# Publications

2021
Shuo Kong, Volker Ossenkopf-Okada, Héctor G. Arce, John Bally, Álvaro Sánchez-Monge, Peregrine McGehee, Sümeyye Suri, Ralf S. Klessen, John M. Carpenter, Dariusz C. Lis, Fumitaka Nakamura, Peter Schilke, Rowan J. Smith, Steve Mairs, Alyssa Goodman, and María José Maureira. 2021. “The CARMA-NRO Orion Survey: Filament Formation via Collision-induced Magnetic Reconnectionthe Stick in Orion A.” The Astrophysical Journal, 906, Pp. 80. Publisher's VersionAbstract
A unique filament is identified in the Herschel maps of the Orion A giant molecular cloud. The filament, which we name the Stick, is ruler-straight and at an early evolutionary stage. Transverse position–velocity diagrams show two velocity components closing in on the Stick. The filament shows consecutive rings/forks in C18O (1−0) channel maps, which is reminiscent of structures generated by magnetic reconnection. We propose that the Stick formed via collision-induced magnetic reconnection (CMR). We use the magnetohydrodynamics code Athena++ to simulate the collision between two diffuse molecular clumps, each carrying an antiparallel magnetic field. The clump collision produces a narrow, straight, dense filament with a factor of >200 increase in density. The production of the dense gas is seven times faster than freefall collapse. The dense filament shows ring/fork-like structures in radiative transfer maps. Cores in the filament are confined by surface magnetic pressure. CMR can be an important dense-gas-producing mechanism in the Galaxy and beyond.
Cameren Swiggum, Elena D’Onghia, João Alves, Josefa Großschedl, Michael Foley, Catherine Zucker, Stefan Meingast, Boquan Chen, and Alyssa Goodman. 2021. “Evidence for Radial Expansion at the Core of the Orion Complex with Gaia EDR3.” arXiv, 2101, 10380.Abstract
We present a phase-space study of two stellar groups located at the core of the Orion complex: Briceño-1 and Orion Belt Population-near (OBP-near). We identify the groups with the unsupervised clustering algorithm, Shared Nearest Neighbor (SNN), which previously identified twelve new stellar substructures in the Orion complex. For each of the two groups, we derive the 3D space motions of individual stars using Gaia EDR3 proper motions supplemented by radial velocities from Gaia DR2, APOGEE-2, and GALAH DR3. We present evidence for radial expansion of the two groups from a common center. Unlike previous work, our study suggests that evidence of stellar group expansion is confined only to OBP-near and Briceño-1 whereas the rest of the groups in the complex show more complicated motions. Interestingly, the stars in the two groups lie at the center of a dust shell, as revealed via an extant 3D dust map. The exact mechanism that produces such coherent motions remains unclear, while the observed radial expansion and dust shell suggest that massive stellar feedback could have influenced the star formation history of these groups.
Anika Schmiedeke, Jaime E. Pineda, Paola Caselli, Héctor G. Arce, Gary A Fuller, Alyssa A. Goodman, María José Maureira, Stella S. R. Offner, Dominique Segura-Cox, and Daniel Seifried. 2021. “Dissecting the super-critical filaments embedded in the 0.5 pc subsonic region of Barnard 5.” arXiv, 2101, 00248. Publisher's VersionAbstract
We characterize in detail the two ~0.3 pc long filamentary structures found within the subsonic region of Barnard 5. We use combined GBT and VLA observations of the molecular lines NH3(1,1) and (2,2) at a resolution of 1800 au, as well as JCMT continuum observations at 850 and 450 μm at a resolution of 4400 au and 3000 au, respectively. We find that both filaments are highly super-critical with a mean mass per unit length, M/L, of ~80 M⊙ pc−1, after background subtraction, with local increases reaching values of ~150 M⊙ pc−1. This would require a magnetic field strength of ~500 μG to be stable against radial collapse.
We extract equidistant cuts perpendicular to the spine of the filament and fit a modified Plummer profile as well as a Gaussian to each of the cuts. The filament widths (deconvolved FWHM) range between 6500-7000 au (~0.03 pc) along the filaments. This equals ~2.0 times the radius of the flat inner region. We find an anti-correlation between the central density and this flattening radius, suggestive of contraction. Further, we also find a strong correlation between the power-law exponent at large radii and the flattening radius. We note that the measurements of these three parameters fall in a plane and derive their empirical relation. Our high-resolution observations provide direct constraints of the distribution of the dense gas within super-critical filaments showing pre- and protostellar activity.
Natalie Grasser, Sebastian Ratzenböck, João Alves, Josefa Großschedl, Stefan Meingast, Catherine Zucker, Alvaro Hacar, Charles Lada, Alyssa Goodman, Marco Lombardi, John C. Forbes, Immanuel M. Bomze, and Torsten Möller. 2021. “The $$\rho$$ Oph region revisited with Gaia EDR3.” arXiv, 2101, 12200.Abstract
Context. Young and embedded stellar populations are important probes of the star formation process. Paradoxically, we have a better census of nearby embedded young populations than the slightly more evolved optically visible young populations. The high accuracy measurements and all-sky coverage of Gaia data are about to change this situation. Aims. This work aims to construct the most complete sample to date of YSOs in the ρ Oph region. Methods. We compile a catalog of 761 Ophiuchus YSOs from the literature and crossmatch it with the Gaia EDR3, Gaia-ESO and APOGEE-2 surveys. We apply a multivariate classification algorithm to this catalog to identify new, co-moving population candidates. Results. We find 173 new YSO candidates in the Gaia EDR3 catalog belonging to the ρ Oph region. The new sources appear to be mainly Class III M-stars and substellar objects and are less extincted than the known members. We find 19 previously unknown sources with disks. The analysis of the proper motion distribution of the entire sample reveals a well-defined bimodality, implying two distinct populations sharing a similar 3D volume. The first population comprises young stars' clusters around the ρ Ophiuchi star and the main Ophiuchus clouds (L1688, L1689, L1709). In contrast, the second population is older (∼ 10 Myr), dispersed, has a distinct proper motion, and is likely part of the Upper-Sco group. The two populations are moving away from each other at about 3.8 km/s, and will no longer overlap in about 4 Myr. Finally, we flag 47 sources in the literature as impostors, which are sources that exhibit large deviations from the average distance and proper motion properties of the ρ Oph population. Our results show the importance of accurate 3D space and motion information for improved stellar population analysis.
2020
Catherine Zucker, Joshua S. Speagle, Edward F. Schlafly, Gregory M. Green, Douglas P. Finkbeiner, Alyssa Goodman, and João Alves. 1/2020. “A Compendium of Distances to Molecular Clouds in the Star Formation Handbook.” Astronomy and Astrophysics, 633, Pp. A51.Abstract
Accurate distances to local molecular clouds are critical for understanding the star and planet formation process, yet distance measurements are often obtained inhomogeneously on a cloud-by-cloud basis. We have recently developed a method that combines stellar photometric data with Gaia DR2 parallax measurements in a Bayesian framework to infer the distances of nearby dust clouds to a typical accuracy of ∼5%. After refining the technique to target lower latitudes and incorporating deep optical data from DECam in the southern Galactic plane, we have derived a catalog of distances to molecular clouds in Reipurth (2008, Star Formation Handbook, Vols. I and II) which contains a large fraction of the molecular material in the solar neighborhood. Comparison with distances derived from maser parallax measurements towards the same clouds shows our method produces consistent distances with ≲10% scatter for clouds across our entire distance spectrum (150 pc-2.5 kpc). We hope this catalog of homogeneous distances will serve as a baseline for future work. Table A.1 is also available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz- bin/cat/J/A+A/633/A51. It is also available on the Harvard Dataverse at http://https://doi.org/1 0.7910/DVN/07L7YZ An interactive 3D version of Fig. 2 is available at http://https://www.aanda.org
Hope How-Huan Chen, Stella S. R. Offner, Jaime E. Pineda, Alyssa A. Goodman, Andreas Burkert, Adam Ginsburg, and Spandan Choudhury. 2020. “Core Formation, Coherence and Collapse: A New Core Evolution Paradigm Revealed by Machine Learning.” arXiv, 2006, Pp. 07325.Abstract
We study the formation, evolution and collapse of dense cores by tracking density structures in a magnetohydrodynamic (MHD) simulation. We identify cores using the dendrogram algorithm and utilize machine learning techniques, including principal component analysis (PCA) and the k-means clustering algorithm to analyze the full density and velocity dispersion profiles of these cores. We find that there exists an evolutionary sequence consisting of three distinct phases: i) the formation of turbulent density structures (Phase I), ii) the dissipation of turbulence and the formation of coherent cores (Phase II), and iii) the transition to protostellar cores through gravitational collapse (Phase III). In dynamically evolving molecular clouds, the existence of these three phases corresponds to the coexistence of three populations of cores with distinct physical properties. The prestellar and protostellar cores frequently analyzed in previous studies of observations and simulations belong to the last phase in this evolutionary picture. We derive typical lifetimes of 1.4±1.0×105 yr, 3.3±1.4×105 yr and 3.3±1.4×105 yr, respectively for Phase I, II and III. We find that cores can form from both converging flows and filament fragmentation and that cores may form both inside and outside the filaments. We then compare our results to previous observations of coherent cores and provide suggestions for future observations to study cores belonging to the three phases.
Jorma Harju, Jaime E. Pineda, Anton I. Vasyunin, Paola Caselli, Stella S. R. Offner, and Alyssa A. Goodman. 2020. “Efficient Methanol Production on the Dark Side of a Prestellar Core.” The Astrophysical Journal, 895, Pp. 101.Abstract
We present Atacama Large Millimeter/submillimeter Array maps of the starless molecular cloud core Ophiuchus/H-MM1 in the lines of deuterated ammonia (ortho-), methanol (), and sulfur monoxide (SO). The dense core is seen in  emission, whereas the  and SO distributions form a halo surrounding the core. Because methanol is formed on grain surfaces, its emission highlights regions where desorption from grains is particularly efficient. Methanol and sulfur monoxide are most abundant in a narrow zone that follows the eastern side of the core. This side is sheltered from the stronger external radiation field coming from the west. We show that photodissociation on the illuminated side can give rise to an asymmetric methanol distribution but that the stark contrast observed in H-MM1 is hard to explain without assuming enhanced desorption on the shaded side. The region of the brightest emission has a wavy structure that rolls up at one end. This is the signature of Kelvin–Helmholtz instability occurring in sheared flows. We suggest that in this zone, methanol and sulfur are released as a result of grain–grain collisions induced by shear vorticity.
Spandan Choudhury, Jaime E. Pineda, Paola Caselli, Adam Ginsburg, Stella S. R. Offner, Erik Rosolowsky, Rachel K. Friesen, Felipe O. Alves, Ana Chacón-Tanarro, Anna Punanova, Elena Redaelli, Philip Helen Kirk, C. Myers, Peter G. Martin, Yancy Shirley, Michael Chun-Yuan Chen, Alyssa A. Goodman, and James Di Francesco. 2020. “Ubiquitous NH$$_{3}$$ supersonic component in L1688 coherent cores.” A&A, 640, Pp. L6. Publisher's VersionAbstract

Context. Star formation takes place in cold dense cores in molecular clouds. Earlier observations have found that dense cores exhibit subsonic non-thermal velocity dispersions. In contrast, CO observations show that the ambient large-scale cloud is warmer and has supersonic velocity dispersions.

Aims. We aim to study the ammonia (NH3) molecular line profiles with exquisite sensitivity towards the coherent cores in L1688 in order to study their kinematical properties in unprecedented detail.

Methods. We used NH3 (1,1) and (2,2) data from the first data release (DR1) in the Green Bank Ammonia Survey (GAS). We first smoothed the data to a larger beam of 1′ to obtain substantially more extended maps of velocity dispersion and kinetic temperature, compared to the DR1 maps. We then identified the coherent cores in the cloud and analysed the averaged line profiles towards the cores.

Results. For the first time, we detected a faint (mean NH3(1,1) peak brightness < 0.25 K in TMB), supersonic component towards all the coherent cores in L1688. We fitted two components, one broad and one narrow, and derived the kinetic temperature and velocity dispersion of each component. The broad components towards all cores have supersonic linewidths (ℳS ≥ 1). This component biases the estimate of the narrow dense core component’s velocity dispersion by ≈28% and the kinetic temperature by ≈10%, on average, as compared to the results from single-component fits.

Conclusions. Neglecting this ubiquitous presence of a broad component towards all coherent cores causes the typical single-component fit to overestimate the temperature and velocity dispersion. This affects the derived detailed physical structure and stability of the cores estimated from NH3 observations.

Kaustav K. Das, Catherine Zucker, Joshua S. Speagle, Alyssa Goodman, Gregory M. Green, and João Alves. 2020. “Constraining the distance to the North Polar Spur with Gaia DR2.” MNRAS, 498, Pp. 5863-5872.Abstract
The North Polar Spur (NPS) is one of the largest structures observed in the Milky Way in both the radio and soft X-rays. While several predictions have been made regarding the origin of the NPS, modelling the structure is difficult without precise distance constraints. In this paper, we determine accurate distances to the southern terminus of the NPS and towards latitudes ranging up to 55°. First, we fit for the distance and extinction to stars towards the NPS using optical and near-infrared photometry and Gaia Data Release 2 astrometry. We model these per-star distance–extinction estimates as being caused by dust screens at unknown distances, which we fit for using a nested sampling algorithm. We then compare the extinction to the Spur derived from our 3D dust modelling with integrated independent measures from XMM–Newton X-ray absorption and H I column density measures. We find that we can account for nearly 100 per cent of the total column density of the NPS as lying within 140 pc for latitudes >26° and within 700 pc for latitudes <11°. Based on the results, we conclude that the NPS is not associated with the Galactic Centre or the Fermi bubbles. Instead, it is likely associated, especially at higher latitudes, with the Scorpius–Centaurus association.
João Alves, Catherine Zucker, Alyssa A. Goodman, Joshua S. Speagle, Stefan Meingast, Thomas Robitaille, Douglas P. Finkbeiner, Edward F. Schlafly, and Gregory M. Green. 2020. “A Galactic-Scale Gas Wave in the Solar Neighbourhood.” Nature, 578, Pp. 237–239. Publisher's VersionAbstract
For the past 150 years, the prevailing view of the local interstellar medium has been based on a peculiarity known as the Gould Belt1-4, an expanding ring of young stars, gas and dust, tilted about 20 degrees to the Galactic plane. However, the physical relationship between local gas clouds has remained unknown because the accuracy in distance measurements to such clouds is of the same order as, or larger than, their sizes5-7. With the advent of large photometric surveys8 and the astrometric survey9, this situation has changed10. Here we reveal the three- dimensional structure of all local cloud complexes. We find a narrow and coherent 2.7-kiloparsec arrangement of dense gas in the solar neighbourhood that contains many of the clouds thought to be associated with the Gould Belt. This finding is inconsistent with the notion that these clouds are part of a ring, bringing the Gould Belt model into question. The structure comprises the majority of nearby star-forming regions, has an aspect ratio of about 1:20 and contains about three million solar masses of gas. Remarkably, this structure appears to be undulating, and its three-dimensional shape is well described by a damped sinusoidal wave on the plane of the Milky Way with an average period of about 2 kiloparsecs and a maximum amplitude of about 160 parsecs.
T. K. Sridharan, Shmuel Bialy, Raymond Blundell, Andrew Burkhardt, Thomas Dame, Sheperd Doeleman, Douglas Finkbeiner, Alyssa Goodman, and alia. 2020. “A Prospective ISRO-CfA Himalayan Sub-millimeter-wave Observatory Initiative.” arXiv, 2008, 07453. Publisher's VersionAbstract
The Smithsonian Astrophysical Observatory (SAO), a member of the Center for Astrophysics | Harvard and Smithsonian, is in discussions with the Space Applications Centre (SAC) of the Indian Space Research Organization (ISRO) and its partners in the newly formed Indian Sub-millimetre-wave Astronomy Alliance (ISAA), to collaborate in the construction of a sub-millimeter-wave astronomy observatory in the high altitude deserts of the Himalayas, initially at the 4500 m Indian Astronomical Observatory, Hanle. Two primary science goals are targeted. One is the mapping of the distribution of neutral atomic carbon, and the carbon monoxide (CO) molecule in higher energy states, in large parts of the Milky Way, and in selected external galaxies. Such studies would advance our understanding of molecular hydrogen present in the interstellar medium, but partly missed by existing observations; and characterize Galaxy-wide molecular cloud excitation conditions, through multi-level CO observations. Stars form in interstellar clouds of molecular gas and dust, and these observations would allow research into the formation and destruction processes of such molecular clouds and the life cycle of galaxies. As the second goal, the observatory would add a new location to the global Event Horizon Telescope (EHT) network, which lacks a station in the Himalayan longitudes. This addition would enhance the quality of the images synthesized by the EHT, support observations in higher sub-millimeter wave bands, sharpening its resolving ability, improve its dynamic imaging capability and add weather resilience to observing campaigns. In the broader context, this collaboration can be a starting point for a wider, mutually beneficial scientific exchange between the Indian and US astronomy communities, including a potential future EHT space component.
Che-Yu Chen, Erica A. Behrens, Jasmin E. Washington, Laura M. Fissel, Rachel K. Friesen, Zhi-Yun Li, Jaime E. Pineda, and alia. 2020. “Relative Alignment between Dense Molecular Cores and Ambient Magnetic Field: The Synergy of Numerical Models and Observations.” arXiv E-Prints.Abstract

The role played by magnetic field during star formation is an important topic in astrophysics. We investigate the correlation between the orientation of star-forming cores (as defined by the core major axes) and ambient magnetic field directions in 1) a 3D MHD simulation, 2) synthetic observations generated from the simulation at different viewing angles, and 3) observations of nearby molecular clouds. We find that the results on relative alignment between cores and background magnetic field in synthetic observations slightly disagree with those measured in fully 3D simulation data, which is partly because cores identified in projected 2D maps tend to coexist within filamentary structures, while 3D cores are generally more rounded. In addition, we examine the progression of magnetic field from pc- to core-scale in the simulation, which is consistent with the anisotropic core formation model that gas preferably flow along the magnetic field toward dense cores. When comparing the observed cores identified from the GBT Ammonia Survey (GAS) and Planck polarization-inferred magnetic field orientations, we find that the relative core-field alignment has a regional dependence among different clouds. More specifically, we find that dense cores in the Taurus molecular cloud tend to align perpendicular to the background magnetic field, while those in Perseus and Ophiuchus tend to have random (Perseus) or slightly parallel (Ophiuchus) orientations with respect to the field. We argue that this feature of relative core-field orientation could be used to probe the relative significance of the magnetic field within the cloud.

2019
Hope How-Huan Chen, Jaime E. Pineda, Stella S. R. Offner, Alyssa A. Goodman, Andreas Burkert, Rachel K. Friesen, Erik Rosolowsky, Samantha Scibelli, and Yancy Shirley. 12/2019. “Droplets. II. Internal Velocity Structures and Potential Rotational Motions in Pressure-Dominated Coherent Structures.” The Astrophysical Journal, 886, Pp. 119. Publisher's VersionAbstract
We present an analysis of the internal velocity structures of the newly identified sub-0.1 pc coherent structures, droplets, in L1688 and B18. By fitting 2D linear velocity fields to the observed maps of velocity centroids, we determine the magnitudes of linear velocity gradients and examine the potential rotational motions that could lead to the observed velocity gradients. The results show that the droplets follow the same power-law relation between the velocity gradient and size found for larger-scale dense cores. Assuming that rotational motion giving rise to the observed velocity gradient in each core is a solid-body rotation of a rotating body with a uniform density, we derive the “net rotational motions” of the droplets. We find a ratio between rotational and gravitational energies, β, of ∼0.046 for the droplets, and when including both droplets and larger-scale dense cores, we find β ∼ 0.039. We then examine the alignment between the velocity gradient and the major axis of each droplet, using methods adapted from the histogram of relative orientations introduced by Soler et al. We find no definitive correlation between the directions of velocity gradients and the elongations of the cores. Lastly, we discuss physical processes other than rotation that may give rise to the observed velocity field.
Ian W. Stephens, Tyler L. Bourke, Michael M. Dunham, Philip C. Myers, Riwaj Pokhrel, John J. Tobin, Héctor G. Arce, and et al. 12/2019. “Mass Assembly of Stellar Systems and Their Evolution with the SMA (MASSES)—Full Data Release.” The Astrophysical Journal. Supplement Series, 245, Pp. 21. Publisher's VersionAbstract
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 (̃4″ or ̃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), 13CO(2-1), C18O(2-1), and N2D+(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 C18O(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 C18O(2-1) line widths are 1.45 km s-1, which is higher than the C18O line widths detected toward Perseus filaments and cores. We find that N2D+(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 C18O(2-1) and N2D+(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://da taverse.harvard.edu/dataverse/MASSES.
Catherine Zucker, Rowan Smith, and Alyssa Goodman. 12/2019. “Synthetic Large-Scale Galactic Filaments: On Their Formation, Physical Properties, and Resemblance to Observations.” The Astrophysical Journal , 887, Pp. 186. Publisher's VersionAbstract
Using a population of large-scale filaments extracted from an AREPO simulation of a Milky Way─like galaxy, we seek to understand the extent to which observed large-scale filament properties (with lengths ≳100 pc) can be explained by galactic dynamics alone. From an observer’s perspective in the disk of the galaxy, we identify filaments forming purely due to galactic dynamics, without the effects of feedback or local self-gravity. We find that large-scale galactic filaments are intrinsically rare, and we estimate that at maximum approximately one filament per kpc2 should be identified in projection, when viewed from the direction of our Sun in the Milky Way. In this idealized scenario, we find filaments in both the arm and interarm regions and hypothesize that the former may be due to gas compression in the spiral potential wells, with the latter due to differential rotation. Using the same analysis pipeline applied previously to observations, we analyze the physical properties of large-scale galactic filaments and quantify their sensitivity to projection effects and galactic environment (i.e., whether they lie in the arm or interarm regions). We find that observed “Giant Molecular Filaments” are consistent with being non-self- gravitating structures dominated by galactic dynamics. Straighter, narrower, and denser “Bone-like” filaments, like the paradigmatic Nessie filament, have similar column densities, velocity gradients, and galactic plane heights (z ≈ 0 pc) to those in our simple model, but additional physical effects (such as feedback and self-gravity) must be invoked to explain their lengths and widths.
P. Udomprasert, H. Houghton, S. Sunbury, J. Plummer, E. Wright, A. Goodman, E. Johnson, H. Zhang, A. Vaishampayan, and K. Cho. 11/2019. “Visualizing Seasons and Moon Phases with WorldWide Telescope.” Advancing Astronomy for All: ASP 2018 ASP Conference Series, 524, Pp. 125. Publisher's VersionAbstract

WorldWide Telescope (WWT) is a powerful visualization program that allows users to connect Earth-based and space-based views of the Sun- Earth-Moon system. By blending hands-on physical activities with WWT's virtual models, students can visualize spatially complex concepts like seasons, Moon phases, and eclipses. In this workshop, we will demonstrate how WWT and the physical models are used together in our WWT ThinkSpace curriculum, developed with funding from the National Science Foundation. We will also present student learning outcomes based on written assessments and student interviews.

Hope How-Huan Chen, Jaime E. Pineda, Alyssa A. Goodman, Andreas Burkert, Stella S. R. Offner, Rachel K. Friesen, Philip C. Myers, and et al. 6/2019. “Droplets. I. Pressure-Dominated Coherent Structures in L1688 and B18.” The Astrophysical Journal, 877, Pp. 93. Publisher's VersionAbstract
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, we identify regions of high density and near-constant, almost-thermal velocity dispersion. We reveal 18 coherent structures are revealed, 12 in L1688 and 6 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 statistically study the coherent structures in molecular clouds. The identified coherent structures have a typical radius of 0.04 pc and a typical mass of 0.4 M ☉, generally smaller than previously known coherent cores identified by Goodman et al., Caselli et al., and Pineda et al. 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 (V LSR) 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.
Laura M. Fissel, Peter A. R. Ade, Francesco E. Angilè, Peter Ashton, Steven J. Benton, Che-Yu Che, Maria Cunningham, and et al. 6/2019. “Relative Alignment between the Magnetic Field and Molecular Gas Structure in the Vela C Giant Molecular Cloud Using Low- and High-Density Tracers.” The Astrophysical Journal , 878, Pp. 110. Publisher's VersionAbstract

We compare the magnetic field orientation for the young giant molecular cloud Vela C inferred from 500 μm polarization maps made with the BLASTPol balloon-borne polarimeter to the orientation of structures in the integrated line emission maps from Mopra observations. Averaging over the entire cloud we find that elongated structures in integrated line-intensity or zeroth-moment maps, for low-density tracers such as 12CO and 13CO J → 1 - 0, are statistically more likely to align parallel to the magnetic field, while intermediate- or high-density tracers show (on average) a tendency for alignment perpendicular to the magnetic field. This observation agrees with previous studies of the change in relative orientation with column density in Vela C, and supports a model where the magnetic field is strong enough to have influenced the formation of dense gas structures within Vela C. The transition from parallel to no preferred/perpendicular orientation appears to occur between the densities traced by 13CO and by C18O J → 1 - 0. Using RADEX radiative transfer models to estimate the characteristic number density traced by each molecular line, we find that the transition occurs at a molecular hydrogen number density of approximately 103 cm-3. We also see that the Centre Ridge (the highest column density and most active star-forming region within Vela C) appears to have a transition at a lower number density, suggesting that this may depend on the evolutionary state of the cloud.

Aneta Siemiginowska, Gwendolyn Eadie, Ian Czekala, Eric Feigelson, Eric B. Ford, Vinay Kashyap, Michael Kuhn, and et al. 5/2019. “The Next Decade of Astroinformatics and Astrostatistics.” Bulletin of the American Astronomical Society, 51, Pp. 355. Publisher's VersionAbstract

Over the past century, major advances in astronomy and astrophysics have been driven by improvements in instrumentation. With the amassing of high quality data from new telescopes it is becoming clear that research in astrostatistics and astroinformatics will be necessary to develop new methodology needed in astronomy.

J.D. Soler, H. Beuther, M. Rugel, Y. Wang, P. C. Clark, S. C. O. Glover, P. F. Goldsmith, and et al. 2/2019. “Histogram of Oriented Gradients: A Technique for the Study of Molecular Cloud Formation.” Astronomy and Astrophysics, 622, Pp. A166. Publisher's VersionAbstract

We introduce the histogram of oriented gradients (HOG), a tool developed for machine vision that we propose as a new metric for the systematic characterization of spectral line observations of atomic and molecular gas and the study of molecular cloud formation models. In essence, the HOG technique takes as input extended spectral-line observations from two tracers and provides an estimate of their spatial correlation across velocity channels. We characterized HOG using synthetic observations of HI and 13CO (J = 1 → 0) emission from numerical simulations of magnetohydrodynamic (MHD) turbulence leading to the formation of molecular gas after the collision of two atomic clouds. We found a significant spatial correlation between the two tracers in velocity channels where vHI ≈ v13CO, almost independent of the orientation of the collision with respect to the line of sight. Subsequently, we used HOG to investigate the spatial correlation of the HI, from The HI/OH/recombination line survey of the inner Milky Way (THOR), and the 13CO (J = 1 → 0) emission from the Galactic Ring Survey (GRS), toward the portion of the Galactic plane 33°.75 ≤l ≤ 35°.25 and |b| ≤ 1°.25. We found a significant spatial correlation between the two tracers in extended portions of the studied region. Although some of the regions with high spatial correlation are associated with HI self-absorption (HISA) features, suggesting that it is produced by the cold atomic gas, the correlation is not exclusive to this kind of region. The HOG results derived for the observational data indicate significant differences between individual regions: some show spatial correlation in channels around vHI ≈ v13CO while others present spatial correlations in velocity channels separated by a few kilometers per second. We associate these velocity offsets to the effect of feedback and to the presence of physical conditions that are not included in the atomic-cloud-collision simulations, such as more general magnetic field configurations, shear, and global gas infall.