c^2$. The Compton $y$ parameter is of order unity. We then compare derived and observed or inferred spectra, millimeter images, and jet powers. Simulations with $M/M_{\odot} = 3.3\times10^{9}$ are in conflict with observations. These simulations produce millimeter images that are too small, while the low-spin simulation also overproduces X-rays. For $M/M_{\odot} = 6.2\times10^{9}$, both simulations agree with constraints on radio/IR/X-ray fluxes and millimeter image sizes. Simulation jet power is a factor $10^2-10^3$ below inferred values, a possible consequence of the modest net magnetic flux in our models. | The supermassive black hole (SMBH) at the center of the massive elliptical galaxy M87, hereafter simply M87, has been a classic observational target from the millimeter to the $\gamma$-ray for decades. M87 is a valuable laboratory for studying radiatively inefficient accretion flows (RIAF; \citealt{Ichimaru1977, NarayanYi1994, YuanNarayan2014}), jet launching, and the phenomenology of low-luminosity active galactic nuclei (LLAGN), which dominate the population of local SMBHs (\citealt{GreeneHo2007}). Apart from details of the accretion disk, the appearance of a black hole is set by its mass $M$ and dimensionless spin parameter $a_{\star}$. The two leading methods for determining the mass of M87, stellar-dynamical measurements (e.g.\ \citealt{Gebhardt+2011}, who find $M = 6.6\times10^9 M_{\odot}$ for distance $D=17.9$ Mpc) and gas-dynamical measurements (e.g.\ \citealt{Walsh+2013}, who find $M = 3.5\times10^9 M_{\odot}$, also for $D=17.9$ Mpc) currently disagree by a factor $\approx 2$. Note that in this work we prefer $D=16.7$ Mpc (\citealt{Blakeslee+2009}). The spin of M87 is uncertain (although see \citealt{Doeleman+2012} for an argument based on very long baseline interferometry (VLBI) favoring $a_{\star} \gtrsim 0.5$). M87 is detectable at essentially all observed wavelengths: the radio (e.g.\ \citealt{Hada+2011, de Gasperin+2012, Doeleman+2012}), IR (e.g.\ \citealt{Shi+2007, Asmus+2014}), optical/UV (e.g.\ \citealt{Sparks+1996}), X-ray (e.g.\ \citealt{Bohringer+2001, WilsonYang2002, Di Matteo+2003}), and $\gamma$-ray (e.g.\ \citealt{Abdo+2009, Abramowski+2012}). Constructing broadband spectra of LLAGN, however, leads to difficulties: (1) different frequency bands use different observational techniques, leading to inconsistent aperture sizes (2) LLAGN exhibit variability, often on the timescale at which observations at different frequencies may be performed (3) The jet of M87 exhibits several bright knots, especially HST-1 (e.g.\ \citealt{Perlman+2011}). \cite{Prieto+2016} have addressed these issues for M87, creating an optimal set of contemporaneous measurements that led to the identification of two states of accretion: quiescence and outburst. In both cases, the spectrum is nearly flat and featureless across almost 10 decades in frequency, in contrast to typical RIAF models, which contain distinct Compton bumps, at least for a thermal electron distribution function (e.g.\ \citealt{Narayan+1998, Moscibrodzka+2009}). Along with Sagittarius A* (Sgr A*), the Milky Way's SMBH, M87 is one of the two event horizons sufficiently large on the sky for resolved VLBI imaging by the Event Horizon Telescope (EHT, e.g.\ \citealt{Doeleman+2012}). Sgr A* and M87 form a serendipitous pair for studying RIAFs. Despite masses and accretion rates (in Eddington units) differing by several orders of magnitude, and Sgr A* possibly being nearly edge-on (e.g.\ \citealt{Moscibrodzka+2009,Dexter+2010,Shcherbakov+2012}) while M87 is nearly face-on (e.g.\ \citealt{HeinzBegelman1997}), the two sources have approximately the same synchrotron peak frequency. Although Sgr A*'s event horizon is somewhat larger on the sky, particularly if lower measurements for the M87 mass are correct, M87 remains an attractive target for two reasons: (1) intrinsic variability is long compared to the timescale of a global VLBI observation (2) there is only modest interstellar scattering between Earth and M87, in contrast to Sgr A* (\citealt{Bower+2006}). Radio VLBI observations of M87 have already achieved beam sizes of the order of a few Schwarzschild radii (\citealt{Doeleman+2012}), implying a compact population of hot electrons near the black hole, in agreement with previous RIAF models (e.g.\ \citealt{Esin+1997}, \citealt{Moscibrodzka+2009, YuanNarayan2014}). Accretion onto black holes is probably mediated at least in part by angular momentum transport due to the turbulent state resulting from the saturation of the magnetorotational instability (MRI; \citealt{BalbusHawley1991}). The magnetic field may also generate long-range correlations in the accretion disk (\citealt{GuanGammie2011}) and produce jets (\citealt{BlandfordZnajek1977}). These features strongly motivate global general relativistic magnetohydrodynamic (MHD) models of accretion. As an example of the importance of general relativity for M87, \cite{Dexter+2012} have argued that emission is counterjet dominated through gravitational lensing, a purely relativistic effect. Significant progress in modeling RIAFs has been made through numerical simulations (e.g.\ \citealt{DeVilliers+2003, McKinneyGammie2004, Narayan+2012, White+2016}), which allow for a self-consistent treatment of the turbulent stress, as well as capturing the effects of large-scale components of the magnetic field (\citealt{Tchekhovskoy+2011, McKinney+2012}). At very low accretion rates, $\dot{m} \equiv \dot{M}/\dot{M}_{\mathrm{Edd}} \ll 1$ (where the Eddington rate $\dot{M}_{\mathrm{Edd}}$ $\equiv 2.2\times10^{-8}$ $(M/M_{\odot})~M_{\odot}~\mathrm{yr}^{-1}$, i.e.\ we adopt a nominal efficiency $\eta = 0.1$), RIAFs are Coulomb collisionless \citep{MahadevanQuataert1997, Ryan+2017, Sadowski+2017}. Even for such collisionless flows, simple fluid model closures may be sufficient to accurately evolve the total fluid \citep{Foucart+2017}. However, the electron thermodynamics are probably set by Larmor-scale heating and velocity space instabilities (\citealt{Quataert1998, SironiNarayan2015}) which are not captured in ideal MHD. Magnetic reconnection may also play a role in electron heating (\citealt{Rowan+2017}), as well as accelerating nonthermal electrons (e.g.\ \citealt{SironiSpitkovsky2014}) which may have observational consequences for infrared variability and low-frequency radio emission (\citealt{Ozel+2000, Yuan+2003, Chael+2017}). The generic consequence of electron heating through kinetic turbulent dissipation is probably hot protons and somewhat cooler electrons (\citealt{Quataert1998}). Despite the absence of Coulomb collisions, each population may nonetheless be approximately thermal due to kinetic instabilities that feed off distribution function anisotropies, particularly at higher $\beta \equiv 8 \pi n k_B T / B^2$ (\citealt{Kunz+2014, Riquelme+2015, Kunz+2016}). The electron heating probably depends on the local plasma conditions (\citealt{Howes2010}). While the electron temperature in RIAF simulations is often set to a prescribed fraction of the total internal energy (\citealt{Moscibrodzka+2009, Drappeau+2013, Chan+2015, Moscibrodzka+2016}), \cite{Ressler+2015} have developed a method to combine advection and heating based on implicit dissipation in numerical general relativistic magnetohydrodynamic (GRMHD) schemes to self-consistently evolve the electron temperature, which we extended to include Coulomb coupling in \cite{Ryan+2017} (see also \citealt{Sadowski+2017} for a similar method). Post-processing of nonradiative GRMHD simulations is now a standard technique for interpreting LLAGN observations, particularly for Sgr A* where the accretion rate is so low that radiative feedback on the flow dynamics and energetics is negligible (\citealt{Dibi+2012}). GRMHD models, electron physics, accretion rate, black hole spin, and observer angle are all constrained through spectra, variability, polarization, and imaging (e.g.\ \citealt{Moscibrodzka+2009, Dexter+2012, Dolence+2012, Shcherbakov+2012, Drappeau+2013, ShcherbakovMcKinney2013, Moscibrodzka+2014, Chan+2015, Ball+2016, Medeiros+2017, Ressler+2017}). A significant challenge to numerical models of M87 has been the apparent importance of radiative processes to the thermodynamics of the accretion flow. Previous efforts applying nonradiative GRMHD simulations to M87 have had difficulty demonstrating self-consistency \citep{Moscibrodzka+2011, Dexter+2012}. Near the black hole, both synchrotron emission and Compton upscattering cool the electrons. More recently, \cite{Moscibrodzka+2016} achieved reasonable radiative efficiencies, but required a proton-to-electron temperature ratio $T_p/T_e=100$ in the midplane, relatively high compared to those preferred for unambiguously nonradiative models (e.g.\ Sgr A*, \citealt{Moscibrodzka+2014}); for $T_p/T_e < 40$ in the midplane, the M87 models overproduced X-ray emission. While optically thin synchrotron emission is easily incorporated, the Compton $y$ parameter is probably $\sim 1$ for M87 (e.g.\ \citealt{Dexter+2012}). Compton scattering globally couples the disk electrons through the radiative transfer equation. Global GRMHD models with self-consistent radiation transport are therefore strongly motivated. The importance of radiative cooling to electron temperatures and observable radiation in RIAFs above some $\dot{m}$ has long been recognized (e.g.\ \citealt{Esin+1997, Xie+2010, Niedzwiecki+2012}). However, computational expense and algorithmic complexity have restricted the inclusion of radiative transport into GRMHD calculations. \cite{Ohsuga+2009} and \cite{OhsugaMineshige2011} studied the first global radiation MHD models of accretion disks, using a diffusion model for radiation transport to demonstrate the anticipated transition from RIAFs to radiation-dominated thin disks (\citealt{ShakuraSunyaev1973}) with increasing $\dot{m}$. Subsequent work used local models for radiative cooling (\citealt{FragileMeier2009, Dibi+2012, Wu+2016}), or a fluid model for radiation to yield a general relativistic radiation magnetohydrodynamic (GRRMHD) model in axisymmetry (\citealt{Sadowski+2017}) and 3D (\citealt{SadowskiGaspari2017}). Simulations have generally confirmed the picture of a RIAF perturbed by radiative cooling, although details of the transition to radiatively efficient thin disks are still uncertain. We have developed a numerical method, \bhlight{}, for solving the GRRMHD equations with a Monte Carlo method to provide a direct solution to the frequency-dependent radiative transport equation, including emission, absorption, and Compton scattering (\citealt{Ryan+2015}). We introduced \ebhlight{} to include the electron heating scheme of \cite{Ressler+2015} with the Coulomb coupling in \cite{Ryan+2017}. Surveying $\dot{m}$ for $M=10^8M_{\odot}$, \cite{Ryan+2017} found radiative cooling to be significant for $\dot{m} \gtrsim 10^{-5}$, with high-energy spectra progressively hardening and previously distinct Compton bumps merging to form a smooth power-law tail with increasing $\dot{m}$. \ebhlight{} allows us to model optically thin RIAFs in axisymmetry without substantial approximation to the radiation physics, although our model remains sensitive to the electron thermodynamics, and to our assumption that the electron distribution function is thermal. In this work we study a suite of global axisymmetric GRRMHD \ebhlight{} simulations to interpret time-averaged spectral and imaging observations of M87. In Section \ref{sec:eqns} we describe the governing equations, and in Section \ref{sec:numerics} we describe our numerical implementation and present a test of our code. Section \ref{sec:results} presents our models and results. Section \ref{sec:discussion} discusses these results in the context of current and upcoming observations, and Section \ref{sec:conclusion} concludes. | \label{sec:conclusion} We have presented two-temperature GRRMHD models of the inner accretion flow of M87. Along the way, we considered the interplay of dissipative heating, Coulomb coupling, and radiative cooling in RIAFs at $\dot{M}/\dot{M}_{\rm Edd} \sim 10^{-5}$. We found that Compton $y$ parameters $\sim 1$ for these models, consistent with previous estimates. We find that Coulomb heating dominates dissipative heating for electrons for $r \gtrsim 10 r_{\rm G}$. We have demonstrated that radiative cooling is important for the inner region of the M87 accretion flow in our model. For black hole masses bracketing the observationally preferred values and high and low black hole spins, we have derived synthetic observations of spectra and 230 GHz images. Acknowledging uncertainties in our chosen net magnetic field and electron heating model, we exclude a low black hole mass, $M/M_{\odot} = 3.3\times10^9$, through radio image sizes, and the low-mass, low-spin model through overproduction of X-rays. $M/M_{\odot} = 6.2\times10^9$ simulations satisfy radio/IR/X-ray emission and image size. However, jet power is always a factor $10^2-10^3$ lower than previously inferred values. This is probably a consequence of the absence of a strong large-scale poloidal field in our initial conditions. Our model is axisymmetric, which not only limits our time integration window but also renders variability information unreliable. Similar modeling in three spatial dimensions is a critical future direction, albeit much more expensive, especially given our procedure for determining the optimal accretion rate through a series of simulations. | 18 | 8 | 1808.01958 |
1808 | 1808.10255_arXiv.txt | In this work, we apply multichannel singular spectrum analysis (MSSA), a data-adaptive, multivariate, non-parametric technique that simultaneously exploits the spatial and temporal correlations of the input data to extract common modes of variability, to investigate the intermediate quasi-periodicities of the \fe\ green coronal emission line at 530.3 nm for the period between 1944 and 2008. Our analysis reveals several significant mid-term periodicities in a range from about one to four years that are consistent with the so-called quasi-biennial oscillations (QBOs), which have been detected by several authors using different data sets and analysis methods. These QBOs display amplitudes varying significantly with time and latitude over the six solar cycles (18 to 23) covered by this study. A clear North--South asymmetry is detected both in their intensity and period distribution, with a net predominance of spectral power in the active-region belt of the northern hemisphere. On the other hand, while the QBOs with periods $\gtrsim 1.7$ years are particularly intense around the polar regions and therefore related to the global magnetic field, the ones with shorter periods are mainly generated at mid-latitudes, in correspondence with the emergence of active regions. Our findings indicate that the North--South asymmetry manifested in the uneven latitudinal distribution of QBOs is a fundamental, albeit puzzling, characteristic of solar activity. | Solar activity fluctuates with time, exhibiting a wide range of periodicities from scales of a few minutes up to thousands of years as evinced from direct measurements and from proxies based on cosmogenic isotopes. Except for the nearly periodic 11-year activity cycle, due to the emergence of sunspots in a butterfly-shaped pattern, and the 27-day synodic period, due to solar rotation, the question of the origin and evolution of all other periodicities is still under debate. Among the latter, variations at timescales between about one and four years, the so-called quasi-biennial oscillations (QBOs), and variations at timescales of several months, the so-called Rieger-type periodicities (Rieger {\it et al.}, 1984), are gaining increasing attention. These oscillatory modes appear ubiquitous in observations pertaining to the Sun and have been detected in activity proxies that are sensitive to the solar interior, the solar atmosphere, the corona, and even the interplanetary medium (see Bazilevskaya {\it et al.}, 2014 for a recent review). The 11-year sunspot cycle is attributed to the large-scale solar dynamo mechanism operating in the solar interior, but the physical reason for the occurrence of these shorter periodicities, also referred to as intermediate- or mid-term quasi-periodicities, is not completely clear. In this respect, a number of candidate mechanisms have been proposed in the literature. One plausible explanation for their origin involves the possible presence of two different dynamo processes acting in the deep and near-surface layers of the convective zone that are responsible, respectively, for the sunspot cycle and the shorter variations (see, {\it e.g.}, Benevolenskaya, 1998; Fletcher {\it et al.}, 2010; Obridko and Badalyan, 2014; Beaudoin {\it et al.}, 2016). Indeed, periodic variations of 1.3 years in the differential rotation of the deep interior have been discovered by helioseismology (Howe {\it et al.}, 2000) suggesting that the QBOs might be sub-surface in origin. Further support for the dual-dynamo hypothesis has been found in the temporal analysis of stellar cycle data for Sun-like stars (Baliunas {\it et al.}, 1995; Ol\'ah {\it et al.}, 2009; Metcalfe {\it et al.}, 2013; Egeland {\it et al.}, 2015). Alternatively, non-dynamo-based interpretations have also been proposed, involving hydrodynamic (HD) Rossby-type waves (Wolff, 1992; Lou, 2000; Sturrock {\it et al.}, 2015) and magnetic Rossby waves in the solar tachocline leading to the periodic emergence of magnetic flux at the solar surface due to magnetic buoyancy (Zaqarashvili {\it et al.}, 2010). Notwithstanding the above, intermediate-range periodicities are not continuously detected in solar data sets and it is also possible that the bulk of these variations is merely attributable to stochastic processes of magnetic-flux emergence through the photosphere and interaction, through magnetic reconnection, with existing coronal structures and plasma flows (see Wang and Sheeley (2003) for a discussion of this topic). For this reason, the use of a specific technique that is able to distinguish pure oscillatory signals from colored noise at an appropriate significance level (say, $>99\,\%$) can be an important tool for determining the actual presence and evolution of these oscillations. The purpose of this article is to investigate the presence of QBOs in a set of intensity data of the green corona covering as long as six full solar cycles (18 through 23) from 1944 to 2008. The green emission forbidden line of the solar corona (\fe, 530.3 nm) is the brightest coronal line in the optical range and is mainly detected in dense loops and loop clusters of the inner corona at a temperature of about $2 \times 10^6$ K. Its intensity depends on the temperature and density of the coronal plasma, both parameters being modulated by the solar magnetic field. The emission from this line is thus a useful tracer of large coronal structures in both the quiet and the active corona. An advantage of the green-line emission is that it can be acquired on a daily basis almost simultaneously at the solar limb over all heliographic latitudes, thus allowing analysis of the spatio--temporal evolution of solar activity based on uniform data for the entire solar surface. Previous work has already established the presence of intermediate oscillations in the green corona through various techniques, such as the proper orthogonal decomposition (POD) analysis (Vecchio and Carbone, 2009), the empirical mode decomposition (EMD), and the Morlet wavelet transform (Deng {\it et al.}, 2005). Here, we apply an alternative, advanced data-driven method, the multichannel singular spectrum analysis (MSSA) algorithm (see Ghil {\it et al.}, 2002 for a comprehensive review) that is specifically designed to empirically infer the characteristics of the space--time variations of complex systems and identify coherent space--time patterns within a given set of data. The MSSA technique is particularly suitable to investigate the presence of intermediate-range oscillations in the large multivariate data set represented by the green-corona observations. Unlike traditional spectrum analysis, where the basis functions are sinusoidal functions, MSSA has the advantage of being determined from estimates of the lagged cross-covariance where the basis functions are data-adaptive, empirical, and orthogonal. If an oscillation is superimposed on colored noise with power around the frequency of the given oscillation, MSSA is able to distinguish between the part due to the oscillation and the part attributable to the noise. By applying a Monte Carlo significance test, it is finally possible to establish whether the detected oscillations can be distinguished from colored noise with a specified confidence interval. The remainder of the article is organized as follows: Section 2 presents the data set. Section 3 describes the detailed methodology of MSSA. In Section 4, we analyze the \fe\ data set with MSSA and identify spatio--temporal patterns of variability. In Section 5 the results are discussed and conclusions are summarized in Section 6. | We analyzed the spatio--temporal dynamics of the \fe\ green coronal emission line at 530.3 nm in the time interval from 1944 to 2008 to investigate the intermediate-range periodicities by means of a data-adaptive, multivariate technique called multichannel singular spectrum analysis (MSSA). Our analysis revealed the presence of significant oscillatory modes, with periods in a range from about one to four years, which are consistent with the so-called quasi-biennial oscillations (QBOs) that have been detected by several authors using different data sets and analysis methods. QBOs of the coronal \fe\ emission intensity are present in each of the six cycles considered in this study, although not continuously (see Figure 7). A clear North--South asymmetry is detected both in their intensity and period distribution, with a net predominance of spectral power in the active region belt of the northern hemisphere. Moreover, the amplitude of variations in the QBO range changes significantly with time between different cycles. Apart from their intermittent nature, being more or less excited depending on the individual solar cycle under study, QBOs have been found to have a frequency-dependent, uneven distribution in heliographic latitude. In particular, higher-frequency oscillatory modes, with periods between about 1.2 and 1.7 years, are mainly (but not exclusively) concentrated in the northern hemisphere. Intermediate oscillatory modes with periods between about 1.7 and 2.7 years are almost exclusively excited in the southern pole and are present in four of the six solar cycles examined in this study (but quite faint in the previous Solar Cycle 23), with the exception of Solar Cycles 20 and 21. Finally, lower-frequency oscillatory modes, with periods between about 2.7 and 4.0 years, are present in both hemispheres within the magnetically active streamer belt and enhanced in the northern hemisphere, especially at the Pole. In summary, while the QBOs with longer and intermediate periods are particularly powerful around the polar regions, and therefore related to the global magnetic field, the ones with shorter periods appear to be mainly generated at mid-latitudes, in correspondence to the emergence of active regions. This indicates that the group of QBO components with periods $\gtrsim 1.7$ years must be related to different physical mechanisms than the process generating the emergence of active regions, thus supporting the double-dynamo hypothesis or non-dynamo interpretations based on Rossby-type waves. Our findings indicate that the North--South asymmetry manifested in the uneven latitudinal distribution of QBOs is a fundamental, albeit puzzling, characteristic of solar activity. Our results can thus provide more constraints on dynamo models put forward by theoreticians to describe the different components of the solar cycle. In conclusion, we have introduced an alternative approach to extracting and describing the evolution of quasi-biennial oscillations from coronal \fe\ intensity time series, thus showing that MSSA is a viable and complementary tool for exploring the spatio--temporal behavior of intermediate oscillations from multivariate coronal time series. | 18 | 8 | 1808.10255 |
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