arXiv subjects

John J. Tobin

Publications and source records attributed to John J. Tobin.

At least 127 records · Page 7Linked to original sources

arXiv2018

The Envelope Kinematics and a Possible Disk Around the Class 0 Protostar within BHR7

We present a characterization of the protostar embedded within the BHR7 dark cloud, based on both photometric measurements from the near-infrared to millimeter and interferometric continuum and molecular line observations at millimeter wavelengths. We find that this protostar is a Class 0 system, the youngest class of protostars, measuring its bolometric temperature to be 50.5~K, with a bolometric luminosity of 9.3~L$_{\odot}$. The near-infrared and \textit{Spitzer} imaging show a prominent dark lane from dust extinction separating clear bipolar outflow cavities. Observations of $^{13}$CO ($J=2\rightarrow1$), C$^{18}$O ($J=2\rightarrow1$), and other molecular lines with the Submillimeter Array (SMA) exhibit a clear rotation signature on scales $<$1300~AU. The rotation can be traced to an inner radius of $\sim$170~AU and the rotation curve is consistent with an R$^{-1}$ profile, implying that angular momentum is being conserved. Observations of the 1.3~mm dust continuum with the SMA reveal a resolved continuum source, extended in the direction of the dark lane, orthogonal to the outflow. The deconvolved size of the continuum indicates a radius of $\sim$100~AU for the continuum source at the assumed distance of 400~pc. The visibility amplitude profile of the continuum emission cannot be reproduced by an envelope alone and needs a compact component. Thus, we posit that the resolved continuum source could be tracing a Keplerian disk in this very young system. If we assume that the continuum radius traces a Keplerian disk (R$\sim$120~AU) the observed rotation profile is consistent with a protostar mass of 1.0~$M_{\odot}$.

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arXiv2018

ALMA's Polarized View of 10 Protostars in the Perseus Molecular Cloud

We present 870 $μ$m ALMA dust polarization observations of 10 young Class 0/I protostars in the Perseus Molecular Cloud. At $\sim$ 0.35$"$ (80 au) resolution, all of our sources show some degree of polarization, with most (9/10) showing significantly extended emission in the polarized continuum. Each source has incredibly intricate polarization signatures. In particular, all three disk-candidates have polarization vectors roughly along the minor axis, which is indicative of polarization produced by dust scattering. On $\sim$ 100 au scales, the polarization is at a relatively low level ($\lesssim 1\%$) and is quite ordered. In sources with significant envelope emission, the envelope is typically polarized at a much higher ($\gtrsim 5\%$) level and has a far more disordered morphology. We compute the cumulative probability distributions for both the small (disk-scale) and large (envelope-scale) polarization percentage. We find that the two are intrinsically different, even after accounting for the different detection thresholds in the high/low surface brightness regions. We perform Kolmogorov-Smirnov and Anderson-Darling tests on the distributions of angle offsets of the polarization from the outflow axis. We find disk-candidate sources are different from the non-disk-candidate sources. We conclude that the polarization on the 100 au scale is consistent with the signature of dust scattering for disk-candidates and that the polarization on the envelope-scale in all sources may come from another mechanism, most likely magnetically aligned grains.

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arXiv2017

Hierarchical fragmentation in the Perseus molecular cloud: From the cloud scale to protostellar objects

We present a study of hierarchical structure in the Perseus molecular cloud, from the scale of the entire cloud ($\gtrsim$10 pc) to smaller clumps ($\sim$1 pc), cores ($\sim$0.05-0.1 pc), envelopes ($\sim$300-3000 AU) and protostellar objects ($\sim$15 AU). We use new observations from the Submillimeter Array (SMA) large project "Mass Assembly of Stellar Systems and their Evolution with the SMA (MASSES)" to probe the envelopes, and recent single-dish and interferometric observations from the literature for the remaining scales. This is the first study to analyze hierarchical structure over five scales in the same cloud complex. We compare the number of fragments with the number of Jeans masses in each scale to calculate the Jeans efficiency, or the ratio of observed to expected number of fragments. The velocity dispersion is assumed to arise either from purely thermal motions, or from combined thermal and non-thermal motions inferred from observed spectral line widths. For each scale, thermal Jeans fragmentation predicts more fragments than observed, corresponding to inefficient thermal Jeans fragmentation. For the smallest scale, thermal plus non-thermal Jeans fragmentation also predicts too many protostellar objects. However at each of the larger scales thermal plus non-thermal Jeans fragmentation predicts fewer than one fragment, corresponding to no fragmentation into envelopes, cores, and clumps. Over all scales, the results are inconsistent with complete Jeans fragmentation based on either thermal or thermal plus non-thermal motions. They are more nearly consistent with inefficient thermal Jeans fragmentation, where the thermal Jeans efficiency increases from the largest to the smallest scale.

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arXiv2017

The ALMA early science view of FUor/EXor objects - V: continuum disc masses and sizes

Low-mass stars build a significant fraction of their total mass during short outbursts of enhanced accretion known as FUor and EXor outbursts. FUor objects are characterized by a sudden brightening of ~5 magnitudes at visible wavelengths within one year and remain bright for decades. EXor objects have lower amplitude outbursts on shorter timescales. Here we discuss a 1.3 mm ALMA mini-survey of eight outbursting sources (three FUor, four EXor, and the borderline object V1647 Ori) in the Orion Molecular Cloud. While previous papers in this series discuss the remarkable molecular outflows observed in the three FUor objects and V1647 Ori, here we focus on the continuum data and the differences and similarities between the FUor and EXor populations. We find that FUor discs are significantly more massive (~80-600 M_JUP) than the EXor objects (~0.5-40 M_JUP ). We also report that the EXor sources lack the prominent outflows seen in the FUor population. Even though our sample is small, the large differences in disc masses and outflow activity suggest that the two types of objects represent different evolutionary stages. The FUor sources seem to be rather compact (Rc < 20-40 au) and to have a smaller characteristic radius for a given disc mass when compared to T Tauri stars. V1118 Ori, the only known close binary systemin our sample, is shown to host a disc around each one of the stellar components. The disc around HBC 494 is asymmetric, hinting at a structure in the outer disc or the presence of a second disc.

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arXiv2017

The VLA Nascent Disk And Multiplicity Survey of Perseus Protostars (VANDAM). III. Extended Radio Emission from Protostars in Perseus

Centimeter continuum emission from protostars offers insight into the innermost part of the outflows, as shock-ionized gas produces free-free emission. We observed a complete population of Class 0 and I protostars in the Perseus molecular cloud at 4.1 cm and 6.4 cm with resolution and sensitivity superior to previous surveys. From a total of 71 detections, 8 sources exhibit resolved emission at 4.1 cm and/or 6.4 cm. In this paper we focus on this sub-sample, analyzing their spectral indices along the jet, and their alignment with respect to the large-scale molecular outflow. Spectral indices for fluxes integrated toward the position of the protostar are consistent with free-free thermal emission. The value of the spectral index along a radio jet decreases with distance from the protostar. For six sources, emission is well aligned with the outflow central axis, showing that we observe the ionized base of the jet. This is not the case for two sources, where we note misalignment of the emission with respect to the large-scale outflow. This might indicate that the emission does not originate in the radio jet, but rather in an ionized outflow cavity wall or disk surface. For five of the sources, the spectral indices along the jet decrease well below the thermal free-free limit of -0.1 with $>2σ$ significance. This is indicative of synchrotron emission, meaning that high energy electrons are being produced in the outflows close to the disk. This result can have far-reaching implications for the chemical composition of the embedded disks.

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arXiv2017

Protoplanetary Disks in $ρ$ Ophiuchus as Seen From ALMA

We present a high angular resolution ($\sim 0.2^{\prime\prime}$), high sensitivity ($σ\sim 0.2$ mJy) survey of the 870 $μ$m continuum emission from the circumstellar material around 49 pre-main sequence stars in the $ρ$ Ophiuchus molecular cloud. Because most millimeter instruments have resided in the northern hemisphere, this represents the largest high-resolution, millimeter-wave survey of the circumstellar disk content of this cloud. Our survey of 49 systems comprises 63 stars; we detect disks associated with 29 single sources, 11 binaries, 3 triple systems and 4 transition disks. We present flux and radius distributions for these systems; in particular, this is the first presentation of a reasonably complete probability distribution of disk radii at millimeter-wavelengths. We also compare the flux distribution of these protoplanetary disks with that of the disk population of the Taurus-Auriga molecular cloud. We find that disks in binaries are both significantly smaller and have much less flux than their counterparts around isolated stars. We compute truncation calculations on our binary sources and find that these disks are too small to have been affected by tidal truncation and posit some explanations for this. Lastly, our survey found 3 candidate gapped disks, one of which is a newly identified transition disk with no signature of a dip in infrared excess in extant observations.

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arXiv2017

Chemical and kinematic complexity of the very young star-forming region Serpens Main observed with ALMA

The youngest low-mass protostars are known to be chemically rich, accreting matter most vigorously, and producing the most powerful outflows. Molecules are unique tracers of these phenomena. We use ALMA to study several outflow sources in the Serpens Main region. The most luminous source, Ser-SMM1, shows the richest chemical composition, but some complex molecules are also present in S68N. No emission from complex organics is detected toward Ser-emb 8N, which is the least luminous in the sample. We discuss whether these differences reflect an evolutionary effect or whether they are due to different physical structures. We also analyze the outflow structure from these young protostars by comparing emission of CO and SiO. EHV molecular jets originating from SMM1-a,b and Ser-emb 8N contrast with no such activity from S68N, which on the other hand presents a complex outflow structure.

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arXiv2017

Unveiling the physical and chemical conditions in the young disk around L1527

Planets form in disks around young stars. The planet formation process may start when the protostar and disk are still deeply embedded within their infalling envelope. However, unlike more evolved protoplanetary disks, the physical and chemical structure of these young embedded disks are still poorly constrained. We have analyzed ALMA data for $^{13}$CO, C$^{18}$O and N$_2$D$^+$ to constrain the temperature structure, one of the critical unknowns, in the disk around L1527. The spatial distribution of $^{13}$CO and C$^{18}$O, together with the kinetic temperature derived from the optically thick $^{13}$CO emission and the non-detection of N$_2$D$^+$, suggest that this disk is warm enough ($\gtrsim$ 20 K) to prevent CO freeze-out.

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arXiv2017

The ALMA Early Science View of FUor/EXor objects. IV. Misaligned Outflows in the Complex Star-forming Environment of V1647 Ori and McNeil's Nebula

We present Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of the star-forming environment surrounding V1647 Ori, an outbursting FUor/EXor pre-MS star. Dust continuum and the (J = 2 - 1) $^{12}$CO, $^{13}$CO, C$^{18}$O molecular emission lines were observed to characterize the V1647 Ori circumstellar disc and any large scale molecular features present. We detect continuum emission from the circumstellar disc and determine a radius r = 40 au, inclination i = 17$^{\circ}$$^{+6}_{-9}$ and total disc mass of M$_{\mathrm{disk}}$ of ~0.1 M$_{\odot}$. We do not identify any disc structures associated with nearby companions, massive planets or fragmentation. The molecular cloud environment surrounding V1647 Ori is both structured and complex. We confirm the presence of an excavated cavity north of V1647 Ori and have identified dense material at the base of the optical reflection nebula (McNeil's Nebula) that is actively shaping its surrounding environment. Two distinct outflows have been detected with dynamical ages of ~11,700 and 17,200 years. These outflows are misaligned suggesting disc precession over ~5500 years as a result of anisotropic accretion events is responsible. The collimated outflows exhibit velocities of ~2 km s$^{-1}$, similar in velocity to that of other FUor objects presented in this series but significantly slower than previous observations and model predictions. The V1647 Ori system is seemingly connected by an "arm" of material to a large unresolved structure located ~20$"$ to the west. The complex environment surrounding V1647 Ori suggests it is in the early stages of star formation which may relate to its classification as both an FUor and EXor type object.

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arXiv2017

IN-SYNC. V. Stellar kinematics and dynamics in the Orion A Molecular Cloud

The kinematics and dynamics of young stellar populations enable us to test theories of star formation. With this aim, we continue our analysis of the SDSS-III/APOGEE IN-SYNC survey, a high resolution near infrared spectroscopic survey of young clusters. We focus on the Orion A star-forming region, for which IN-SYNC obtained spectra of $\sim2700$ stars. In Paper IV we used these data to study the young stellar population. Here we study the kinematic properties through radial velocities ($v_r$). The young stellar population remains kinematically associated with the molecular gas, following a $\sim10\:{\rm{km\:s}}^{-1}$ gradient along filament. However, near the center of the region, the $v_r$ distribution is slightly blueshifted and asymmetric; we suggest that this population, which is older, is slightly in foreground. We find evidence for kinematic subclustering, detecting statistically significant groupings of co-located stars with coherent motions. These are mostly in the lower-density regions of the cloud, while the ONC radial velocities are smoothly distributed, consistent with it being an older, more dynamically evolved cluster. The velocity dispersion $σ_v$ varies along the filament. The ONC appears virialized, or just slightly supervirial, consistent with an old dynamical age. Here there is also some evidence for on-going expansion, from a $v_r$--extinction correlation. In the southern filament, $σ_v$ is $\sim2$--$3$ times larger than virial in the L1641N region, where we infer a superposition along the line of sight of stellar sub-populations, detached from the gas. On the contrary, $σ_v$ decreases towards L1641S, where the population is again in agreement with a virial state.

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arXiv2017

Alignment Between Protostellar Outflows and Filamentary Structure

We present new Submillimeter Array (SMA) observations of CO(2-1) outflows toward young, embedded protostars in the Perseus molecular cloud as part of the Mass Assembly of Stellar Systems and their Evolution with the SMA (MASSES) survey. For 57 Perseus protostars, we characterize the orientation of the outflow angles and compare them with the orientation of the local filaments as derived from $Herschel$ observations. We find that the relative angles between outflows and filaments are inconsistent with purely parallel or purely perpendicular distributions. Instead, the observed distribution of outflow-filament angles are more consistent with either randomly aligned angles or a mix of projected parallel and perpendicular angles. A mix of parallel and perpendicular angles requires perpendicular alignment to be more common by a factor of $\sim$3. Our results show that the observed distributions probably hold regardless of the protostar's multiplicity, age, or the host core's opacity. These observations indicate that the angular momentum axis of a protostar may be independent of the large-scale structure. We discuss the significance of independent protostellar rotation axes in the general picture of filament-based star formation.

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arXiv2017

The Herschel Orion Protostar Survey: Luminosity and Envelope Evolution

The Herschel Orion Protostar Survey obtained well-sampled 1.2 - 870 micron spectral energy distributions (SEDs) of over 300 protostars in the Orion molecular clouds, home to most of the young stellar objects (YSOs) in the nearest 500 pc. We plot the bolometric luminosities and temperatures for 330 Orion YSOs, 315 of which have bolometric temperatures characteristic of protostars. The histogram of bolometric temperature is roughly flat; 29% of the protostars are in Class 0. The median luminosity decreases by a factor of four with increasing bolometric temperature; consequently, the Class 0 protostars are systematically brighter than the Class I protostars, with a median luminosity of 2.3 L_sun as opposed to 0.87 L_sun. At a given bolometric temperature, the scatter in luminosities is three orders of magnitude. Using fits to the SEDs, we analyze how the luminosities corrected for inclination and foreground reddening relate to the mass in the inner 2500 AU of the best-fit model envelopes. The histogram of envelope mass is roughly flat, while the median corrected luminosity peaks at 15 L_sun for young envelopes and falls to 1.7 L_sun for late-stage protostars with remnant envelopes. The spread in luminosity at each envelope mass is three orders of magnitude. Envelope masses that decline exponentially with time explain the flat mass histogram and the decrease in luminosity, while the formation of a range of stellar masses explains the dispersion in luminosity.

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arXiv2017

Protostellar accretion traced with chemistry. High resolution C18O and continuum observations towards deeply embedded protostars in Perseus

Context: Understanding how accretion proceeds is a key question of star formation, with important implications for both the physical and chemical evolution of young stellar objects. In particular, very little is known about the accretion variability in the earliest stages of star formation. Aims: To characterise protostellar accretion histories towards individual sources by utilising sublimation and freeze-out chemistry of CO. Methods: A sample of 24 embedded protostars are observed with the Submillimeter Array (SMA) in context of the large program "Mass Assembly of Stellar Systems and their Evolution with the SMA" (MASSES). The size of the C$^{18}$O emitting region, where CO has sublimated into the gas-phase, is measured towards each source and compared to the expected size of the region given the current luminosity. The SMA observations also include 1.3 mm continuum data, which are used to investigate whether a link can be established between accretion bursts and massive circumstellar disks. Results: Depending on the adopted sublimation temperature of the CO ice, between 20% and 50% of the sources in the sample show extended C$^{18}$O emission indicating that the gas was warm enough in the past that CO sublimated and is currently in the process of refreezing; something which we attribute to a recent accretion burst. Given the fraction of sources with extended C$^{18}$O emission, we estimate an average interval between bursts of 20000-50000 yr, which is consistent with previous estimates. No clear link can be established between the presence of circumstellar disks and accretion bursts, however the three closest known binaries in the sample (projected separations <20 AU) all show evidence of a past accretion burst, indicating that close binary interactions may also play a role in inducing accretion variability.

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arXiv2017

Star Formation Under the Outflow: The Discovery of a Non-Thermal Jet from OMC-2 FIR 3 and its Relationship to the Deeply Embedded FIR 4 Protostar

We carried out multiwavelength (0.7-5 cm), multiepoch (1994-2015) Very Large Array (VLA) observations toward the region enclosing the bright far-IR sources FIR 3 (HOPS 370) and FIR 4 (HOPS 108) in OMC-2. We report the detection of 10 radio sources, seven of them identified as young stellar objects. We image a well-collimated radio jet with a thermal free-free core (VLA 11) associated with the Class I intermediate-mass protostar HOPS 370. The jet presents several knots (VLA 12N, 12C, 12S) of non-thermal radio emission (likely synchrotron from shock-accelerated relativistic electrons) at distances of ~7,500-12,500 au from the protostar, in a region where other shock tracers have been previously identified. These knots are moving away from the HOPS 370 protostar at ~ 100 km/s. The Class 0 protostar HOPS 108, which itself is detected as an independent, kinematically decoupled radio source, falls in the path of these non-thermal radio knots. These results favor the previously proposed scenario where the formation of HOPS 108 has been triggered by the impact of the HOPS 370 outflow with a dense clump. However, HOPS 108 presents a large proper motion velocity of ~ 30 km/s, similar to that of other runaway stars in Orion, whose origin would be puzzling within this scenario. Alternatively, an apparent proper motion could result because of changes in the position of the centroid of the source due to blending with nearby extended emission, variations in the source shape, and /or opacity effects.

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arXiv2017

ALMA Observations of Starless Core Substructure in Ophiuchus

Compact substructure is expected to arise in a starless core as mass becomes concentrated in the central region likely to form a protostar. Additionally, multiple peaks may form if fragmentation occurs. We present ALMA Cycle 2 observations of 60 starless and protostellar cores in the Ophiuchus molecular cloud. We detect eight compact substructures which are >15 arcsec from the nearest Spitzer YSO. Only one of these has strong evidence for being truly starless after considering ancillary data, e.g., from Herschel and X-ray telescopes. An additional extended emission structure has tentative evidence for starlessness. The number of our detections is consistent with estimates from a combination of synthetic observations of numerical simulations and analytical arguments. This result suggests that a similar ALMA study in the Chamaeleon I cloud, which detected no compact substructure in starless cores, may be due to the peculiar evolutionary state of cores in that cloud.

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arXiv2016

The Gould's Belt Distances Survey (GOBELINS) I. Trigonometric parallax distances and depth of the Ophiuchus complex

We present the first results of the Gould's Belt Distances Survey (GOBELINS), a project aimed at measuring the proper motion and trigonometric parallax of a large sample of young stars in nearby regions using multi-epoch Very Long Baseline Array (VLBA) radio observations. Enough VLBA detections have now been obtained for 16 stellar systems in Ophiuchus to derive their parallax and proper motion. This leads to distance determinations for individual stars with an accuracy of 0.3 to a few percent. In addition, the orbits of 6 multiple systems were modelled by combining absolute positions with VLBA (and in some cases, near infrared) angular separations. Twelve stellar systems are located in the dark cloud Lynds 1688, the individual distances for this sample are highly consistent with one another, and yield a mean parallax for Lynds 1688 of $\varpi=7.28\pm0.06$ mas, corresponding to a distance $d=137.3\pm1.2$ pc. This represents an accuracy better than 1%. Three systems for which astrometric elements could be measured are located in the eastern streamer (Lynds 1689) and yield an estimate of $\varpi=6.79\pm0.16$ mas, corresponding to a distance $d=147.3\pm3.4$ pc. This suggests that the eastern streamer is located about 10 pc farther than the core, but this conclusion needs to be confirmed by observations (currently being collected) of additional sources in the eastern streamer. From the measured proper motions, we estimate the one-dimensional velocity dispersion in Lynds 1688 to be 2.8$\pm$1.8 and 3.0$\pm$2.0 ${\rm km~s}^{-1}$, in R.A. and DEC., respectively, these are larger than, but still consistent within $1σ$, with those found in other studies.

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arXiv2016

Radio Measurements of the stellar proper motions in the core of the Orion Nebula Cluster

Using multi-epoch VLA observations, covering a time baseline of 29.1 years, we have measured the proper motions of 88 young stars with compact radio emission in the core of the Orion Nebula Cluster (ONC) and the neighboring BN/KL region. Our work increases the number of young stars with measured proper motion at radio frequencies by a factor of 2.5 and enables us to perform a better statistical analysis of the kinematics of the region than was previously possible. Most stars (79 out of 88) have proper motions consistent with a Gaussian distribution centered on $\overline{μ_α\cosδ}=1.07\pm0.09\quad{\rm mas\,yr^{-1}}$, and $\overline{μ_δ}=-0.84\pm0.16\quad{\rm mas\,yr^{-1}}$, with velocity dispersions of $σ_α=1.08\pm0.07\quad{\rm mas\,\,yr^{-1}},$ $σ_δ=1.27\pm0.15\quad{\rm mas\,\,yr^{-1}}$. We looked for organized movements of these stars but found no clear indication of radial expansion/contraction or rotation. The remaining nine stars in our sample show peculiar proper motions that differ from the mean proper motions of the ONC by more than 3-$σ$. One of these stars, V 1326 Ori, could have been expelled from the Orion Trapezium 7,000 years ago. Two could be related to the multi-stellar disintegration in the BN/KL region, in addition to the previously known sources BN, I and n. The others either have high uncertainties (so their anomalous proper motions are not firmly established) or could be foreground objects.

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arXiv2016

The Gould's Belt Distances Survey (GOBELINS) III. The distance to the Serpens/Aquila Molecular Complex

We report on new distances and proper motions to seven stars across the Serpens/Aquila complex. The observations were obtained as part of the Gould's Belt Distances Survey (GOBELINS) project between September 2013 and April 2016 with the Very Long Baseline Array (VLBA). One of our targets is the proto-Herbig AeBe object EC 95, which is a binary system embedded in the Serpens Core. For this system, we combined the GOBELINS observations with previous VLBA data to cover a total period of ~8 years, and derive the orbital elements and an updated source distance. The individual distances to sources in the complex are fully consistent with each other, and the mean value corresponds to a distance of $436.0\pm9.2$~pc for the Serpens/W40 complex. Given this new evidence, we argue that Serpens Main, W40 and Serpens South are physically associated and form a single cloud structure.

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