Lyman-$α$ Observations of High Radial Velocity Low-Mass Stars Ross 1044 and Ross 825
Adam C. Schneider Evgenya L. Shkolnik Travis S. Barman R. Parke Loyd





Adam C. Schneider
aschneid10@gmail.com


School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85282, USA


School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85282, USA


Lunar & Planetary Laboratory, University of Arizona, 1629 E. University Blvd, Tucson, AZ 85721, USA


School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85282, USA



The discovery of habitable zone (HZ) planets around low-mass stars has highlighted the need for a comprehensive understanding of the radiation environments in which such planets reside. Of particular importance is knowledge of the far-ultraviolet (FUV) radiation, as low-mass stars are typically much more active than solar-type stars and the proximity of their HZs can be one tenth the distance. The vast majority of the flux emitted by low-mass stars at FUV wavelengths occurs in the Lyman-α line at 1216 Å. However, measuring a low-mass star's Lyman-α emission directly is almost always impossible because of the contaminating effects of interstellar hydrogen and geocoronal airglow.  We observed Ross 825 (K3) and Ross 1044 (M0), two stars with exceptional radial velocities, with the STIS spectrograph aboard the Hubble Space Telescope (HST).  Their radial velocities resulted in significant line shifts, allowing for a more complete view of their Lyman-α line profiles.  We provide an updated relation between effective temperature and Lyman-α flux using Gaia DR2 astrometry as well as updated, model-independent relationships between Lyman-α flux and UV flux measurements from the Galaxy Evolution Explorer (GALEX) for low-mass stars.  These new relations, in combination with GALEX's considerable spatial coverage, provide substantial predictive power for the Lyman-α environments for thousands of nearby, low-mass stars. 







§ INTRODUCTION


Both K and M type stars have distinct advantages for studying exoplanets.  M dwarfs are the most abundant stars in the Galaxy <cit.>, have large planet occurrence rates <cit.>, have numerous observational advantages for exoplanet characterization, and have therefore emerged as targets for current and future exoplanet investigations.  K dwarfs, while not as numerous as M dwarfs, also have substantial planet occurrence rates <cit.> and the reduced X-ray and UV radiation incident on HZ planet atmospheres around K dwarfs has led some to suggest that K dwarfs might be the most promising targets for the detection of biology outside of our solar system <cit.>.  For these reasons, understanding the properties of both K and M dwarfs is an essential component of current and upcoming planet-hunting missions such as TESS and PLATO <cit.>. 

The UV radiation emitted by low-mass stars can chemically modify, ionize, and even erode a planetary atmosphere over time, drastically affecting their habitability <cit.>. Furthermore, several studies have shown that incident UV flux could lead to the formation of abiotic oxygen and ozone, resulting in potential false-positive biosignatures <cit.>.  The most prominent FUV emission line, Lyman-α (; λ1215.67 Å), comprises ∼37%–75% of the total 1150–3100 Å flux from most late-type stars <cit.>.  Because  can affect the photodissociation of important molecules such as H_2O and CH_4, any photochemical models assessing potential biosignatures or atmospheric abundances will require accurate  host star flux estimates (e.g., ).         

In almost all cases,  cannot be directly observed because of 1) attenuation of photons by optically thick Hydrogen absorption located in the intervening interstellar medium (ISM) and 2) contamination from Earth's own  geocoronal airglow.  In fact, there exists only one complete  spectrum in the HST archive, that of Kapteyn's star (an M1 subdwarf; ).  With radial velocity (RV) of 245 km s^-1, its stellar  emission feature is shifted out of the geocoronal line core and ISM absorption.  

For all other stars,  emission may only be estimated.  <cit.> reconstructed  fluxes by determining interstellar Hydrogen column densities and velocities inferred from deuterium and metal lines.  Alternatively, reconstructions have been performed by fitting 1 or 2 Gaussians to what remains of the wings of the  profile <cit.>.  Such reconstructions have been used extensively to produce correlations between  fluxes and other spectral emission lines <cit.> and broadband UV photometry from GALEX <cit.>. These correlations have been used to evaluate the capabilities of different spectral types to develop and sustain life (e.g., ).  

The lack of observational  constraints on low-mass stars has limited progress in predicting the environments that surround them.  In an effort to further inform models of low-mass stellar UV flux levels, we have extended the observational sample of stars for which the shape and majority of  flux can be measured beyond just Kapteyn's star. We identified two low-mass targets, Ross 825 (K3) and Ross 1044 (M2).  

lcccccccc
Target Sample

Property     Ross 825     Ref.     Ross 1044     Ref.      
2MASS Name     21111696+3331272     1     15032457+0346574     1  

Spectral Type     K3     2     M0     3  

π (mas)     10.17±0.08     4     26.9 ±8.4     5  

μ_α (mas yr^-1)     502.63 ±0.17     4     -899.1±4.7     5  

μ_δ (mas yr^-1)     159.31 ±0.29     4     695.3±4.6     5  

RV (km s^-1)     -340.17±0.67     4     -169.55±1.80      4  

Age (Gyr)     >10     3     >10     3 

(1) 2MASS <cit.>; (2) <cit.>; (3) This work; (4) <cit.>; (5) <cit.>





§ TARGET SAMPLE


Targets for this program were chosen based on their spectral types (later than K0), distances (within 100 pc), and large absolute radial velocities.  We chose a radial velocity threshold of 150 km s^-1, which results in a ≈0.6 Å wavelength shift of the  emission line.  Such stars are rare because the vast majority of low-mass stars in the solar neighborhood have absolute radial velocities less than 50 km s^-1.  <cit.> found an average radial velocity of 1.6 km s^-1 with a standard deviation of 23.9 km s^-1 for a sample of 159 M dwarfs within 25 pc.  

We identified two targets, Ross 825 and Ross 1044, that satisfied these criteria and were bright enough to acquire a suitable signal-to-noise ratio (S/N) in a reasonable amount of HST time.  The properties of these two stars are summarized below.



 §.§ Ross 825


Ross 825 was first identified as a high proper motion star by <cit.>, who estimated a total proper motion of 048 yr^-1.  <cit.> compiled spectral classifications determined by G. P. Kuiper and published a spectral type of K3 for Ross 825.  Ross 825 was also noted as a close visual double in that work.  The radial velocity of Ross 825 was first measured in <cit.>, who determined a value of -340.68 km s^-1.

Gaia observations <cit.> of Ross 825 confirmed the binary nature of this source (separation ≈16) and its radial velocity (-340.16±0.67 km s^-1).  It is also resolved as a binary in the STIS acquisition images acquired through this investigation. <cit.> provided an effective temperature estimate (T_ eff) of 4680±177 K, which corresponds to a spectral type between K3 and K4 <cit.>, consistent with its measured spectral type.  <cit.> provided Stromgren ubvy–β photometry of Ross 825 and determined a photometric metallicity of [Fe/H] = -1.28 based on empirical relations.  An [Fe/H] value of -1.28 implies an age >10 Gyr according to the models of <cit.>, which is supported by its large absolute radial velocity.  Relevant properties of Ross 825 are summarized in Table 1. 



 §.§ Ross 1044


<cit.> identified Ross 1044 as a high proper motion star with a total proper motion of 127 yr^-1.  A spectral type of K7 was first published for this star in <cit.>.  <cit.> determined a near-infrared spectral type of M2 for Ross 1044.  We determine a spectral type of M0 by comparing the publicly available optical spectrum from the Palomar/MSU survey <cit.> to SDSS standards <cit.>.  Ross 1044 was found to have a large absolute radial velocity in the Palomar/MSU nearby star spectroscopic survey (-159.1 km s^-1; ).  This radial velocity was confirmed in <cit.> (-174±5 km s^-1) and Gaia DR2 (-169.55±1.80 km s^-1).   Ross 1044 was also found to have a low metallicity ([Fe/H] = -1.01±0.21; ), which is consistent with an age >10 Gyr <cit.>. This star also showed no clear signature of Hα either in emission or absorption in the optical spectrum from the Palomar/MSU survey <cit.>.  Note that while Ross 1044 has a measured radial velocity in Gaia DR2, there is no parallax available.  This is likely because Ross 1044 has a visibility_periods_used value of 5 in the Gaia DR2 catalog, which does not satisfy the criterion needed for a five-parameter solution in equation 11 of <cit.>. Two parallax measurements exist in the literature: 47.0±4.1 mas <cit.> and 26.9±8.4 mas <cit.>.  The “quality of interagreement” is listed as “Poor” in <cit.>, and we therefore adopt the <cit.> parallax for this work.  Relevant properties of Ross 1044 are summarized in Table 1.  






§ OBSERVATIONS


We observed Ross 825 and Ross 1044 during HST Cycle 25 with the Space Telescope Imaging Spectrograph (STIS) aboard HST (PID #15286).   For these observations, STIS is preferred over the Cosmic Origins Spectrograph (COS) because COS is a slitless spectrograph, which results in heavy contamination in the  region from geocoronal emission.  We employed the G140M grating (R ∼11400) with a 52× 01 slit.  For each observation, we set the central wavelength to 1222 Å.  Observing dates and total exposure times are provided in Table 2.  

For several of the visits for each object, the HST calstis pipeline failed to locate the correct location of the star.  For both stars, we determined trace positions manually and reran the calstis pipeline for every visit.  For Ross 825, one visit was especially noisy and a trace position could not be determined.  These data are not included in the final spectrum.  Final spectra were produced by coadding the spectra for each visit together for each star and are shown in Figure 1.  While the binary nature of Ross 825 was seen in the STIS acquisition images, the  spectrum comes solely from the primary component of this system.  All subsequent analysis of Ross 825 corresponds exclusively to the primary object of this pair.


lcccccccc
HST/STIS Observations

Object     Obs. Dates     Exposure Time 

     UTC     (s)  
Ross 825     30 July 2018, 3 Aug. 2018     17079  

Ross 1044     26 Nov. 2018, 30 Nov. 2018     15186 





§ ANALYSIS




 §.§  Line Profiles


For both Ross 825 and Ross 1044, the majority of their  emission is directly measured with our STIS observations.  However, neither target is immune to intervening ISM absorption.  For this reason, ISM modeling is necessary to retrieve the intrinsic  flux for both stars.  Following <cit.>, we model the ISM using Voigt profiles for both atomic hydrogen and deuterium.  We fix the D I/ H I ratio to 1.5 × 10^-5 ().  We fix the radial velocity of the  emission to the stellar radial velocity measurements given in Table 1.  Free parameters then include the intrinsic  flux, the column density of hydrogen, and the radial velocity of the intervening ISM.  While there may be multiple ISM components along the line of sight to each star, we model them as a single component.

Whether or not intrinsic  line profiles contain a self-reversal, making them single- or double-peaked, is a large source of uncertainty for  reconstructions, leading to flux differences as large as 30% <cit.>.  For strong lines, such as , the emission in the wings and the core of the line can form in different atmospheric layers, leading to self-reversals in the core <cit.>.  While the Sun shows a self-reversal in its  core, <cit.> argue that M dwarf  profiles should be single-peaked because M dwarf Mg II profiles show signs of self-absorption.  However, we note that the presence of reversals in observed Mg II line cores of M dwarfs is also a consequence of spectral resolution.  <cit.> presented a STIS/E230H spectrum of the M2 star GJ 832, which shows clear signs of reversals in its Mg II line cores.  These reversals are not seen in the MUSCLES NUV spectra (see Figure 5 of ), which were taken with the COS/G230L setting, which has ∼1 order of magnitude less spectral resolution than STIS/E230H.

Our STIS spectra have resolutions similar to COS/G230L.  All models are convolved with the STIS line spread function to compare to observations.  For Ross 1044, the  line shows a clear Gaussian shape, and the peak of the  emission occurs at the same RV as the stellar RV measurement.  Ross 825, on the other hand, has  emission peaking approximately 60 km s^-1 away from the measured stellar RV, suggestive of a double-peaked profile.  We therefore model our M dwarf target (Ross 1044) without a self-reversal and explore models with and without self-reversals for our K dwarf target (Ross 825).  Note that a similar RV offset between peak  emission and the stellar RV is seen for Kepler-444 (spectral type = K1), which has a radial velocity of -121.4 km s^-1 <cit.>. <cit.> found that the  emission for Kepler-444 is best fit by a double-peaked profile.    

For Ross 1044 we use a model consisting of two Gaussians, one strong and narrow and the other weaker and broad, following <cit.>.  We also fit the Ross 825 data with two Gaussians, though the second Gaussian in this case is subtracted from the first to account for the presence of a self-reversal in the line core.  We find posterior distributions for each model parameter using the emcee package <cit.>.  Fore each fit, we run 1000 walkers with 1000 steps, treating the first 300 steps as the burn in sample.  For both objects, we also performed a fit with a single Gaussian, and the two-Gaussian solution is preferred in both cases according to the Bayesian Information Criterion (BIC).  For Ross 825, we also attempted a three-Gaussian fit – broad and narrow profiles as with Ross 1044 and a third component subtracted out.  Again, the BIC prefers the two-Gaussian model compared to the three-Gaussian case.  Our model fits imply that our STIS observed spectra recovered ∼74% and ∼63% of the total intrinsic  flux for Ross 1044 and Ross 825, respectively.  These percentages are significantly higher than that typically seen for field M dwarfs, which is usually ≲30% <cit.>. The best fitting profiles are shown in Figure 1 with best fit parameters given in Table 3 and the resulting  fluxes are provided in Table 6. 

llrlrcccc
emcee Best Fit Model Parameters

     2cRoss 1044     2cRoss 825

2-3
4-5
Model Parameter     Prior     Value     Prior     Value
Amplitude 1 (10^-14 ergs s^-1 cm^-2 Å^-1)     𝒰(0,10)     3.75^+0.34_-1.46     𝒰(0,10)     1.86^+1.82_-0.96  

Sigma 1 (km s^-1)     𝒰(0,5)     0.24±0.01     𝒰(0,10)     0.27^+0.12_-0.02

Amplitude 2a (10^-14 ergs s^-1 cm^-2 Å^-1)     𝒰(0,5)     1.55^+1.82_-0.96     𝒰(0,5)     0.31^+0.61_-0.20 

Sigma 2a (km s^-1)     𝒰(0,25)     0.35^+1.57_-0.12     𝒰(0,5)     0.24^+0.17_-0.03 

RV_ ISM (km s^-1)    𝒰(-50,50)     16.32^+5.18_-3.93     𝒰(-50,50)     -13.39^+30.63_-24.77  

10^18 cm^-2 / d b (pc)     𝒰(4,50)     13.58^+3.28_-2.46     𝒰(4,50)     5.95^+2.72_-1.44 

aThe Gaussian described by Amplitude 2 and Sigma 2 was added to the first Gaussian in the case of Ross 1044, and subtracted in the case of Ross 825.
bRange of H I column densities taken from <cit.>.





§ DISCUSSION




 §.§  Variability of Ross 1044


Ross 1044 has sufficient S/N in each of the six visits to investigate for variability in the strength of the  line.  In Figure 2, we plot the the individual spectra from each visit, as well as our combined final spectrum.  While the general shape and flux is similar for each of the six visits, the total flux for the first three visits is less than that from the last three visits.  We quantify this by summing up the total flux in each visit, then normalizing to the total flux from the final combined spectrum, with uncertainties calculated in a Monte Carlo fashion.  We find that the total flux varied by ∼20% between the smallest and largest flux values measured.  Note that there exists a STIS “breathing” effect, which can cause variability within HST orbits <cit.>.  This effect should be mitigated by the use of spectra averaged over each visit.  The  variability of Ross 1044 is consistent with that seen in the  line of GJ 1132, where a variability up to 22% is seen <cit.> and Kepler-444, where variability of 20–40% is seen between visits <cit.>.  This is also consistent with the variability seen in other UV spectral lines in <cit.>, where variability from 1–41% is measured. The number and cadence of our observations prevents us from determining the cause of Ross 1044's variability (e.g., rotational modulations, flares).  A dedicated monitoring campaign could pinpoint the cause of this variability. 

 




 §.§  Correlations

Correlating intrinsic  fluxes with stellar properties gives us insight into the behavior of high energy emission of stars in various evolutionary and temperature regimes.  Relationships between stellar properties and  flux values can also provide a powerful means to predict  fluxes for stars without  observations.  



  §.§.§  Behavior with Effective Temperature


<cit.> showed a correlation between stellar effective temperature () and  flux.  We revisit this relationship here to include additional measurements published since that paper as well as Ross 825 and Ross 1044.  These additions increase the entire sample size by ∼30%, with 47  measurements used in <cit.> and 61 used in this work, two of which are new measurements.  We also use updated  measurements as well as more precise parallaxes from Gaia DR2 <cit.>. Table 6 lists these properties (, age, parallax) and intrinsic  fluxes for all stars with  flux measurements.  For Kapteyn's Star, <cit.> directly measured a  flux of 5.32 × 10^-13 erg s^-1 cm^-2, though no uncertainty is provided for this measurement.  We assume an uncertainty of 5% for this directly measured line. 

Figure 3 shows the  flux for each star in Table 6 scaled to 1 AU versus their effective temperatures.  While <cit.> partitions different stellar samples based on their rotation rates, we instead choose to use age because age measurements are more readily available for this sample.  We note that determining precise stellar ages is a challenging process, and many different methods can be used depending on the star in question and the data available for that star.  All ages used in this work are provided in Table 6.  For more information on how a particular age was determined, please refer to the reference provided in the table. There is a clear difference between young (<1 Gyr) and old (>1 Gyr) samples in Figure 3.   emission is typically much stronger from young stars than old stars with similar  values.  For old stars, a very clear trend of decreasing  flux strength with decreasing  can be seen.  We fit this trend using a least-squares optimization approach, with uncertainties handled in a Monte Carlo fashion.  We find:


    log_10(F_ Lyα)_ 1 AU = (6.754±0.263)×10^-4() - 2.639±0.137 .


The RMS scatter about the fit is ∼0.21 dex.  We also plot in Figure 3 the  flux scaled to the HZ for each target.  The distance to the HZ for each star is calculated using their effective temperatures and radii listed in Table 6 and equations 4 and 5 of <cit.> and the updated coefficients in <cit.>.  The HZ distance is taken as the average value between the distances to the moist greenhouse and maximum greenhouse boundaries.  While the relation between  flux and  flattens out for this sample, it is notable that the slope of this relation positive, implying that, in general, the  flux in the HZ increases with decreasing .  The relationship between  HZ flux and  is:


    log_10(F_ Lyα)_HZ = (-1.601±0.243)×10^-4() + 1.804±0.126 .


The RMS scatter about the fit is ∼0.19 dex.





  §.§.§ Predicting  Fluxes with GALEX Photometry


Because  emission is impossible to measure directly for almost all stars, and even indirect methods, such as reconstructions, require valuable HST time, correlations between  fluxes and other activity indicators are immensely valuable as predictive tools.  <cit.> used GALEX photometry and reconstructed  emission profiles from <cit.>, <cit.>, and <cit.> to show trends between NUV and FUV photometry and the reconstructed  fluxes for K and M dwarfs.  We update these correlations with precision parallaxes from Gaia and new and updated  fluxes from this work, <cit.>, <cit.>, and <cit.>.  The new NUV relation contains three objects not included in the <cit.> relation; Ross 1044, Kapteyn's Star, and GJ 176.  We also produce these new correlations using flux at 1 AU instead of excess surface flux as in <cit.>, which avoids any uncertainties from model photospheric flux and radius estimates.  Thus for the NUV relation, we only include stars where the photospheric NUV flux is negligible, namely M stars (at all ages) and young K stars (, ).  

Table 4 summarizes all observed  flux measurements for M stars and young K stars with NUV detections in GALEX.  Note that there are many young K dwarfs from Table 4 that were observed by GALEX, however the majority of the sources were saturated (NUV < 15 mag), and are thus not included here.  For objects that have detections from multiple GALEX surveys, the magnitudes presented in Tables 4 and 5 are the weighted average of their flux densities, converted back to magnitude.  Figure 4 shows a comparison of the the  flux values versus GALEX NUV flux values at 1 AU.  We perform a least-squares fit to the data and find the following relation between  flux and GALEX NUV fluxes: 


    log_10(F_ Lyα) = (0.701±0.019)* log_10(F_ NUV) + 0.193±0.043 .


Uncertainties for the fit are determined in a Monte Carlo fashion.  The RMS scatter about this fit is 0.10 dex.

Some nearby, low-mass stars are too bright for reliable GALEX NUV measurements.  For these stars, it may be possible to use FUV fluxes.  We plot GALEX FUV fluxes versus  fluxes in Figure 4, with FUV data given in Table 5.  All K and M dwarfs with unsaturated GALEX FUV measurements are included in the FUV relation, with the total number of stars used increasing from 10 in <cit.> to 13 in this work with the addition of HD 97658, HD 40307, and GJ 176.  We perform a fit using the same methods as for GALEX NUV data and find:


    log_10(F_ Lyα) = (0.742±0.038)* log_10(F_ FUV) + 0.945±0.053 .


The RMS scatter about this fit is 0.13 dex.  The utility of these correlations can be far-reaching, as GALEX has archived flux measurements for thousands of late-K and M dwarfs in the solar neighborhood (e.g., , , , , , ).

lrrrrrrrr
GALEX NUV and  Fluxes for M Dwarfs and Young K Dwarfs

Name     Spectral     Age     plxa          NUV 

     Type     (Gyr)     (mas)     (erg cm^-2 s^-1)     (mag)  
PW And     K2     0.15^+0.05_-0.02     35.29 ± 0.05     (2.31±0.46)×10^-12     15.317 ± 0.012 
 
Speedy Mic     K3     0.03±0.01     14.98 ± 0.27     (1.85±0.37)×10^-12     15.469 ± 0.009 
 
Ross 1044     M0     >10     26.9 ± 8.4     (2.54^+0.14_-0.11)×10^-14     21.571 ± 0.095 

Kapteyn's Star     sdM1     11.5^+0.5_-1.5     254.23 ± 0.03     (5.27±0.26)×10^-13     19.103 ± 0.048 

AU Mic     M1     0.02±0.01     102.83 ± 0.05     (1.07±0.04)×10^-11     15.614 ± 0.010 
 
GJ 832     M2     8.4     201.41 ± 0.04     (9.5±0.6)×10^-13     18.393 ± 0.036 
 
GJ 176     M2     4.0±0.3     105.56 ± 0.07     (3.9±0.2)×10^-13     18.825 ± 0.052 
 
GJ 436     M3     4.2±0.3     102.50 ± 0.09     (2.1±0.3)×10^-13     20.685 ± 0.198 
 
AD Leo     M3     0.025-0.3     201.37 ± 0.07     (8.07±0.20)×10^-12     15.814 ± 0.017 
 
GJ 876     M4     9.51±0.58     213.87 ± 0.08     (3.9±0.4)×10^-13     20.149 ± 0.087 
 
Prox Cen     M5.5     5.3±0.3     768.50 ± 0.20     (4.37±0.07)×10^-12     18.601 ± 0.086 
 
aParallaxes from Gaia <cit.>, with the exception of Ross 1044 <cit.>.


 

lrrrrrrrr
GALEX FUV and  Fluxes for K and M Dwarfs

Name     Spectral     Age     plx          FUV

     Type     (Gyr)     (mas)     (erg cm^-2 s^-1)     (mag)  
DX Leo     K0     0.25±0.05     55.32 ± 0.06     (2.31±0.46)×10^-12     18.188 ± 0.053  
 
HR 1925     K1     0.5±0.1     81.43 ± 0.05     (4.57±0.91)×10^-12     18.695 ± 0.091  
 
HD 97658     K1     9.7±2.8     46.35 ± 0.05     (9.1^+9.9_-4.5)×10^-13     21.568 ± 0.308  
 
EP Eri     K2     0.2±0.1     96.54 ± 0.04     (6.05±1.21)×10^-12     17.779 ± 0.018  
 
ϵ Eri     K2     0.5±0.1     312.22 ± 0.47     (6.1±0.2)×10^-11     16.049 ± 0.023  
 
LQ Hya     K2     0.07^+0.03_-0.02     54.68 ± 0.07     (4.01±0.80)×10^-12     17.217 ± 0.037  
 
HD 40307     K2.5     6.9±4.0     77.27 ± 0.03     (2.0^+2.2_-0.9)×10^-13     21.728 ± 0.346  
 
Speedy Mic     K3     0.03±0.01     14.98 ± 0.27     (1.85±0.37)×10^-12     18.604 ± 0.079  
 
ϵ Ind     K5     1.6±0.2     274.80 ± 0.25     (3.10±0.62)×10^-11     17.881 ± 0.047  
 
AU Mic     M1     0.02±0.01     102.83 ± 0.05     (1.07±0.04)×10^-11     17.436 ± 0.038  
 
GJ 832     M2     8.4     201.41 ± 0.04     (9.5±0.6)×10^-13     20.687 ± 0.168  
 
GJ 176     M2     4.0±0.3     105.56 ± 0.07     (3.9±0.2)×10^-13     21.130 ± 0.347  
 
GJ 876     M4     9.51±0.58     213.87 ± 0.08     (3.9±0.4)×10^-13     22.560 ± 0.360  
 




§ CONCLUSION

We presented HST/STIS observations of Ross 1044 and Ross 825, two low-mass stars with sufficient to directly observe the majority of each star's intrinsic  flux (∼74% for Ross 1044 and ∼63% for Ross 825).  We combined these new  measurements with updated astrometric information and physical parameters to explore the relationship between  flux and .  We also constructed new correlations between  fluxes and GALEX UV photometry.

Astrometric missions such as Gaia will soon reveal numerous additional nearby stars with radial velocities for which such observations are possible.  High-RV targets are the only way to directly observe the majority of the  flux from low mass stars.  While the exact RV value needed depends on a target's distance and the ISM properties towards that target, a minimum absolute value of ∼100 km s^-1, corresponding to a wavelength shift of ≈0.4 Å, is likely necessary.   Of the ∼73 thousand stars within 50 pc with radial velocity measurements in the Gaia DR2 catalog, only 120 (0.16%) have radial velocities >100 km s^-1.  Each of these objects are thus invaluable as laboratories for investigating the UV environments of nearby stars.  



rrrrrrrrrrrrrrr
Properties of All Stars with Intrinsic  Fluxes

R.A.     Dec.     Other     Spectral     ref     T_ eff     ref     Radius     ref     plxa          ref     Age     ref 

()     ()     Name     Type         (K)         R_⊙          (mas)     (erg cm^-2 s^-1)         (Gyr)       
New
317.823653042     +33.524935535     Ross 825     K3     1     4680±177     19     0.57^+0.15_-0.07     15     10.17±0.08     (2.65^+0.38_-0.76)×10^-15     3     >10     3 

225.848498243     +03.785579277     Ross 1044     M0     3     3754±95     25     0.380±0.029     25     26.9±8.4     (2.54^+0.14_-0.11)×10^-14     3     >10     3 

Old
067.695503172     +16.148537247     SAO 93981     F2     8     6567±49     26     1.474^+0.058_-0.047     27     22.12±0.06     (1.41±0.28)×10^-12     63     1.16±0.82     38 

…    …    Procyon     F5IV-V     7     6648±71     21     2.13±0.07     19     284.56±1.26     (1.47±0.03)×10^-10     65     1.97^+0.19_-0.23     21 

066.577555282     +21.470267340     SAO 76609     F8V     9     6376±80     23     1.30±0.02     15     20.67±0.07     (2.70±0.54)×10^-13     63     0.63±0.05     41 

076.377476718     -57.472196938     ζ Dor     F9V     4     6147±53     21     1.07^+0.02_-0.07     15     86.02±0.15     (7.99±1.60)×10^-12     63     0.68±0.47     38  

184.446711916     -36.093975066     HR 4657     F9V     4     6258±39     21     1.096±0.033     27     44.74±0.81     (1.28±0.26)×10^-12     63     1.8±0.5     37 

330.795021930     +18.884241930     V376 Peg     F9V     9     6071±20     22     1.18±0.02     15     20.68±0.05     (1.50±0.30)×10^-13     63     3.83^+0.98_-0.70     22 

066.900016757     +15.589088762     V993 Tau     G0     8     6197±94     23     1.27^+0.03_-0.02     15     20.93±0.07     (5.90±1.18)×10^-13     63     0.63±0.05     41  

088.594850554     +20.275864770     χ Ori     G0V     4     5898±25     17     0.9791±0.0091     17     113.12±0.32     (1.27±0.25)×10^-11     63     0.3±0.1     37 

238.171482022     +42.454228071     χ Her     G0     1     5890±53     28     1.7090±0.0200     29     63.16±0.15     (2.04±0.41)×10^-12     63     6.85^+0.42_-0.50     22 

271.599395673     -36.019748261     HR 6748     G0V     4     5932±80     23     0.98^+0.01_-0.04     15     58.13±0.12     (3.73±0.75)×10^-12     63     0.44±0.19     40 

…    …    Sun     G2V     …    5778     …    1.0     …    …    5.95-9.15b     65     4.566±0.005     66 

…    …    α Cen A     G2V     4     5793±7     17     1.2329±0.0037     17     754.81±4.11     (1.01±0.20)×10^-10     63     5.3±0.3     34 

112.676660493     -37.339135172     HR 2882     G2V     4     5830±76     22     0.93^+0.02_-0.01     15     45.93±0.03     (2.76±0.55)×10^-12     63     0.35±0.07     40  

168.133469825     +35.813428317     HR 4345     G2     1     5906±29     28     0.99±0.03     30     44.14±0.04     (2.09±0.42)×10^-12     63     0.45±0.02     40 

049.841563537     +03.370603059     κ Cet     G4     1     5723±76     17     0.9193±0.0247     17     109.34±0.31     (8.44±1.70)×10^-12     63     0.6±0.2     22 

129.458268090     -06.806694022     SAO 136111     G5V     7     5836±45     28     1.00±0.02     15     41.08±0.04     (1.34±0.27)×10^-12     63     0.51±0.14     40 

093.438506782     -23.861458285     HR 2225     G6.5V     4     5651±43     28     0.890±0.04     18     59.76±0.02     (3.45±0.69)×10^-12     63     0.28–0.36     39 

199.596454977     -18.315774489     61 Vir     G7V     4     5538±13     18     0.987±0.005     18     117.57±0.24     (1.69±0.34)×10^-12     63     9.41^+1.31_-3.15     22  

222.848032870     +19.100275013     ξ Boo A     G7V     7     5483±32     17     0.8627±0.0107     17     148.52±0.24     (1.85±0.37)×10^-11     63     0.2±0.1     37 

001.655138540     +29.020738353     HR 8     G8V     7     5327±39     17     0.9172±0.0090     17     72.58±0.05     (4.77±0.95)×10^-12     63     0.3±0.1     22 
 
220.129108915     -16.209576638     SAO 158720     G8V     4     5574±50     23     0.87^+0.02_-0.04     15     42.09±0.04     (1.44±0.29)×10^-12     63     0.62±0.07     40 

321.170945318     -68.227107394     SAO 254993     G8V     4     5480±80     23     0.87±0.01     15     48.06±0.34     (2.47±0.49)×10^-12     63     0.33±0.08     40 

026.009302877     -15.933798651     τ Cet     G8.5V     4     5290±39     17     0.8154±0.0122     17     277.52±0.52     (9.99±2.0)×10^-12     63     5.6±1.2     37 

201.437998838     +56.970541983     SAO 28753     G9V     7     5308±36     28     0.84±0.03     30     46.17±0.02     (1.62±0.32)×10^-12     63     0.33±0.10     40 

300.182122282     +22.709775924     HD 189733     K2V     7     5019±23     44     0.76±0.01     44     50.57±0.03     (7.48±1.50)×10^-13     65     6.4^+4.8_-4.2     30 

085.334752342     +53.478804107     HR 1925     K0V     7     5243±32     28     0.850±0.050     18     81.43±0.05     (4.57±0.91)×10^-12     63     0.5±0.1     22 
 
063.808266649     -07.667602660     40 Eri A     K0.5V     4     5147±14     18     0.805±0.004     18     198.57±0.51     (6.93±1.39)×10^-12     63     11.76^+1.92_-5.19     22 
 
143.181619543     +26.987467661     DX Leo     K1V     4     5315±35     28     0.81±0.02     15     55.32±0.06     (2.31±0.46)×10^-12     63     0.25±0.05     43 
 
168.637658369     +25.710596159     HD 97658     K1     7     5157±29     28     0.72±0.02     30     46.35±0.05     (9.1^+9.9_-4.5)×10^-13     59     9.7±2.8     44 
 
271.365865680     +02.494347134     70 Oph A     K1     1     5407±52     18     0.831±0.004     18     195.22±0.10     (2.14±0.43)×10^-11     63     1.3±0.3     37 

289.752832267     +41.631884026     Kepler-444     K1     1     5053±45     28     0.748±0.006     24     27.41±0.03     (5.47±1.77)×10^-14     60     11.23^+0.91_-0.99     33 

043.135621621     -12.770528318     EP Eri     K1.5V     4     5167±44     21     0.79^+0.04_-0.02     15     96.54±0.04     (6.05±1.21)×10^-12     63     0.2±0.1     22 
 
…    …    α Cen B     K2IV     4     5232±8     17     0.8761±0.0029     17     796.92±25.90     (1.50±0.30)×10^-10     63     5.3±0.3     34 

004.587759006     +30.955409811     PW And     K2V     11     4796±80     23     0.72^+0.01_-0.02     15     35.29±0.05     (2.31±0.46)×10^-12     63     0.15^+0.05_-0.02     45 
 
053.228430601     -09.458171515     ϵ Eri     K2V     4     5077±35     18     0.735±0.005     18     312.22±0.47     (6.1±0.2)×10^-11     59     0.5±0.1     22 
 
057.604601528     +17.246412457     V471 Tau     K2V     10     5291±167     27     0.83±0.03     31     20.96±0.04     (2.87±0.57)×10^-12     63     0.63±0.05     41 
 
143.105446328     -11.184489206     LQ Hya     K2V     12     5376±43     17     1.0029±0.0158     17     54.68±0.07     (4.01±0.80)×10^-12     63     0.07^+0.03_-0.02     43 
 
258.834197764     -26.606646978     36 Oph A     K2     1     5103±29     28     0.76±0.02     15     167.82±0.16     (1.42±0.28)×10^-11     63     1.7±0.4     22 

349.865586265     +79.003831692     V368 Cep     K2V     7     5075±36     28     0.76^+0.02_-0.01     15     52.73±0.03     (2.99±0.60)×10^-12     63     0.09^+0.06_-0.04     43 
 
088.517219072     -60.023728916     HD 40307     K2.5V     4     4925±71     22     0.71±0.01     15     77.27±0.03     (2.0^+2.2_-0.9)×10^-13     59     6.9±4.0     44 
 
311.937623890     -36.595010211     Speedy Mic     K3V     13     4609±80     23     1.42^+0.03_-0.06     15     14.98±0.27     (1.85±0.37)×10^-12     63     0.03±0.01     40 
 
330.871402102     -56.796902312     ϵ Ind     K4V     4     4649±36     21     0.732±0.006     32     274.81±0.25     (3.10±0.62)×10^-11     63     1.6±0.2     21 
 
316.747737594     +38.763411107     61 Cyg A     K5     5     4361±17     18     0.665±0.005     35     285.95±0.10     (1.72±0.34)×10^-11     63     6.0±1.0     35 

147.782122563     -43.504816171     HD 85512     K6V     4     4455±80     23     0.70^+0.02_-0.04     15     88.62±0.04     (1.2^+1.8_-0.5)×10^-12     59     5.61±0.61     50 

077.958661296     -45.043019828     Kapteyn's Star     sdM1     2     3570±80     20     0.291±0.025     20     254.23±0.04     (5.27±0.26)×10^-13     20     11.5^+0.5_-1.5     20 

311.291136883     -31.342450016     AU Mic     M1     6     3588±87     16     0.698±0.021     16     102.83±0.05     (1.07±0.04)×10^-11     61     0.02±0.01     45 
 
259.751060918     -34.997765110     GJ 667C     M1.5V     48     3472±73     6     0.37±0.05     6     138.02±0.09     (5.2±0.9)×10^-13     59     >2     51  

323.391261617     -49.012516898     GJ 832     M2     6     3522±60     6     0.442±0.013     16     201.40±0.04     (9.5±0.6)×10^-13     59     8.4     46 
 
070.735385329     +18.953357960     GJ 176     M2     6     3679±77     18     0.453±0.022     18     105.56±0.07     (3.9±0.2)×10^-13     59     4.0±0.3     18 
 
154.898887344     +19.869815836     AD Leo     M3     6     3308±84     16     0.422±0.013     16     201.37±0.07     (8.07±0.20)×10^-12     61     0.025–0.3     36 
 
175.550536327     +26.703066902     GJ 436     M3     6     3416±53     18     0.455±0.018     18     102.50±0.09     (2.1±0.3)×10^-13     59     4.2±0.3     18 
 
229.856472500     -07.722693495     GJ 581     M3     6     3415±87     16     0.330±0.01     16     158.75±0.05     (1.1^+0.3_-0.2)×10^-12     59     4.1±0.3     18 

258.831399085     +04.960679473     GJ 1214     M4V     49     3008±96     16     0.203±0.008     16     68.27±0.17     (1.3^+1.4_-0.5)×10^-14     59     5–10     52 

343.323973712     -14.266595816     GJ 876     M4     6     3129±19     18     0.376±0.006     18     213.87±0.08     (3.9±0.4)×10^-13     59     9.51±0.58     47 
 
153.709069589     -47.154935771     GJ 1132     M4     1     3216±100     16     0.217±0.008     16     79.25±0.04     (2.9^+0.4_-0.3)×10^-14     64     >5.0     62 

341.702962603     +44.332017083     EV Lac     M5     6     3273±86     16     0.341±0.010     16     198.01±0.04     (2.75±0.55)×10^-12     63     0.025–0.3     36 

217.393465743     -62.676182103     Prox Cen     M5.5     14     2840±88     16     0.154±0.005     16     768.50±0.20     (4.37±0.07)×10^-12     61     5.3±0.3     34 
 
346.626391870     -05.043461802     Trappist-1     M8     53     2550±55     53     0.117±0.004     53     80.45±0.12     (7.5±2.3)×10^-15     53     7.6 ±2.2     54 

278.907460881     +32.994892032     LSR J1835+3259     M8.5     55     2578±39     56     0.137±0.007     57     175.82±0.09     (7.0±3.5)×10^-15     58     …    …

aAll parallaxes come from Gaia <cit.> with the following exceptions: Procyon, α Cen A, α Cen B, and HR 4657 <cit.>, and  Ross 1044 <cit.>.
bThe  flux range given for the Sun encompasses both the active and quiet Sun values given in <cit.>.
(1) <cit.>; (2) <cit.>; (3) This work; (4) <cit.>; (5) <cit.>; (6) <cit.>; (7) <cit.>; (8) <cit.>; (9) <cit.>; (10) <cit.>; (11) <cit.>; (12) <cit.>; (13) <cit.>;  (14) <cit.>; (15) <cit.>; (16) <cit.>; (17) <cit.>; (18) <cit.>; (19) <cit.>; (20) <cit.>; (21) <cit.>; (22) <cit.>; (23) <cit.>; (24) <cit.>; (25) <cit.>; (26) <cit.>; (27) <cit.>; (28) <cit.>; (29) <cit.>; (30) <cit.>; (31) <cit.>; (32) <cit.>; (33) <cit.>; (34) <cit.>; (35) <cit.>; (36) <cit.>; (37) <cit.>; (38) <cit.>; (39) <cit.>; (40) <cit.>; (41) <cit.>; (42) <cit.>; (43) <cit.>; (44) <cit.>; (45) <cit.>; (46) <cit.>; (47) <cit.>; (48) <cit.>; (49) <cit.>; (50) <cit.>; (51) <cit.>; (52) <cit.>; (53) <cit.>; (54) <cit.>; (55) <cit.>; (56) <cit.>; (57) <cit.>; (58) <cit.>; (59) <cit.>; (60) <cit.>; (61) <cit.>; (62) <cit.>; (63) <cit.>; (64) <cit.>; (65) <cit.>; (66) <cit.>  



This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program #15283. Support for program #15283 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.  A. S. and E. S. appreciate support from NASA/Habitable Worlds grant NNX16AB62G.  T. B. was supported in part by NASA HST grant HST-AR-13911 and NASA Habitable Worlds grant NNX16AB62G.This work is based on observations made with the NASA Galaxy Evolution Explorer. GALEX is operated for NASA by the California Institute of Technology under NASA contract NAS5-98034.  This work has made use of data from the European Space Agency (ESA) mission Gaia (<https://www.cosmos.esa.int/gaia>), processed by the Gaia Data Processing and Analysis Consortium (DPAC, <https://www.cosmos.esa.int/web/gaia/dpac/consortium>). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

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