The Near-Infrared Outflow and Cavity of the proto-Brown Dwarf Candidate ISO-Oph 200
Emma Whelan Basmah Riaz Bastien Rouze
astro-ph.SR astro-ph.GA






 

Maynooth University Department of Experimental Physics, National University of Ireland Maynooth, Maynooth Co. Kildare, Ireland
 Max-Planck-Institut fr Extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany



The ISO-Oph 200 NIR Outflow 


In this letter a near-infrared integral field study of a proto-brown dwarf candidate is presented. A ∼ 05 blue-shifted outflow is detected in both H_2 and [Fe II] lines at V_sys = (-35 ± 2) km s^-1 and V_sys = (-51 ± 5) km s^-1 respectively. In addition, slower (∼ ± 10 km s^-1) H_2 emission is detected out to < 54, in the direction of both the blue and red-shifted outflow lobes but along a different position angle to the more compact faster emission. It is argued that the more compact emission is a jet and the extended H_2 emission is tracing a cavity. The source extinction is estimated at A_v = 18 mag ± 1 mag and the outflow extinction at Av = 9 mag ± 0.4 mag. 
The H_2 outflow temperature is calculated to be 1422 K ± 255 K and the electron density of the [Fe II] outflow is measured at ∼ 10000 cm^-3. Furthermore, the mass outflow rate is estimated at Ṁ_out[H2] = 3.8 × 10^-10  yr^-1 and Ṁ_out[FeII] = 1 × 10^-8  yr^-1. Ṁ_out[FeII] takes a Fe depletion of ∼ 88% into account. The depletion is investigated using the ratio of the [FeII] 1.257 μm and [PII] 1.188 μm lines. Using the Paβ and Brγ lines and a range in stellar mass and radius Ṁ_acc is calculated to be (3 - 10) × 10^-8  yr^-1. Comparing these rates puts the jet efficiency in line with predictions of magneto-centrifugal models of jet launching in low mass protostars. This is a further case of a brown dwarf outflow exhibiting analogous properties to protostellar jets.



The Near-Infrared Outflow and Cavity of the proto-Brown Dwarf Candidate ISO-Oph 200Based on Observations collected with SINFONI at the Very Large Telescope on Cerro Paranal (Chile), 
operated by the European Southern Observatory (ESO). Program ID: 097.C-0732(A).
    Whelan, E.T. 1
  Riaz, B. 2
   Rouzé, B. 1
    
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§ INTRODUCTION


Outflows play an important role in the star formation process
and are driven by young stellar objects (YSOs) in the Class 0 to
Class II evolutionary stages <cit.>. Thus, they can
be observed across a large range in wavelength <cit.>.
Also, as different wavelength regimes can offer complementary observational constraints to launching models, multi-wavelength studies have proven important <cit.>. Near-infrared (NIR) integral field observations are a particularly
useful tool for studies of jet launching in Class 0/I sources which
are still very much embedded in their natal clouds <cit.>. The J,H, and K NIR bands cover Fe II forbidden and H_2 ro-vibrational emission lines that are strong tracers of jets. These lines can be used to infer jet properties such as temperature and density and to measure the extinction towards the jet <cit.>. Integral field spectroscopy provides spectro-images which allow the properties of the jets to be mapped as a function of velocity and distance <cit.>. Such studies have found the [Fe II] and H_2 lines to be tracing different components of the jet, with the forbidden emission associated with faster, hot dense material and the molecular emission tracing lower excitation, slower molecular gas <cit.>. Additionally, H_2 is often found to trace cavities which are postulated to be evacuated by a wide-angled wind <cit.>.

Recently, we reported the first detection of a large-scale Herbig-Haro optical jet driven by a Class I proto-brown dwarf (proto-BD) that shows several of the well-known features seen in protostellar HH jets <cit.>. Here we present the results of a near-infrared study of an outflow from a proto-BD candidate, which also shows characteristics similar to protostellar jets. The driving source of the outflow is a YSO named ISO-Oph 200 (α = 16:31:43.8, δ = - 24:55:24.5). The near- to mid-infrared (2-24 μm) spectral slope of the SED for ISO-Oph 200 is consistent with a Class I classification <cit.>. We have conducted physical and chemical modelling of sub-millimeter/millimeter continuum and molecular line data for this object, and find the results to be consistent with an early Stage 0+I evolutionary stage (Riaz et al. in prep). The total (dust + gas) mass for ISO-Oph 200 as derived from the sub-millimeter 850 μm flux is 0.06±0.01  with a bolometric luminosity of ∼ 0.09±0.02 . The mass of the central object in this Class I system can be constrained to within the sub-stellar mass regime using the measured bolometric luminosity and numerical simulations of stellar evolution, as discussed in <cit.>. Considering the very low mass reservoir in the (envelope+disk) for this system, and the presence of an outflow that will further dissipate the envelope material, ISO Oph-200 will likely have a final mass below the sub-stellar limit, and can be considered as a strong candidate proto-BD.












§ OBSERVATIONS AND DATA REDUCTION


ISO-Oph 200 was observed at the Very Large Telescope (VLT) using the integral field spectrograph SINFONI, on April 19 2016 (H and K band, seeing = 1.1), May 12 2016 (J band, seeing = 13), and May 13 2016 (K band, seeing = 055). SINFONI is
a medium resolution instrument with R =2000, 3000, 4000, in the J, H and K bands respectively <cit.>. Observations were made with the 8 × 8 field of view (FOV) and therefore a pixel scale of 0125. The recommended observing mode of ABB^'A^' sky nodding and a jittering between the source and sky positions was chosen. The total exposure times for the J, H and K bands were 2250 s, 240 s and 120 s (both observations) respectively. The SINFONI pipeline was used to produce the wavelength calibrated, sky subtracted datacubes. The wave- length calibration provided by the pipeline was checked using the OH lines and an average shift of 40 km s^-1 was measured across all four cubes. This was comparable to the systematic wavelength shift of 2.2 Å reported by <cit.>. The telluric subtraction and flux calibration were carried out in the usual way using standard star observations <cit.>. The extracted J, H and K band flux calibrated spectra are presented in Figure 1 with the J, H and K magnitudes as measured by the Two Micron All Sky Survey (2MASS) over plotted <cit.>.







§ RESULTS

The fluxes of the lines marked in Figure 1 are given in Table <ref>. Numerous [Fe II] and H_2 lines are detected along with the H I lines Pa , Br  and Br 12, which are known to primarily trace accretion. In Figure 2, spectro-images in the H_2 1-0S(1) and [Fe II] 1.257 μm lines are presented.
A compact blue-shifted outflow is detected in all lines, while larger scale, slower, red and blue-shifted extended emission is detected in H_2 only. Evidence is presented below that this slower more extended emission is a cavity.






 §.§ Extinction and mass accretion rate estimates

The source visual extinction can firstly be derived using the J, H and K magnitudes. Following the method as described in <cit.>, where the source is plotted on a NIR color - color diagram and its position compared to the relevant locus, the source extinction is estimated at A_v = 12 ± 1 mag. A second approach is to estimate A_v simultaneously with L_acc by assuming that L_acc measured from Brγ and Paβ agree (see the following paragraph of this section for a description of how L_acc is calculated. This gives an estimate of A_v = 18 ± 1 mag. Thirdly one can use the outflow lines extracted at the source position to derive the outflow extinction at this point. This can be taken as a lower limit to the source extinction. Forbidden transitions of [Fe II] that arise from the same upper level are used to estimate A_v with the [Fe II] 1.644 μm / 1.25 μm ratio frequently used <cit.>. From this ratio the outflow extinction here is estimated at 9.0 ± 0.4 mag. The Einstein coefficients as reported in <cit.> were adopted along with the extinction law of <cit.>. Considering that the estimate of A_v from the ratio of the [Fe II] 1.644 μm / 1.25 μm lines, is consistent with the jet extinction being a lower limit to the source extinction, the estimate of A_v 18 ± 1 mag is adopted for the source.



The mass accretion rate (Ṁ_acc) was estimated from the luminosity of the Paβ and Brγ lines. The procedure as outlined in <cit.> was followed along with the L_line and L_acc relations of <cit.>. Fluxes were corrected for a source extinction of A_v = 18 mag. For the mass, a range of 0.06  to 0.1  was adopted with corresponding radii of 0.9  and 0.5  respectively. This range is based on estimates of the source luminosity and the cold and hybrid accretion models of <cit.>. The average value of log(Ṁ_acc) is found to be in the range -7.0 ± 0.4 to -7.5 ± 0.4. <cit.> found log(Ṁ_acc) ∼ -6.5 for the very low mass protostar IRS54 (using the Brγ line) therefore the results presented here are consistent with the idea that ISO-Oph 200 is a proto-BD. In <cit.> and <cit.> we investigated Ṁ_acc for the Class I BDs Mayrit 1701117 and Mayrit 1082188, and report values in the range log(Ṁ_acc) = -9.3 to -9. These values were calculated using optical spectra and are likely underestimated. This is due to scattering effects which are more prominent in this wavelength range and the likely obscuration of some component of the accretion emission. Here the Paβ and Brγ emission lines regions were analysed using spectro-astrometry to check for any contribution from the outflow and none was found <cit.>.




 
  
  
   
  
 




 §.§ Outflow morphology, kinematics and physical properties

In Figure 2, continuum subtracted H_2 1-0S(1) and [Fe II] 1.257 μm spectro-images of the source in different velocity bins are presented. All velocities are systemic and the velocity bins were chosen to highlight the outflow features, as outlined in the figure caption. In Table 1 the properties of the outflow features are compared. In the top panels of Figure 2 the emission in a predominantly blue-shifted velocity range is shown. The peak velocity of the H_2 and [Fe II] emission is measured at (-35 ± 2) km s^-1 and (-51 ± 5) km s^-1 respectively. These radial velocities are in line with the radial velocities of Class II BD outflows <cit.>. A 2D fitting of the emission regions shows that the H_2 emission peaks at a distance of 07 from the source along a position angle (PA) of 230^∘ and the [Fe II] emission at a distance of 05 from the source along a position angle (PA) of 215^∘. The emission in this velocity bin is compatible with a small-scale jet where the jet width is not spatially resolved in the observations. The difference between the velocity of the H_2 and [Fe II] jets is consistent with the fact that the H_2 tracers the outer layers of the jet where material is cooler and slower <cit.>. 

 

In the bottom panels the H_2 and [Fe II] emission in the velocity range -96 km s^-1 to 128 km s^-1 is shown. The velocity range was extended to 128 km s^-1 to check for a red-shifted jet lobe along the same PA and with a similar velocity to the blue-shifted lobe. None is detected. However, low velocity (∼ 10 km s^-1), fainter (also see Figure <ref>), red and blue-shifted spatially resolved H_2 emission is seen at distance of 26 to 54 and along a different PA to the faster emission. The main emission features here are marked F1, F2 and F3 in Figure 2. The velocity, morphology and PA of this emission would suggest that it is tracing a cavity around the collimated jet. This emission is not detected in [Fe II] and is discussed further in Section 4.



A H_2 excitation diagram can be constructed by plotting the natural log of the column densities (divided by the statistical weight), of a number of H_2 lines against the upper energy level of the line. This can be used to derive the temperature and to constrain the extinction of the line emitting region <cit.>. In a thermalised gas at a single temperature the points will lie along a straight line, the slope of which is the temperature <cit.>. The extinction is constrained by investigating the scatter of the points about this straight line. Correcting the line fluxes for extinction should reduce the scatter of the points about a linear fit. Therefore, varying A_v and analysing the goodness of the fit leads to an estimate of A_v <cit.>. A H_2 excitation diagram for the ISO-Oph 200 jet was constructed (see Figure <ref>) using the fluxes given in Table 1. For A_v = 9 mag, T = 1442 ± 255 K is measured. This result is in agreement with the temperatures reported by <cit.> for the very low mass protostar IRS54. By varying the extinction it is found that the analysis of the excitation diagrams is in a agreement with the A_v estimates presented in Section 3.1. 

Using the ratios of the [Fe II] lines and the models of <cit.>, the electron density (n_e) can be estimated. Studies have found that the 1.600 μm / 1.644 μm, 1.664 μm / 1.644 μm, and 1.677 μm / 1.644 μm ratios are more accurate tracers of the density than the 1.534 μm / 1.644 μm ratio. However, due to the non-detection of the 1.600 μm, 1.664 μm and 1.677 μm lines here we are limited to using the 1.534 μm / 1.644 μm ratio. From this ratio and the models we estimate ne ∼ 10,000 cm^-3. This is in line with typical jet densities <cit.>.

The mass outflow rate (Ṁ_out) can be investigated by calculating the mass of both H_2 and H (from the [Fe II] lines) in the jet and combining this with the jet velocity and the length of the jet over for which the mass was estimated. Using equations 1 and 2 of <cit.> the column density of H_2 and H are estimated at N_H2 = 3.6 × 10^21 m^-2 and N_H =1.5 × 10^22 m^-2. The fluxes of the H_2 1-0S(1) and [Fe II] 1.644 μm lines were used here and were corrected for the jet extinction (A_v = 9 mag). For the jet velocity the same approach as adopted in <cit.> is used here. That is the line width is taken as a lower limit to the jet velocity. The lower limits to the jet velocity are 100 km s^-1 and 150 km s^-1 for the H_2 and [Fe II] line respectively. For Ṁ_out it is found that Ṁ_out[H2] = 3.8 ∼ 10^-10  yr^-1 and Ṁ_out[FeII] = 1.2 × 10^-9  yr^-1. For Ṁ_out[FeII] a Fe / H solar abundance was assumed. Using the ratio of the [FeII] 1.257 μm and [PII] 1.188 μm lines it is found that 88% ± 14 % of the Fe is still in dust grains in the jet <cit.>. Taking this value of the dust depletion into account places Ṁ_out[FeII] at ∼ 1 × 10^-8  yr^-1. Overall our results put the jet efficiency (Ṁ_out / Ṁ_acc) within the limits of magneto-centrifugal jet launching models <cit.>. Also note that as the mass outflow rate is higher in the [Fe II] than H_2 component of the jet, our results support the argument of <cit.>, that most of the outflow material is transported in the component traced by the atomic rather than molecular lines.








§ DISCUSSION AND CONCLUSIONS


High angular resolution observations of jets from low mass YSOs have provided critical constraints to jet launching models in the form of collimation, rotation, and structural studies for example <cit.>. The difficulty with obtaining such high angular resolution observations, of jets at the lower end of the mass spectrum, has meant that the question of whether there are differences in how BD jets and stellar jets are launched has not yet been well investigated <cit.>. To date, discussion of BD jet launching has focused on a comparison between the kinematical and morphological properties of BD and YSO jets (e.g. asymmetries, multiple velocity components, molecular components) and on measurements of the jet efficiency. High angular resolution is not necessary to measure the jet efficiency and magneto-centrifugal jet launching models set an upper limit of 30% for Ṁ_out / Ṁ_acc <cit.>. Thus, studies of Ṁ_out / Ṁ_acc have been used as a first test of jet launching at the lowest masses <cit.>.

The detection here of a molecular hydrogen jet and cavity adds to the list of ways in which jets at the lowest masses are found to be analogous to stellar jets. Molecular hydrogen emission lines (MHELs) have long been associated with stellar jets with two jet components detected. Both a molecular component to a collimated FEL jet and a component which traces excitation along the walls of a edge-brightened cavity are observed <cit.>. For the latter component, it is theorised that the jet is surrounded by a wide-angled wind which carves out a cavity in the ambient medium and that the shocked H_2 emission is a result of the interaction between the wind and the ambient gas <cit.>. <cit.> give the wide-angled wind opening angles of a number of Class 0/I sources. Values range between 25^∘ and 100^∘ and there is a trend of the opening angle increasing with age. In Figure <ref>, the H_2 cavity emission is shown in green over-plotted on the H_2 jet emission (red). The extended H_2 emission from ISO-Oph 200 is fitted with a bipolar cavity of opening angle of 40^∘, putting our source within the range of the Class I sources of <cit.>. The fact that the extended H_2 emission has different geometry and kinematics to the blue-shifted jet as traced by both H_2 and [Fe II] supports the argument that what we are seeing is a wind-swept cavity. A slow wide-angled wind is a natural by-product of a magneto-hydrodynamic (MHD) disk wind and its detection here is further evidence that MHD jets occur at the lowest masses <cit.>.

Turning now to estimates of jet efficiencies for proto-BDs, it would be reasonable to expect Ṁ_out in proto-BDs to be higher than for Class II BDs, as seen for low mass YSOs <cit.>, and for the jet efficiencies to remain within the limits of leading jet launching models. <cit.> presented some evidence that Ṁ_out in proto-BDs is higher than in Class II BDs but very few sources have been properly investigated to date. The first studies of Ṁ_out / Ṁ_acc in BDs concentrated on Class II sources and values were found to be > 1 which is inconsistent with models. Improvements in data and methods led to a better constrained estimate for one Class II BD ISO-Cha I 217 <cit.>. Observations of proto-BD jets have to date been confined to optical data which has led to problems with estimates of jet efficiencies. Firstly, the jet and source extinction is generally not well known and secondly Ṁ_acc has been under-estimated due to poor sampling of the accretion zone by the accretion tracers. The efficiency study presented here demonstrates the importance of NIR observations for investigating Ṁ_out / Ṁ_acc in BD jets. The [Fe II] and H_2 lines allow both the jet and source extinction to be well studied, the dust depletion in the jet to be investigated, and obscuration effects are limited by the use of the NIR HI lines. These improvements have meant that this is the second case where efficiency has been found to be within accepted limits. Also note that Ṁ_out in ISO-Oph 200 (∼ 10^-9) is higher than in ISO-ChaI 217 (∼ 10^-11) as would be expected.

In conclusion we argue that NIR studies of proto-BD jets are an important tool for investigating jet launching in the BD mass regime. Evidence of outflow components such as wide-angled winds can be looked for, the source and jet extinction can be investigated and measurements of the jet efficiency made. Furthermore, it will be possible in the near future to carry out observations of the jet collimation and structure at high angular resolution with JWST / NIRSPEC, which can provide additional constraints to jet launching models.






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§ EMISSION LINES FLUXES AND EXCITATION DIAGRAM



















