Astrophysics

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.

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.
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.

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.

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.

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.
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.
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
P. Udomprasert, H. Houghton, S. Sunbury, J. Plummer, A. Goodman, and P. Sadler. 2020. “Best Practices in Astronomy Education: Long-Term Retention of Learning Gains with the WWT ThinkSpace Seasons Lab.” Conference Paper, 385, 07.Abstract
 

Seasons is a middle school science topic where students commonly hold strong misconceptions. Ideas about seasons are so resistant to change because the Earth-Sun system is a complex, dynamic system that requires strong spatial reasoning to fully understand. Most "traditional" methods of instruction (lecture, textbook diagrams, passively-watched videos) do not adequately support students in the spatial reasoning tasks needed to understand seasons. The ThinkSpace Seasons Lab specifically targets the necessary spatial reasoning skills and strategies (for example, connecting the position of the Sun in a space-based perspective with the position in the sky in an Earth-based perspective), by blending visualizations from the WorldWide Telescope program and hands-on models. Student understanding of seasons after using the ThinkSpace curriculum increases with large effect size, and we have longitudinal data showing that two years post-instruction, students who used the ThinkSpace curriculum have stronger recollection of key seasons concepts than peers who used "traditional" curricula.

A. Goodman, J. Alves, C. Zucker, J. S. Speagle, S. Meingast, T. Robitaille, D. P. Finkbeiner, E. P. Schlafly, and G. M. Green. 2020. “A New Feature of the Galaxy Revealed by 3D Dust Mapping.” Conference Paper, 385, 05.Abstract
We present a new, very large, highly-elongated, undulating, gaseous structure lying just beyond the current orbit of the Sun in the Milky Way. The structure was discovered using 3D-dust mapping techniques, and it encompasses several star-forming regions formerly associated with "Gould's Belt" and the "Local Arm" of the Galaxy. Gaia data, along with several other large surveys, were critical to this discovery. We will present several three-dimensional views for this new object, and speculate on its origins.
Che-Yu Chen, Erica A. Behrens, Jasmin E. Washington, Laura M. Fissel, Rachel K. Friesen, Zhi-Yun Li, Jaime E. Pineda, and et al. 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.

João Alves, Catherine Zucker, Alyssa A. Goodman, Joshua S. Speagle, Stefan Meingast, Thomas Robitaille, Douglas P. Finkbeiner, Edward F. Schlafly, and Gregory M. Green. 1/2020. “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.
Photo of Milky Way

Professors Offer Insights From Their Fields Amid COVID

July 10, 2020

In this time of profound uncertainty, society can be sure of one thing: more uncertainty. The seemingly opaque path forward for us, individually and collectively, was the Gazette’s topic with three Harvard professors, including Robert Wheeler Willson Professor of Applied Astronomy, Dr. Alyssa Goodman, who shared insights into how uncertainty is viewed in their fields, and the surprising ways in which it’s not necessarily a bad thing. ...

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