arXiv subjects

Mary Barsony

Publications and source records attributed to Mary Barsony.

13 recordsLinked to original sources

arXiv2024

Unveiling two deeply embedded young protostars in the S68N Class 0 protostellar core with JWST/NIRSpec

The near-infrared (NIR) emission of the youngest protostars still needs to be characterized to better understand the evolution of their accretion and ejection activity. We analyze James Webb Space Telescope NIRSpec 1.7 -- 5.3 $\mu$m observations of two deeply embedded sources in the S68N protostellar core in Serpens. The North Central (NC) source exhibits a highly obscured spectrum (A_K ~ 4.8 mag) that is modeled with a pre-main-sequence photosphere and a hot disk component. The photospheric parameters are consistent with a young, low-mass photosphere, as suggested by the low surface gravity, log g of 1.95 $\pm$ 0.15 cm s$^{-2}$. The hot disk suggests that accretion onto the central protostellar embryo is ongoing, although prototypical accretion-tracing emission lines HI are not detected. The South Central (SC) source, which is even more embedded (A_K ~ 8 mag; no continuum is detected shortward of 3.6 $\mu$m) appears to be driving the large-scale S68N protostellar outflow, and launches a collimated hot molecular jet detected in \Ht and CO ro-vibrational lines. Shock modeling of the \Ht (ro)vibrational lines establishes that fast $C$-type shocks ($\geq$ 30 km s$^{-1}$), with high pre-shock density ($\geq$ $10^7$ cm$^{-3}$), and strong magnetic field (b ~ 3--10, where $B = b\,\times\,\sqrt{\textrm{n}_{\textrm{H}} (\textrm{cm}^{-3})}\,\mu\textrm{G}$) best match the data. The bright CO fundamental line forest suggests energetic excitation, with the contribution of non-LTE effects, ie irradiation pumping. Detected OH and CH$^{+}$ ro-vibrational lines support this hypothesis. These two Class 0 protostars seem to be in very young evolutionary stages and still have to acquire the bulk of their final stellar masses. These results demonstrate that JWST enables unprecedented diagnostics of these first stages of the protostellar evolutionary phase.

astro-ph.SR↗

arXiv2024

Resolving Twin Jets and Twin Disks with JWST and ALMA: The Young WL 20 Multiple System

We report the discovery of jets emanating from pre-main-sequence objects exclusively at mid-infrared wavelengths, enabled by the superb sensitivity of JWST's Mid-InfraRed Medium-Resolution Spectrometer (MIRI MRS) instrument. These jets are observed only in lines of [NiII], [FeII], [ArII], and [NeII]. The H$_2$ emission, imaged in eight distinct transitions, has a completely different morphology, exhibiting a wide-angled, biconical shape, symmetrically distributed about the jet axes. Synergistic high-resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations resolve a pair of side-by-side edge-on accretion disks lying at the origin of the twin mid-infrared jets. Assuming coevality of the components of the young multiple system under investigation, the system age is at least (2 $-$ 2.5) $\times$ 10$^6$ yr, despite the discrepantly younger age inferred from the spectral energy distribution of the combined edge-on disk sources. The later system evolutionary stage is corroborated by ALMA observations of CO(2$-$1), $^{13}$CO(2$-1$), and C$^{18}$O(2$-$1), which show no traces of molecular outflows or remnant cavity walls. Consequently, the observed H$_2$ structures must have their origins in wide-angled disk winds, in the absence of any ambient, swept-up gas. In the context of recent studies of protostars, we propose an outflow evolutionary scenario in which the molecular gas component dominates in the youngest sources, whereas the fast, ionized jets dominate in the oldest sources, as is the case for the twin jets discovered in the WL 20 system.

astro-ph.SR↗

arXiv2018

Detection of Photospheric Features in the Near-Infrared Spectrum of a Class 0 Protostar

We present a near-infrared $K$-band $R \simeq 1500$ Keck spectrum of S68N, a Class 0 protostar in the Serpens molecular cloud. The spectrum shows a very red continuum, CO absorption bands, weak or non-existent atomic metal absorptions, and H$_2$ emission lines. The near-IR H$_2$ emission is consistent with excitation in shocks or by X-rays but not by UV radiation. We model the absorption component as a stellar photosphere plus circumstellar continuum emission with wavelength-dependent extinction. A Markov Chain Monte Carlo analysis shows that the most likely model parameters are consistent with a low-temperature, low-gravity photosphere with significant extinction and no more than modest continuum veiling. Its $T_{\mathrm{eff}} \simeq 3260$ K effective temperature is similar to that of older, more evolved pre-main-sequence stars, but its surface gravity log $g \simeq 2.4$ cm s$^{-2}$ is approximately 1 dex lower. This implies that the radius of this protostar is a factor of $\sim 3$ larger than that of $10^6$ yr old T Tauri stars. Its low veiling is consistent with a circumstellar disk having intrinsic near-IR emission that is less than or equal to that of more evolved Class I protostars. Along with the high extinction, this suggests that most of the circumstellar material is in a cold envelope, as expected for a Class 0 protostar. This is the first known detection and analysis of a Class 0 protostar absorption spectrum.

astro-ph.SR↗

arXiv2012

A Significant Population of Candidate New Members of the Rho Ophiuchi Cluster

We present a general method for identifying the pre-main-sequence population of any star-forming region, unbiased with respect to the presence or absence of disks, in contrast to samples selected primarily via their mid-infrared emission from Spitzer surveys. We have applied this technique to a new, deep, wide-field, near-infrared imaging survey of the Rho Ophiuchi cloud core to search for candidate low mass members. In conjunction with published Spitzer IRAC photometry, and least squares fits of model spectra (COND, DUSTY, NextGen, and blackbody) to the observed spectral energy distributions, we have identified 948 candidate cloud members within our 90% completeness limits of J=20.0, H=20.0, and K_S=18.50. This population represents a factor of ~3 increase in the number of known young stellar objects in the Rho Ophiuchi cloud. A large fraction of the candidate cluster members (81% +/- 3%) exhibit infrared excess emission consistent with the presence of disks, thus strengthening the possibility of their being bona fide cloud members. Spectroscopic follow-up will confirm the nature of individual objects, better constrain their parameters, and allow an initial mass function to be derived.

astro-ph.GA↗

arXiv2010

Near-IR Molecular Hydrogen Emission of Protostars: Probing Circumstellar Environments

We present new observations of near-infrared molecular hydrogen (H2) line emission in a sample of 18 Class I and flat-spectrum low mass protostars, primarily in the Tau- Aur and ρ Oph dark clouds. The line emission is extended by up to several arcseconds (several hundred AU) for most objects, and there is little night-to-night variation in line strength coincident with the continuum point source. Flux ratios of H2 v = 2 - 1 S(1) and v = 1 - 0 S(1) lines are consistent with this emission arising in jets or winds in many objects. However, most objects have only small offsets (under 10 km s-1) between their H2 and photospheric radial velocities. No objects have line ratios which are clearly caused solely by UV excitation, but the H2 emission of several objects may be caused by UV or X-ray excitation in the presence of circumstellar dust. There are several objects in the sample whose observed velocities and line fluxes suggest quiescent, non-mechanical origins for their molecular hydrogen emissions. Overall we find the H2 emission properties of these protostars to be similar to the T Tauri stars studied in previous surveys.

astro-ph.SR↗

arXiv2006

Mid-Infrared Observations of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds

We have obtained new mid-infrared observations of 65 Class I/Flat-Spectrum (F.S.) objects in the Perseus, Taurus, Chamaeleon I/II, Rho Ophiuchi, and Serpens dark clouds. We detected 45/48 (94%) of the single sources, 16/16 (100%) of the primary components, and 12/16 (75%) of the secondary/triple components of the binary/multiple objects surveyed. The composite spectral energy distributions (SEDs) for all of our sample sources are either Class I or F.S., and, in 15/16 multiple systems, at least one of the individual components displays a Class I or F.S. spectral index. However, the occurrence of mixed pairings, such as F.S. with Class I, F.S. with Class II, and, in one case, F.S. with Class III, is surprisingly frequent. Such behaviour is not consistent with that of multiple systems among T Tauri stars (TTS), where the companion of a classical TTS also tends to be a classical TTS, although other mixed pairings have been previously observed among Class II objects. Based on an analysis of the spectral indices of the individual binary components, there appears to be a higher proportion of mixed Class I/F.S. systems (65-80%) than that of mixed Classical/Weak-Lined TTS (25-40%), demonstrating that the envelopes of Class I/ F.S. systems are rapidly evolving during this evolutionary phase. We report the discovery of a steep spectral index secondary companion to ISO-ChaI 97, detected for the first time via our mid-infrared observations. In our previous near- infrared imaging survey of binary/multiple Class I/F.S. sources, ISO-ChaI 97 appeared to be single. With a spectral index of Alpha >= 3.9, the secondary component of this system is a member of a rare class of very steep spectral index objects, those with Alpha > 3. Only three such objects have previously been reported, all of which are either Class 0 or Class I.

astro-ph↗

arXiv2006

First Evidence of a Precessing Jet Excavating a Protostellar Envelope

We present new, sensitive, near-infrared images of the Class I protostar, Elias 29, in the Ophiuchus cloud core. To explore the relationship between the infall envelope and the outflow, narrowband H2 1-0 S(1), Br-gamma, and narrowband K-continuum filters were used to image the source with the Wide-Field Infrared Camera on the Hale 5m telescope and with Persson's Auxiliary Nasmyth Infrared Camera on the Baade 6.5 m telescope. The source appears as a bipolar, scattered light nebula, with a wide opening angle in all filters, as is typical for late-stage protostars. However, the pure H2 emission-line images point to the presence of a heretofore undetected precessing jet. It is argued that high-velocity, narrow, precessing jets provide the mechanism for creating the observed wide-angled outflow cavity in this source.

astro-ph↗

arXiv2005

A Mid-Infrared Imaging Survey of Embedded Young Stellar Objects in the Rho Ophiuchi Cloud Core

Results of a comprehensive, new, ground-based mid-infrared imaging survey of the young stellar population of the Rho Ophiuchi cloud are presented. Data were acquired at the Palomar 5-m and at the Keck 10-m telescopes with the MIRLIN and LWS instruments, at 0.25 arcsec and 0.25 arcsec resolutions, respectively. Of 172 survey objects, 85 were detected. Among the 22 multiple systems observed, 15 were resolved and their individual component fluxes determined. A plot of the frequency distribution of the detected objects with SED spectral slope shows that YSOs spend ~400,000 yr in the Flat Spectrum phase, clearing out their remnant infall envelopes. Mid-infrared variability is found among a significant fraction of the surveyed objects, and is found to occur for all SED classes with optically thick disks. Large-amplitude near-infrared variability, also found for all SED classes with optically thick disks, seems to occur with somewhat higher frequency at the earlier evolutionary stages. Although a general trend of mid-infrared excess and NIR veiling exists proceeding through SED classes, with Class I objects generally exhibiting K-veilings > 1, Flat Spectrum objects with K-veilings > 0.58, and Class III objects with K-veilings =0, Class II objects exhibit the widest range of K-band veiling values, 0-4.5. However, the highly variable value of veiling that a single source can exhibit in any of the SED classes in which active disk accretion can take place is striking, and is direct observational evidence for highly time-variable accretion activity in disks. Finally, by comparing mid-infrared vs. near-infrared excesses in a subsample with well-determined effective temperatures and extinction values, disk clearing mechanisms are explored. The results are consistent with disk clearing proceeding from the inside-out.

astro-ph↗

arXiv2004

X-ray Properties of Pre--Main-Sequence Stars in the Orion Nebula Cluster with Known Rotation Periods

We re-analyze all archival Chandra/ACIS observations of the Orion Nebula Cluster (ONC) to study the X-ray properties of a large sample of pre--main-sequence (PMS) stars with optically determined rotation periods. Our goal is to elucidate the origins of X-rays in PMS stars by seeking out connections between the X-rays and the mechanisms most likely driving their production--rotation and accretion. In our sample X-ray luminosity is significantly correlated with stellar rotation, in the sense of decreasing Lx/Lbol with more rapid rotation, suggesting that these stars are in the "super-saturated" regime of the rotation-activity relationship. However, we also find that stars with optical rotation periods are significantly biased to high Lx. This is not the result of magnitude bias in the optical rotation-period sample but rather to the diminishingly small amplitude of optical variations in stars with low Lx. Evidently, there exists in the ONC a population of stars whose rotation periods are unknown and that possess lower average X-ray luminosities than those of stars with known rotation periods. These stars may sample the linear regime of the rotation-activity relationship. Accretion also manifests itself in X-rays, though in a somewhat counterintuitive fashion: While stars with spectroscopic signatures of accretion show harder X-ray spectra than non-accretors, they show lower X-ray luminosities and no enhancement of X-ray variability. We interpret these findings in terms of a common origin for the X-ray emission observed from both accreting and non-accreting stars, with the X-rays from accreting stars simply being attenuated by magnetospheric accretion columns. This suggests that X-rays from PMS stars have their origins primarily in chromospheres, not accretion.

astro-ph↗

arXiv2003

A Near-Infrared Multiplicity Survey of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds

We present new near-IR observations of 76 Class I/flat-spectrum objects in the nearby (d < 320 pc) Perseus, Taurus, Chamaeleon I and II, rho Ophiuchi, and Serpens dark clouds. These observations are part of a larger systematic infrared multiplicity survey of self-embedded objects in the nearest dark clouds. When combined with the results of our previously published near-infrared multiplicity survey, we find a restricted companion star fraction of 14/79 (18% +/- 4%) of the sources surveyed to be binary or higher order multiple systems over a separation range of ~300 - 2000 AU with a magnitude difference $Δ$ K <= 4, and with no correction for background contamination or completeness. This is consistent with the fraction of binary/multiple systems found among older pre-main-sequence T Tauri stars in each of the Taurus, rho Ophiuchi, and Chamaeleon star-forming regions over a similar separation range, as well as the combined companion star fraction for these regions. However, the companion star fraction for solar-type, and lower mass M dwarf, main-sequence stars in the solar neighborhood in this separation range (11% +/- 3%) is approximately one-half that of our sample. Together with multiplicity statistics derived for previously published samples of Class 0 and Class I sources, our study suggests that a significant number of binary/multiple objects may remain to be discovered at smaller separations among our Class I/flat-spectrum sample and/or most of the evolution of binary/multiple systems occurs during the Class 0 phase of early stellar evolution.

astro-ph↗

arXiv2002

Structure of the Mid-Infrared Emitting Disk Around WL16

WL16 is a unique member of the embedded young stellar population in the nearby rho Ophiuchi cloud core: its extended (900 AU diameter), high surface brightness disk is visible only at mid-infrared wavelengths. We present diffraction-limited (~ 0.3 arcsec) images, from 7.9 to 24.5 microns, of WL16 acquired at the Keck II telescope. We find the central star to be a 250 Lsun, 4 Msun, Herbig Ae star, seen through foreground material of the rho Oph cloud core that provides an extinction of Av=31+/-1 magnitudes. WL16's disk is detected through all nine observed passbands. We confirm, therefore, that the emitting particles are composed of both polycyclic aromatic hydrocarbons (PAHs) and very small (5--100 Angstroms) graphitic grains (VSGs). We confirm a resolved (1.5" diameter) core component at 7.9 and 8.8 microns, due to emission from positively charged PAHs. An enhancement in the emission at 12.5 microns at the disk's edges is found for the first time, and signals the presence of larger (>= 50-80 carbon atoms) PAHs and/or more hydrogenated PAHs than those found in the bulk of the disk. We find a disk asymmetry, observed at all nine mid-infrared wavelengths, at projected radii 1"-2.5" (corresponding to 125 AU <= r <= 300 AU) from the central source.

astro-ph↗

arXiv2000

A Luminous Infrared Companion in the Young Triple System WL 20

We present spatially resolved near-infrared and mid-infrared (1-25 microns) imaging of the WL 20 triple system in the nearby (d=125 pc) rho Ophiuchi star-forming cloud core. We find WL 20 to be a new addition to the rare class of "infrared companion systems", with WL 20:E and WL 20:W displaying Class II (T-Tauri star) spectral energy distributions (SEDs) and total luminosities of 0.61 and 0.39 L_sun, respectively, and WL 20:S, the infrared companion, with a Class I (embedded protostellar) SED and a luminosity of 1.0-1.8 L_sun. WL 20:S is found to be highly variable over timescales of years, to be extended (40 AU diameter) at mid-infrared wavelengths, and to be the source of the centimeter emission in the system. The photospheric luminosities, estimated from our data, allow us to compare and test current pre-main-sequence evolutionary tracks. WL 20:E and WL 20:W fall into the region of the H-R diagram in which sources may appear up to twice as old as they actually are using non-accreting tracks, a fact which may reconcile the co-existence of two T-Tauri stars with an embedded protostar in a triple system. The derived masses and observed projected separations of the components of the WL 20 triple system indicate that it is in an unstable dynamical configuration, and may therefore provide an example of dynamical evolution during the pre-main-sequence phase.

astro-ph↗

arXiv1999

From Pre-Stellar Cores to Protostars: The Initial Conditions of Star Formation

The last decade has witnessed significant advances in our observational understanding of the earliest stages of low-mass star formation. The advent of sensitive receivers on large radio telescopes such as the JCMT and IRAM 30m MRT has led to the identification of young protostars at the beginning of the main accretion phase (`Class 0' objects), and has made it possible to probe, for the first time, the inner density structure of pre-collapse cores. Class 0 objects are characterized by strong, centrally-condensed dust continuum emission at submillimeter wavelengths, very little emission shortward of 10 microns, and powerful jet-like outflows. Direct evidence for gravitational infall has been observed toward several of them. They are interpreted as accreting protostars which have not yet accumulated the majority of their final stellar mass. In contrast to protostars, pre-stellar cores have flat inner density profiles, suggesting the initial conditions for fast protostellar collapse depart sometimes significantly from a singular isothermal sphere. In the case of non-singular initial conditions, the beginning of protostellar evolution is expected to feature a brief phase of vigorous accretion/ejection which may coincide with Class 0 objects. In addition, submillimeter continuum imaging surveys of regions of multiple star formation such as Ophiuchus and Serpens suggest a picture according to which each star in an embedded cluster is built from a finite reservoir of mass and the associated IMF is primarily determined at the pre-stellar stage of evolution.

astro-ph↗