arXiv subjects

Philip C. Myers

Publications and source records attributed to Philip C. Myers.

At least 55 records · Page 3Linked to original sources

arXiv2019

Droplets I: Pressure-Dominated Sub-0.1 pc Coherent Structures in L1688 and B18

We present the observation and analysis of newly discovered coherent structures in the L1688 region of Ophiuchus and the B18 region of Taurus. Using data from the Green Bank Ammonia Survey (GAS), we identify regions of high density and near-constant, almost-thermal, velocity dispersion. Eighteen coherent structures are revealed, twelve in L1688 and six in B18, each of which shows a sharp "transition to coherence" in velocity dispersion around its periphery. The identification of these structures provides a chance to study the coherent structures in molecular clouds statistically. The identified coherent structures have a typical radius of 0.04 pc and a typical mass of 0.4 Msun, generally smaller than previously known coherent cores identified by Goodman et al. (1998), Caselli et al. (2002), and Pineda et al. (2010). We call these structures "droplets." We find that unlike previously known coherent cores, these structures are not virially bound by self-gravity and are instead predominantly confined by ambient pressure. The droplets have density profiles shallower than a critical Bonnor-Ebert sphere, and they have a velocity (VLSR) distribution consistent with the dense gas motions traced by NH3 emission. These results point to a potential formation mechanism through pressure compression and turbulent processes in the dense gas. We present a comparison with a magnetohydrodynamic simulation of a star-forming region, and we speculate on the relationship of droplets with larger, gravitationally bound coherent cores, as well as on the role that droplets and other coherent structures play in the star formation process.

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arXiv2019

TRAO Survey of Nearby Filamentary Molecular clouds, the Universal Nursery of Stars (TRAO FUNS) I. Dynamics and Chemistry of L1478 in the California Molecular Cloud

"TRAO FUNS" is a project to survey Gould Belt's clouds in molecular lines. This paper presents its first results on the central region of the California molecular cloud, L1478. We performed On-The-Fly mapping observations using the Taedeok Radio Astronomy Observatory (TRAO) 14m single dish telescope equipped with a 16 multi-beam array covering $\sim$1.0 square degree area of this region using C$^{18}$O (1-0) mainly tracing low density cloud and about 460 square arcminute area using N$_{2}$H$^{+}$ (1-0) mainly tracing dense cores. CS (2-1) and SO $(3_{2}-2_{1})$ were also used simultaneously to map $\sim$440 square arcminute area of this region. We identified 10 filaments by applying the dendrogram technique to the C$^{18}$O data-cube and 8 dense N$_{2}$H$^{+}$ cores by using {\sc FellWalker}. Basic physical properties of filaments such as mass, length, width, velocity field, and velocity dispersion are derived. It is found that L1478 consists of several filaments with slightly different velocities. Especially the filaments which are supercritical are found to contain dense cores detected in N$_{2}$H$^{+}$. Comparison of non-thermal velocity dispersions derived from C$^{18}$O and N$_{2}$H$^{+}$ for the filaments and dense cores indicates that some of dense cores share similar kinematics with those of the surrounding filaments while several dense cores have different kinematics with those of their filaments. This suggests that the formation mechanism of dense cores and filaments can be different in individual filaments depending on their morphologies and environments.

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arXiv2019

The Mass Evolution of Protostellar Disks and Envelopes in the Perseus Molecular Cloud

In the standard picture for low-mass star formation, a dense molecular cloud undergoes gravitational collapse to form a protostellar system consisting of a new central star, a circumstellar disk, and a surrounding envelope of remaining material. The mass distribution of the system evolves as matter accretes from the large-scale envelope through the disk and onto the protostar. While this general picture is supported by simulations and indirect observational measurements, the specific timescales related to disk growth and envelope dissipation remain poorly constrained. In this paper we conduct a rigorous test of a method introduced by Jørgensen et al. (2009) to obtain mass measurements of disks and envelopes around embedded protostars with observations that do not resolve the disk (resolution of $\sim$1000\,AU). Using unresolved data from the recent Mass Assembly of Stellar Systems and their Evolution with the SMA (MASSES) survey, we derive disk and envelope mass estimates for $59$ protostellar systems in the Perseus molecular cloud. We compare our results to independent disk mass measurements from the VLA Nascent Disk and Multiplicity (VANDAM) survey and find a strong linear correlation, suggesting that accurate disk masses can be measured from unresolved observations. Then, leveraging the size of the MASSES sample, we find no significant trend in protostellar mass distribution as a function of age, as approximated from bolometric temperatures. These results may indicate that the disk mass of a protostar is set near the onset of the Class 0 protostellar stage and remains roughly constant throughout the Class I protostellar stage.

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arXiv2019

Magnetic Field Structure of Dense Cores using Spectroscopic Methods

We develop a new ''core field structure'' (CFS) model to predict the magnetic field strength and magnetic field fluctuation profile of dense cores using gas kinematics. We use spatially resolved observations of the nonthermal velocity dispersion from the Green Bank Ammonia survey along with column density maps from SCUBA-2 to estimate the magnetic field strength across seven dense cores located in the L1688 region of Ophiuchus. The CFS model predicts the profile of the relative field fluctuation, which is related to the observable dispersion in direction of the polarization vectors. Within the context of our model we find that all the cores have a transcritical mass-to-flux ratio.

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arXiv2018

Dust Polarization Toward Embedded Protostars in Ophiuchus with ALMA. II. IRAS 16293-2422

We present high-resolution (~ 35 au) ALMA Band 6 1.3 mm dust polarization observations of IRAS 16293. These observations spatially resolve the dust polarization across the two protostellar sources and toward the filamentary structures between them. The dust polarization and inferred magnetic field have complicated structures throughout the region. In particular, we find that the magnetic field is aligned parallel to three filamentary structures. We characterize the physical properties of the filamentary structure that bridges IRAS 16293A and IRAS 16293B and estimate a magnetic field strength of 23-78 mG using the Davis-Chandrasekhar-Fermi method. We construct a toy model for the bridge material assuming that the young stars dominate the mass and gravitational potential of the system. We find that the expected gas flow to each star is of comparable order to the Alfven speed, which suggests that the field may be regulating the gas flow. We also find that the bridging material should be depleted in ~ 1000 yr. If the bridge is part of the natal filament that formed the stars, then it must have accreted new material. Alternatively, the bridge could be a transient structure. Finally, we show that the 1.3 mm polarization morphology of the optically thick IRAS 16293B system is qualitatively similar to dust self-scattering. Based on similar polarization measurements at 6.9 mm, we propose that IRAS 16293B has produced a substantial population of large dust grains with sizes between 200 and 2000 um.

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arXiv2018

Catalogue of High Protostellar Surface Density Regions in Nearby Embedded Clusters

We analyze high-quality stellar catalogs for 24 young and nearby (within 1 kpc) embedded clusters and present a catalogue of 32 groups which have a high concentration of protostars. The median effective radius of these groups is 0.17 pc. The median protostellar and pre-main sequence star surface densities are 46 M_{\odot} pc^{-2} and 11 M_{\odot} pc^{-2}, respectively. We estimate the age of these groups using a model of constant birthrate and random accretion stopping and find a median value of 0.25 Myr. Some groups in Aquila and Serpens, Corona Australia and Ophichus L1688 show high protostellar surface density and high molecular gas surface density, which seem to be undergoing vigorous star formation. These groups provide an excellent opportunity to study initial conditions of clustered star formation. Comparison of protostellar and pre-main-sequence stellar surface densities reveal continuous low-mass star formation of these groups over several Myr in some clouds. For groups with typical protostellar separations of less than 0.4 pc, we find that these separations agree well with the thermal Jeans fragmentation scale. On the other hand, for groups with typical protostellar separations larger than 0.4 pc, these separations are always larger than the associated Jeans length.

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arXiv2018

Magnetic Field Structure in Spheroidal Star-Forming Clouds

A model of magnetic field structure is presented to help test the prevalence of flux freezing in star-forming clouds of various shapes, orientations, and degrees of central concentration, and to estimate their magnetic field strength. The model is based on weak-field flux freezing in centrally condensed Plummer spheres and spheroids of oblate and prolate shape. For a spheroid of given density contrast, aspect ratio, and inclination, the model estimates the local field strength and direction, and the global field pattern of hourglass shape. Comparisons with a polarization simulation indicate typical angle agreement within 1 - 10 degrees. Scalable analytic expressions are given to match observed polarization patterns, and to provide inputs to radiative transfer codes for more accurate predictions. The model may apply to polarization observations of dense cores, elongated filamentary clouds, and magnetized circumstellar disks.

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arXiv2018

High-resolution ALMA Study of the Proto-Brown-Dwarf Candidate L328-IRS

This paper presents our observational attempts to precisely measure the central mass of a proto-brown dwarf candidate, L328-IRS, in order to investigate whether L328-IRS is in the substellar mass regime. Observations were made for the central region of L328-IRS with the dust continuum and CO isotopologue line emission at ALMA band 6, discovering the detailed outflow activities and a deconvolved disk structure of a size of $\sim$87 AU $\times$ 37 AU. We investigated the rotational velocities as a function of the disk radius, finding that its motions between 130 AU and 60 AU are partially fitted with a Keplerian orbit by a stellar object of $\sim0.30 ~\rm\,M_\odot$, while the motions within 60 AU do not follow any Keplerian orbit at all. This makes it difficult to lead a reliable estimation of the mass of L328-IRS. Nonetheless, our ALMA observations were useful enough to well constrain the inclination angle of the outflow cavity of L328-IRS as $\sim 66^{\circ}$, enabling us to better determine the mass accretion rate of $\sim 8.9\times 10^{-7} \rm\,M_\odot~yr^{-1}$. From assumptions that the internal luminosity of L328-IRS is mostly due to this mass accretion process in the disk, or that L328-IRS has mostly accumulated the mass through this constant accretion rate during its outflow activity, its mass was estimated to be $\sim 0.012 - 0.023\rm\,M_\odot$, suggesting L328-IRS to be a substellar object. However, we leave our identification of L328-IRS as a proto-brown dwarf to be tentative because of various uncertainties especially regarding the mass accretion rate.

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arXiv2018

Mass Assembly of Stellar Systems and their Evolution with the SMA - 1.3 mm Subcompact Data Release

We present the Mass Assembly of Stellar Systems and their Evolution with the SMA (MASSES) survey, which uses the Submillimeter Array (SMA) interferometer to map the continuum and molecular lines for all 74 known Class 0/I protostellar systems in the Perseus molecular cloud. The primary goal of the survey is to observe an unbiased sample of young protostars in a single molecular cloud so that we can characterize the evolution of protostars. This paper releases the MASSES 1.3 mm data from the subcompact configuration ($\sim$4$^{\prime\prime}$ or $\sim$1000 au resolution), which is the SMA's most compact array configuration. We release both $uv$ visibility data and imaged data for the spectral lines CO(2-1),$^{13}$CO(2-1), C$^{18}$O(2-1), and N$_2$D$^+$(3-2), as well as for the 1.3 mm continuum. We identify the tracers that are detected toward each source. We also show example images of continuum and CO(2-1) outflows, analyze C$^{18}$O(2-1) spectra, and present data from the SVS 13 star-forming region. The calculated envelope masses from the continuum show a decreasing trend with bolometric temperature (a proxy for age). Typical C$^{18}$O(2-1) linewidths are 1.45 km s$^{-1}$, which is higher than the C$^{18}$O linewidths detected toward Perseus filaments and cores. We find that N$_2$D$^+$(3-2) is significantly more likely to be detected toward younger protostars. We show that the protostars in SVS 13 are contained within filamentary structures as traced by C$^{18}$O(2-1) and N$_2$D$^+$(3-2). We also present the locations of SVS 13A's high velocity (absolute line-of-sight velocities $>$150 km s$^{-1}$) red and blue outflow components. Data can be downloaded from https://dataverse.harvard.edu/dataverse/MASSES .

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arXiv2018

Dust Polarization Toward Embedded Protostars in Ophiuchus with ALMA. I. VLA 1623

We present high resolution (~ 30 au) ALMA Band 6 dust polarization observations of VLA 1623. The VLA 1623 data resolve compact ~ 40 au inner disks around the two protobinary sources, VLA 1623-A and VLA 1623-B, and also an extended ~ 180 au ring of dust around VLA 1623-A. This dust ring was previously identified as a large disk in lower-resolution observations. We detect highly structured dust polarization toward the inner disks and the extended ring with typical polarization fractions ~ 1.7% and ~ 2.4%, respectively. The two components also show distinct polarization morphologies. The inner disks have uniform polarization angles aligned with their minor axes. This morphology is consistent with expectations from dust scattering. By contrast, the extended dust ring has an azimuthal polarization morphology not previously seen in lower-resolution observations. We find that our observations are well fit by a static, oblate spheroid model with a flux-frozen, poloidal magnetic field. We propose that the polarization traces magnetic grain alignment likely from flux freezing on large scales and magnetic diffusion on small scales. Alternatively, the azimuthal polarization may be attributed to grain alignment by the anisotropic radiation field. If the grains are radiatively aligned, then our observations indicate that large (~ 100 um) dust grains grow quickly at large angular extents. Finally, we identify significant proper motion of VLA 1623 using our observations and those in the literature. This result indicates that the proper motion of nearby systems must be corrected for when combining ALMA data from different epochs.

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arXiv2018

Filamentary Fragmentation and Accretion in High-Mass Star-Forming Molecular Clouds

Filamentary structures are ubiquitous in high-mass star-forming molecular clouds. Their relation with high-mass star formation is still to be understood. Here we report interferometric observations toward 8 filamentary high-mass star-forming clouds. A total of 50 dense cores are identified in these clouds, most of which present signatures of high-mass star formation. Five of them are not associated with any star formation indicators, hence are prestellar core candidates. Evolutionary phases of these cores and their linewidths, temperatures, NH$_3$ abundances, and virial parameters are found to be correlated. In a sub-sample of 4 morphologically well-defined filaments, we find that their fragmentation can not be solely explained by thermal or turbulence pressure support. We also investigate distributions of gas temperatures and non-thermal motions along the filaments, and find a spatial correlation between non-thermal linewidths and star formation activities. We find evidence of gas flows along these filaments, and derive an accretion rate along filaments of $\sim$10$^{-4}$ M$_\odot$ yr$^{-1}$. These results suggest a strong relationship between massive filaments and high-mass star formation, through i) filamentary fragmentation in very early evolutionary phases to form dense cores, ii) accretion flows along filaments that are important for the growth of dense cores and protostars, and iii) enhancement of non-thermal motion in the filaments by the feedback or accretion during star formation.

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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

Intensity-Corrected Herschel Observations of Nearby Isolated Low-Mass Clouds

We present intensity-corrected Herschel maps at 100 um, 160 um, 250 um, 350 um, and 500 um for 56 isolated low-mass clouds. We determine the zero-point corrections for Herschel PACS and SPIRE maps from the Herschel Science Archive (HSA) using Planck data. Since these HSA maps are small, we cannot correct them using typical methods. Here, we introduce a technique to measure the zero-point corrections for small Herschel maps. We use radial profiles to identify offsets between the observed HSA intensities and the expected intensities from Planck. Most clouds have reliable offset measurements with this technique. In addition, we find that roughly half of the clouds have underestimated HSA-SPIRE intensities in their outer envelopes relative to Planck, even though the HSA-SPIRE maps were previously zero-point corrected. Using our technique, we produce corrected Herschel intensity maps for all 56 clouds and determine their line-of-sight average dust temperatures and optical depths from modified black body fits. The clouds have typical temperatures of ~ 14-20 K and optical depths of ~ 1e-5 - 1e-3. Across the whole sample, we find an anti-correlation between temperature and optical depth. We also find lower temperatures than what was measured in previous Herschel studies, which subtracted out a background level from their intensity maps to circumvent the zero-point correction. Accurate Herschel observations of clouds are key to obtain accurate density and temperature profiles. To make such future analyses possible, intensity-corrected maps for all 56 clouds are publicly available in the electronic version.

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arXiv2017

The Green Bank Ammonia Survey: Observations of Hierarchical Dense Gas Structures in Cepheus-L1251

We use Green Bank Ammonia Survey observations of NH$_3$ (1,1) and (2,2) emission with 32'' FWHM resolution from a ~ 10 pc$^{2}$ portion of the Cepheus-L1251 molecular cloud to identify hierarchical dense gas structures. Our dendrogram analysis of the NH$_3$ data results in 22 top-level structures, which reside within 13 lower-level, parent structures. The structures are compact (0.01 pc $\lesssim R_{eff} \lesssim$ 0.1 pc) and are spatially correlated with the highest H$_2$ column density portions of the cloud. We also compare the ammonia data to a catalog of dense cores identified by higher-resolution (18.2'' FWHM) Herschel Space Observatory observations of dust continuum emission from Cepheus-L1251. Maps of kinetic gas temperature, velocity dispersion, and NH$_3$ column density, derived from detailed modeling of the NH$_3$ data, are used to investigate the stability and chemistry of the ammonia-identified and Herschel-identified structures. We show that the dust and dense gas in the structures have similar temperatures, with median $T_{dust}$ and $T_K$ measurements of 11.7 $\pm$ 1.1 K and 10.3 $\pm$ 2.0 K, respectively. Based on a virial analysis, we find that the ammonia-identified structures are gravitationally dominated, yet may be in or near a state of virial equilibrium. Meanwhile, the majority of the Herschel-identified dense cores appear to be not bound by their own gravity and instead confined by external pressure. CCS $(2_0-1_0)$ and HC$_5$N $(9-8)$ emission from the region reveal broader line widths and centroid velocity offsets when compared to the NH$_3$ (1,1) emission in some cases, likely due to these carbon-based molecules tracing the turbulent outer layers of the dense cores.

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arXiv2017

The Green Bank Ammonia Survey: Dense Cores Under Pressure in Orion A

We use gas temperature and velocity dispersion data from the Green Bank Ammonia Survey and core masses and sizes from the James Clerk Maxwell Telescope Gould Belt Survey to estimate the virial states of dense cores within the Orion A molecular cloud. Surprisingly, we find that almost none of the dense cores are sufficiently massive to be bound when considering only the balance between self-gravity and the thermal and non-thermal motions present in the dense gas. Including the additional pressure binding imposed by the weight of the ambient molecular cloud material and additional smaller pressure terms, however, suggests that most of the dense cores are pressure confined.

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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

An Estimation of the Star Formation Rate in the Perseus Complex

We present the results of our investigation of the star-forming potential in the Perseus star-forming complex. We build on previous starless core, protostellar core, and young stellar object (YSO) catalogs from Spitzer, Herschel, and SCUBA observations in the literature. We place the cores and YSOs within seven star-forming clumps based on column densities greater than 5x10^21 cm^-2. We calculate the mean density and free-fall time for 69 starless cores as 5.55x10^-19 gcm^-3 and 0.1 Myr,respectively, and we estimate the star formation rate for the near future as 150 Msun Myr^-1. According to Bonnor Ebert stability analysis, we find that majority of starless cores in Perseus are unstable. Broadly, these cores can collapse to form the next generation of stars. We found a relation between starless cores and YSOs, where the numbers of young protostars (Class 0 + Class I) are similar to the numbers of starless cores. This similarity, which shows a one-to-one relation, suggests that these starless cores may form the next generation of stars with approximately the same formation rate as the current generation, as identified by the Class 0 and Class I protostars. It follows that if such a relation between starless cores and any YSO stage exists, the SFR values of these two populations must be nearly constant. In brief, we propose that this one-to-one relation is an important factor in better understanding the star formation process within a cloud.

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arXiv2017

The Green Bank Ammonia Survey (GAS): First Results of NH3 mapping the Gould Belt

We present an overview of the first data release (DR1) and first-look science from the Green Bank Ammonia Survey (GAS). GAS is a Large Program at the Green Bank Telescope to map all Gould Belt star-forming regions with $A_V \gtrsim 7$ mag visible from the northern hemisphere in emission from NH$_3$ and other key molecular tracers. This first release includes the data for four regions in Gould Belt clouds: B18 in Taurus, NGC 1333 in Perseus, L1688 in Ophiuchus, and Orion A North in Orion. We compare the NH$_3$ emission to dust continuum emission from Herschel, and find that the two tracers correspond closely. NH$_3$ is present in over 60\% of lines-of-sight with $A_V \gtrsim 7$ mag in three of the four DR1 regions, in agreement with expectations from previous observations. The sole exception is B18, where NH$_3$ is detected toward ~ 40\% of lines-of-sight with $A_V \gtrsim 7$ mag. Moreover, we find that the NH$_3$ emission is generally extended beyond the typical 0.1 pc length scales of dense cores. We produce maps of the gas kinematics, temperature, and NH$_3$ column densities through forward modeling of the hyperfine structure of the NH$_3$ (1,1) and (2,2) lines. We show that the NH$_3$ velocity dispersion, $σ_v$, and gas kinetic temperature, $T_K$, vary systematically between the regions included in this release, with an increase in both the mean value and spread of $σ_v$ and $T_K$ with increasing star formation activity. The data presented in this paper are publicly available.

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