# Astrophysics

Michelle Ntampaka, Camille Avestruz, Steven Boada, Joao Caldeira, Jessi Cisewski-Kehe, Rosanne Di Stefano, Cora Dvorkin, August E. Evrard, Arya Farahi, Doug Finkbeiner, Shy Genel, Alyssa Goodman, and alia. 2019. “The Role of Machine Learning in the Next Decade of Cosmology.” Bulletin of the American Astronomical Society, 51, 14. Publisher's VersionAbstract
In recent years, machine learning (ML) methods have remarkably improved how cosmologists can interpret data. The next decade will bring new opportunities for data-driven cosmological discovery, but will also present new challenges for adopting ML methodologies and understanding the results. ML could transform our field, but this transformation will require the astronomy community to both foster and promote interdisciplinary research endeavors.
C. Zucker, J. S. Speagle, E. Schlafly, G. M. Green, D. P. Finkbeiner, A. Goodman, and J. Alves. 2019. “VizieR Online Data Catalog: Distances to molecular clouds in SFR (Zucker+, 2020).” VizieR Online Data Catalog, J/A+A/633/A51.Abstract

Distances to ~60 star-forming regions in Reipurth (2008, Star Formation Handbook, vols I and II) have been computed using stellar photometry and Gaia DR2 parallax measurements. Usually, several distance estimates are taken across each cloud.

For each sightline, the median distance (d50) is provided, plus the 16th and 84th percentiles on the distance probability distribution function. There is an additional systematic uncertainty, which is unknown but estimated to be ~5% in distance for clouds <1.5kpc, ~10% in distance for clouds >1.5kpc, and ~7% in distance for the southern clouds Lupus, Chamaeleon, and Corona Australis. These should be added in quadrature with the statistical uncertainties reported in the table. In addition to the distances, ancillary model parameters used in our fit are also included (e.g. the amount of foreground extinction "f"). See Section 3.2.1 and Section 3.2.2 in Zucker et al. (2019ApJ...879..125Z) for a complete description of model parameters.
H. Houghton, P. Udomprasert, S. Sunbury, E. Wright, A. Goodman, E. Johnson, and A. Bishop. 2019. “Cultivating Curiosity with Life in the Universe and WorldWide Telescope.” Astronomical Society of the Pacific, 524, 273.Abstract
When students encounter complex topics like the search for extraterrestrial life, questions abound - thoughtful, unpredictable, and often profound. Despite this thriving curiosity, the first step to be able to explore complex questions is developing the capacity to verbalize a meaningful question. The WorldWide Telescope Ambassadors team designed an out-of-school curriculum called Life in the Universe, which engages middle school-aged students in the science and scientific process of the search for distant life. Students practice generating meaningful questions, which will guide them through the science content, as groups of students build to culminating capstone projects. Results from surveys administered to participating students indicate gains in curiosity in science, as well as in seeing oneself as successful in science.
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.
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.
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.
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.
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.
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.
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.

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.
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.

# Astronomy Magazine Talks to the Team Who Discovered the Radcliffe Wave

December 1, 2020

Astronomy Magazine talks to the team from Harvard’s Radcliffe Institute for Advanced Study and Harvard-Smithsonian Center for Astrophysics (CfA) about their serendipitous discovery of the Radcliffe Wave, a massive interconnected stream of stellar nurseries, molecular clouds, and supernovae that snakes through the Milky Way galaxy - and how history, art and science came together to enable this paradigm-changing discovery.  Read the full article ...