arXiv subjects

John J. Tobin

Publications and source records attributed to John J. Tobin.

At least 145 records · Page 8Linked to original sources

arXiv2016

The Gould's Belt Distances Survey (GOBELINS) II. Distances and structure towards the Orion Molecular Clouds

We present the results of the Gould's Belt Distances Survey (GOBELINS) of young star forming regions towards the Orion Molecular Cloud Complex. We detected 36 YSOs with the Very Large Baseline Array (VLBA), 27 of which have been observed in at least 3 epochs over the course of 2 years. At least half of these YSOs belong to multiple systems. We obtained parallax and proper motions towards these stars to study the structure and kinematics of the Complex. We measured a distance of 388$\pm$5 pc towards the Orion Nebula Cluster, 428$\pm$10 pc towards the southern portion L1641, 388$\pm$10 pc towards NGC 2068, and roughly $\sim$420 pc towards NGC 2024. Finally, we observed a strong degree of plasma radio scattering towards $λ$ Ori.

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arXiv2016

Revolutionizing our View of Protostellar Multiplicity and Disks: The VLA Nascent Disk and Multiplicity (VANDAM) Survey of the Perseus Molecular Cloud

There is substantial evidence for disk formation taking place during the early stages of star formation and for most stars being born in multiple systems; however, protostellar multiplicity and disk searches have been hampered by low resolution, sample bias, and variable sensitivity. We have conducted an unbiased, high-sensitivity Karl G. Jansky Very Large Array (VLA) survey toward all known protostars (n = 94) in the Perseus molecular cloud (d~230 pc), with a resolution of ~15 AU (0.06") at 8 mm. We have detected candidate protostellar disks toward 17 sources (with 12 of those in the Class 0 stage) and we have found substructure on < 50AU scales for three Class 0 disk candidates, possibly evidence for disk fragmentation. We have discovered 16 new multiple systems (or new components) in this survey; the new systems have separations < 500 AU and 3 by < 30 AU. We also found a bi-modal distribution of separations, with peaks at ~75 AU and ~3000 AU, suggestive of formation through two distinct mechanisms: disk and turbulent fragmentation. The results from this survey demonstrate the necessity and utility of uniform, unbiased surveys of protostellar systems at millimeter and centimeter wavelengths.

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arXiv2016

Characterizing the Youngest Herschel-detected Protostars II. Molecular Outflows from the Millimeter and the Far-infrared

We present CARMA CO (J=1-0) observations and Herschel PACS spectroscopy, characterizing the outflow properties toward extremely young and deeply embedded protostars in the Orion molecular clouds. The sample comprises a subset of the Orion protostars known as the PACS Bright Red Sources (PBRS) (Stutz et al. 2013). We observed 14 PBRS with CARMA and 8 of these 14 with Herschel, acquiring full spectral scans from 55 micron to 200 micron. Outflows are detected in CO (J=1-0) from 8 of 14 PBRS, with two additional tentative detections; outflows are also detected from the outbursting protostar HOPS 223 (V2775 Ori) and the Class I protostar HOPS 68. The outflows have a range of morphologies, some are spatially compact, <10000 AU in extent, while others extend beyond the primary beam. The outflow velocities and morphologies are consistent with being dominated by intermediate inclination angles (80 deg > i > 20 deg). This confirms the interpretation of the very red 24 micron to 70 micron colors of the PBRS as a signpost of high envelope densities, with only one (possibly two) cases of the red colors resulting from edge-on inclinations. We detect high-J (J_up > 13) CO lines and/or H_2O lines from 5 of 8 PBRS and only for those with detected CO outflows. The far-infrared CO rotation temperatures of the detected PBRS are marginally colder (~230 K) than those observed for most protostars (~300 K), and only one of these 5 PBRS has detected [OI] 63 micron emission. The high envelope densities could be obscuring some [OI] emission and cause a ~20 K reduction to the CO rotation temperatures.

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arXiv2016

CARMA Large Area Star Formation Survey: Dense Gas in the Young L1451 Region of Perseus

We present a 3 mm spectral line and continuum survey of L1451 in the Perseus Molecular Cloud. These observations are from the CARMA Large Area Star Formation Survey (CLASSy), which also imaged Barnard 1, NGC 1333, Serpens Main and Serpens South. L1451 is the survey region with the lowest level of star formation activity---it contains no confirmed protostars. HCO+, HCN, and N2H+ (J=1-0) are all detected throughout the region, with HCO+ the most spatially widespread, and molecular emission seen toward 90% of the area above N(H_2) column densities of 1.9x10^21 cm^-2. HCO+ has the broadest velocity dispersion, near 0.3 km/s on average, compared to ~0.15 km/s for the other molecules, thus representing a range from supersonic to subsonic gas motions. Our non-binary dendrogram analysis reveals that the dense gas traced by each molecule has similar hierarchical structure, and that gas surrounding the candidate first hydrostatic core (FHSC), L1451-mm, and other previously detected single-dish continuum clumps have similar hierarchical structure; this suggests that different sub-regions of L1451 are fragmenting on the pathway to forming young stars. We determined the three-dimensional morphology of the largest detectable dense gas structures to be relatively ellipsoidal compared to other CLASSy regions, which appeared more flattened at largest scales. A virial analysis shows the most centrally condensed dust structures are likely unstable against collapse. Additionally, we identify a new spherical, centrally condensed N2H+ feature that could be a new FHSC candidate. The overall results suggest L1451 is a young region starting to form its generation of stars within turbulent, hierarchical structures.

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arXiv2016

Do siblings always form and evolve simultaneously? Testing the coevality of multiple protostellar systems through SEDs

Multiplicity is common in field stars and among protostellar systems. Models suggest two paths of formation: turbulent fragmentation and protostellar disk fragmentation. We attempt to find whether or not the coevality frequency of multiple protostellar systems can help to better understand their formation mechanism. The coevality frequency is determined by constraining the relative evolutionary stages of the components in a multiple system. SEDs for known multiple protostars in Perseus were constructed from literature data. Herschel PACS photometric maps were used to sample the peak of the SED for systems with separations >7", a crucial aspect in determining the evolutionary stage of a protostellar system. Inclination effects and the surrounding envelope and outflows were considered to decouple source geometry from evolution. This together with the shape and derived properties from the SED was used to determine each system's coevality as accurately as possible. SED models were used to examine the frequency of non-coevality that is due to geometry. We find a non-coevality frequency of 33+/-10% from the comparison of SED shapes of resolved multiple systems. Other source parameters suggest a somewhat lower frequency of non-coevality. The frequency of apparent non-coevality that is due to random inclination angle pairings of model SEDs is 17+/-0.5%. Observations of the outflow of resolved multiple systems do not suggest significant misalignments within multiple systems. Effects of unresolved multiples on the SED shape are also investigated. We find that 1/3 of the multiple protostellar systems sampled here are non-coeval, which is more than expected from random geometric orientations. The other 2/3 are found to be coeval. Higher order multiples show a tendency to be non-coeval. The frequency of non-coevality found here is most likely due to formation and enhanced by dynamical evolution.

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arXiv2016

An ALMA Search for Substructure, Fragmentation, and Hidden Protostars in Starless Cores in Chamaeleon I

We present an Atacama Large Millimeter/submillimeter Array (ALMA) 106 GHz (Band 3) continuum survey of the complete population of dense cores in the Chamaeleon I molecular cloud. We detect a total of 24 continuum sources in 19 different target fields. All previously known Class 0 and Class I protostars in Chamaeleon I are detected, whereas all of the 56 starless cores in our sample are undetected. We show that the Spitzer+Herschel census of protostars in Chamaeleon I is complete, with the rate at which protostellar cores have been misclassified as starless cores calculated as <1/56, or < 2%. We use synthetic observations to show that starless cores collapsing following the turbulent fragmentation scenario are detectable by our ALMA observations when their central densities exceed ~10^8 cm^-3, with the exact density dependent on the viewing geometry. Bonnor-Ebert spheres, on the other hand, remain undetected to central densities at least as high as 10^10 cm^-3. Our starless core non-detections are used to infer that either the star formation rate is declining in Chamaeleon I and most of the starless cores are not collapsing, matching the findings of previous studies, or that the evolution of starless cores are more accurately described by models that develop less substructure than predicted by the turbulent fragmentation scenario, such as Bonnor-Ebert spheres. We outline future work necessary to distinguish between these two possibilities.

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arXiv2016

Misalignment of Outflow Axes in the Proto-Multiple Systems in Perseus

We investigate the alignment between outflow axes in nine of the youngest binary/multiple systems in the Perseus Molecular Cloud. These systems have typical member spacing larger than 1000 AU. For outflow identification, we use 12CO(2-1) and 12CO(3-2) data from a large survey with the Sub- millimeter Array: Mass Assembly of Stellar Systems and their Evolution with the SMA (MASSES). The distribution of outflow orientations in the binary pairs is consistent with random or preferentially anti-aligned distributions, demonstrating that these outflows are misaligned. This result suggests that these systems are possibly formed in environments where the distribution of angular momentum is complex and disordered, and these systems do not come from the same co-rotating structures or from an initial cloud with aligned vectors of angular momentum.

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arXiv2016

Spectroscopic Binaries in the Orion Nebula Cluster and NGC 2264

We examine the spectroscopic binary population for two massive nearby regions of clustered star formation, the Orion Nebula Cluster and NGC 2264, supplementing the data presented by Tobin et al. (2009, 2015) with more recent observations and more extensive analysis. The inferred multiplicity fraction up to 10 AU based on these observations is $5.3\pm 1.2$% for NGC 2264 and $5.8\pm 1.1$% for the ONC; they are consistent with the distribution of binaries in the field in the relevant parameter range. Eight of the multiple systems in the sample have enough epochs to make an initial fit for the orbital parameters. Two of these sources are double-lined spectroscopic binaries; for them we determine the mass ratio. Our reanalysis of the distribution of stellar radial velocities towards these clusters presents a significantly better agreement between stellar and gas kinematics than was previously thought.

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arXiv2016

The VLA Nascent Disk and Multiplicity Survey of Perseus Protostars (VANDAM). II. Multiplicity of Protostars in the Perseus Molecular Cloud

We present a multiplicity study of all known protostars (94) in the Perseus molecular cloud from a Karl G. Jansky Very Large Array (VLA) survey at Ka-band (8 mm and 1 cm) and C-band (4 cm and 6.6 cm). The observed sample has a bolometric luminosity range between 0.1 L$_{\odot}$ and $\sim$33 L$_{\odot}$, with a median of 0.7 L$_{\odot}$. This multiplicity study is based on the Ka-band data, having a best resolution of $\sim$0.065" (15 AU) and separations out to $\sim$43" (10000 AU) can be probed. The overall multiplicity fraction (MF) is found to be of 0.40$\pm$0.06 and the companion star fraction (CSF) is 0.71$\pm$0.06. The MF and CSF of the Class 0 protostars are 0.57$\pm$0.09 and 1.2$\pm$0.2, and the MF and CSF of Class I protostars are both 0.23$\pm$0.08. The distribution of companion separations appears bi-modal, with a peak at $\sim$75 AU and another peak at $\sim$3000 AU. Turbulent fragmentation is likely the dominant mechanism on $>$1000 AU scales and disk fragmentation is likely to be the dominant mechanism on $<$200 AU scales. Toward three Class 0 sources we find companions separated by $<$30 AU. These systems have the smallest separations of currently known Class 0 protostellar binary systems. Moreover, these close systems are embedded within larger (50 AU to 400 AU) structures and may be candidates for ongoing disk fragmentation.

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arXiv2016

The VLA Nascent Disk and Multiplicity Survey: First Look at Resolved Candidate Disks around Class 0 and I Protostars in the Perseus Molecular Cloud

We present the first dust emission results toward a sample of seven protostellar disk candidates around Class 0 and I sources in the Perseus molecular cloud from the VLA Nascent Disk and Multiplicity (VANDAM) survey with ~0.05'' or 12 AU resolution. To examine the surface brightness profiles of these sources, we fit the Ka-band 8 mm dust-continuum data in the u,v-plane to a simple, parametrized model based on the Shakura-Sunyaev disk model. The candidate disks are well-fit by a model with a disk-shaped profile and have masses consistent with known Class 0 and I disks. The inner-disk surface densities of the VANDAM candidate disks have shallower density profiles compared to disks around more evolved Class II systems. The best-fit model radii of the seven early-result candidate disks are R_c > 10 AU; at 8 mm, the radii reflect lower limits on the disk size since dust continuum emission is tied to grain size and large grains radially drift inwards. These relatively large disks, if confirmed kinematically, are inconsistent with theoretical models where the disk size is limited by strong magnetic braking to < 10 AU at early times.

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arXiv2015

IN-SYNC IV - The Young Stellar Population in the Orion A Molecular Cloud

We present the results of the SDSS APOGEE INfrared Spectroscopy of Young Nebulous Clusters program (IN-SYNC) survey of the Orion A molecular cloud. This survey obtained high resolution near infrared (NIR) spectroscopy of about 2700 young pre-main sequence stars throughout the region, acquired across five distinct fields spanning 6deg field of view (FOV). With these spectra, we have measured accurate stellar parameters (T_eff, log g, v sin i) and extinctions, and placed the sources in the Hertzsprung-Russel Diagram (HRD). We have also extracted radial velocities for the kinematic characterization of the population. We compare our measurements with literature results for a sub-sample of targets in order to assess the performances and accuracy of the survey. Source extinction shows evidence for dust grains that are larger than those in the diffuse interstellar medium (ISM): we estimate an average R_V=5.5 in the region. Importantly, we find a clear correlation between HRD inferred ages and spectroscopic surface-gravity inferred ages. This clearly indicates a real spread of stellar radii at fixed temperature, and together with additional correlations with extinction and with disk presence, strongly suggests a real spread of ages large than a few Myr. Focussing on the young population around NGC1980 iota Ori, which has previously been suggested to be a separate, foreground, older cluster, we confirm its older (5Myr) age and low A_V, but considering that its radial velocity distribution is indistinguishable from the Orion A's population, we suggest that NGC1980 is part of Orion A's star formation activity. Based on their stellar parameters and kinematic properties, we identify 383 new candidate members of Orion A, most of which are diskless sources in areas of the region poorly studied by previous works.

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arXiv2015

Evolution of Mass Outflow in Protostars

We have surveyed 84 Class 0, Class I, and flat-spectrum protostars in mid-infrared [Si II], [Fe II] and [S I] line emission, and 11 of these in far-infrared [O I] emission. We use the results to derive their mass outflow rates. Thereby we observe a strong correlation of mass outflow rates with bolometric luminosity, and with the inferred mass accretion rates of the central objects, which continues through the Class 0 range the trend observed in Class II young stellar objects. Along this trend from large to small mass-flow rates, the different classes of young stellar objects lie in the sequence Class 0 -- Class I/flat-spectrum -- Class II, indicating that the trend is an evolutionary sequence in which mass outflow and accretion rates decrease together with increasing age, while maintaining rough proportionality. The survey results include two which are key tests of magnetocentrifugal outflow-acceleration mechanisms: the distribution of the outflow/accretion branching ratio b, and limits on the distribution of outflow speeds. Neither rule out any of the three leading outflow-acceleration, angular-momentum-ejection mechanisms, but they provide some evidence that disk winds and accretion-powered stellar winds (APSWs) operate in many protostars. An upper edge observed in the branching-ratio distribution is consistent with the upper bound of b = 0.6 found in models of APSWs, and a large fraction (0.31) of the sample have branching ratio sufficiently small that only disk winds, launched on scales as large as several AU, have been demonstrated to account for them.

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arXiv2015

Mass Assembly of Stellar Systems and Their Evolution with the SMA (MASSES). Multiplicity and the Physical Environment in L1448N

We present continuum and molecular line observations at 230 GHz and 345 GHz from the Sub-millimeter Array (SMA) toward three protostars in the Perseus L1448N region. The data are from the large project "Mass Assembly of Stellar Systems and their Evolution with the SMA" (MASSES). Three dust continuum sources, Source B, Source NW, and Source A, are detected at both frequencies. These sources have corresponding emission peaks in C18O (J=2-1), 13CO (J=2-1), and HCO+ (J=4-3), and have offsets with N2D+ (J=3-2) peaks. High angular resolution data from a complimentary continuum survey with the Karl G. Jansky Very Large Array show that Source B is associated with three 8 mm continuum objects, Source NW with two, and Source A remains single. These results suggest that multiplicity in L1448N exists at different spatial scales from a few thousand AU to < 100 AU. Velocity gradients in each source obtained from two-dimensional fits to the SMA C18O emission are found to be perpendicular to within 20 degrees of the outflow directions as revealed by 12CO (J=2-1). We have observed that Sources B and NW with multiplicity have higher densities than Source A without multiplicity. This suggests that thermal Jeans fragmentation can be relevant in the fragmentation process. However, we have not observed a difference in the ratio between rotational and gravitational energy between sources with and without multiplicity. We also have not observed a trend between non-thermal velocity dispersions and the level of fragmentation. Our study has provided the first direct and comprehensive comparison between multiplicity and core properties in low-mass protostars, although based on small number statistics.

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arXiv2015

The Gould's Belt Very Large Array Survey II: The Serpens region

We present deep ($\sim 17~μ$Jy) radio continuum observations of the Serpens molecular cloud, the Serpens south cluster, and the W40 region obtained using the Very Large Array in its A configuration. We detect a total of 146 sources, 29 of which are young stellar objects (YSOs), 2 are BV stars and 5 more are associated with phenomena related to YSOs. Based on their radio variability and spectral index, we propose that about 16 of the remaining 110 unclassified sources are also YSOs. For approximately 65% of the known YSOs detected here as radio sources, the emission is most likely non-thermal, and related to stellar coronal activity. As also recently observed in Ophiuchus, our sample of YSOs with X-ray counterparts lies below the fiducial Güdel & Benz relation. Finally, we analyze the proper motions of 9 sources in the W40 region. This allows us to better constrain the membership of the radio sources in the region.

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arXiv2015

A Sub-arcsecond Survey Toward Class 0 Protostars in Perseus: Searching for Signatures of Protostellar Disks

We present a CARMA 1.3 mm continuum survey toward 9 Class 0 protostars in the Perseus molecular cloud at $\sim$0.3$^{\prime\prime}$ (70 AU) resolution. This study approximately doubles the number of Class 0 protostars observed with spatial resolutions $<$ 100 AU at millimeter wavelengths, enabling the presence of protostellar disks and proto-binary systems to be probed. We detect flattened structures with radii $>$ 100 AU around 2 sources (L1448 IRS2 and Per-emb-14) and these sources may be strong disk candidates. Marginally-resolved structures with position angles within 30$^{\circ}$ of perpendicular to the outflow are found toward 3 protostars (L1448 IRS3C, IRAS 03282+3035, and L1448C) and are considered disk candidates. Two others (L1448 IRS3B and IRAS 03292+3039) have resolved structure, possibly indicative of massive inner envelopes or disks; L1448 IRS3B also has a companion separated by 0.9$^{\prime\prime}$ ($\sim$210 AU). IC348-MMS does not have well-resolved structure and the candidate first hydrostatic core L1451-MMS is marginally resolved on 1$^{\prime\prime}$ scales. The strong disk candidate sources were followed-up with C$^{18}$O ($J=2\rightarrow1$) observations, detecting velocity gradients consistent with rotation, but it is unclear if the rotation is Keplerian. We compare the observed visibility amplitudes to radiative transfer models, finding that visibility amplitude ratios suggest a compact component (possibly a disk) is necessary for 5 of 9 Class 0 sources; envelopes alone may explain the other 4 systems. We conclude that there is evidence for the formation of large disks in the Class 0 phase with a range of radii and masses dependent upon their initial formation conditions.

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arXiv2015

Kinematic and Spatial Substructure in NGC 2264

We present an expanded kinematic study of the young cluster NGC 2264 based upon optical radial velocities measured using multi-fiber echelle spectroscopy at the 6.5 meter MMT and Magellan telescopes. We report radial velocities for 695 stars, of which approximately 407 stars are confirmed or very likely members. Our results more than double the number of members with radial velocities from F{\H u}r{é}sz et al., resulting in a much better defined kinematic relationship between the stellar population and the associated molecular gas. In particular, we find that there is a significant subset of stars that are systematically blueshifted with respect to the molecular ($^{13}$CO) gas. The detection of Lithium absorption and/or infrared excesses in this blue-shifted population suggests that at least some of these stars are cluster members; we suggest some speculative scenarios to explain their kinematics. Our results also more clearly define the redshifted population of stars in the northern end of the cluster; we suggest that the stellar and gas kinematics of this region are the result of a bubble driven by the wind from O7 star S Mon. Our results emphasize the complexity of the spatial and kinematic structure of NGC 2264, important for eventually building up a comprehensive picture of cluster formation.

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arXiv2015

HOPS 383: An Outbursting Class 0 Protostar in Orion

We report the dramatic mid-infrared brightening between 2004 and 2006 of HOPS 383, a deeply embedded protostar adjacent to NGC 1977 in Orion. By 2008, the source became a factor of 35 brighter at 24 microns with a brightness increase also apparent at 4.5 microns. The outburst is also detected in the submillimeter by comparing APEX/SABOCA to SCUBA data, and a scattered-light nebula appeared in NEWFIRM K_s imaging. The post-outburst spectral energy distribution indicates a Class 0 source with a dense envelope and a luminosity between 6 and 14 L_sun. Post-outburst time-series mid- and far-infrared photometry shows no long-term fading and variability at the 18% level between 2009 and 2012. HOPS 383 is the first outbursting Class 0 object discovered, pointing to the importance of episodic accretion at early stages in the star formation process. Its dramatic rise and lack of fading over a six-year period hint that it may be similar to FU Ori outbursts, although the luminosity appears to be significantly smaller than the canonical luminosities of such objects.

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arXiv2014

The Magnetic Field in the Class 0 Protostellar Disk of L1527

We present subarcsecond (~0.35") resolved observations of the 1.3 mm dust polarization from the edge-on circumstellar disk around the Class 0 protostar L1527. The inferred magnetic field is consistent with a dominantly toroidal morphology; there is no significantly detected vertical poloidal component to which observations of an edge-on disk are most sensitive. This suggests that angular momentum transport in Class 0 protostars (when large amounts of material are fed down to the disk from the envelope and accreted onto the protostar) is driven mainly by magnetorotational instability rather than magnetocentrifugal winds at 50 AU scales. In addition, with the data to date there is an early, tentative trend that R>30 AU disks have so far been found in Class 0 systems with average magnetic fields on the 1000 AU scale strongly misaligned with the rotation axis. The absence of such a disk in the aligned case could be due to efficient magnetic braking that disrupts disk formation. If this is the case, this implies that candidate Class 0 disk systems could be identified by the average magnetic field direction at ~1000 AU spatial scales.

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