| Tuesday, September 03, 2019 |
03:30pm |
Postdoc Colloquium |
Speakers:
Will Best, The University of Texas at Austin
Probing the Low-Mass End of the Initial Mass Function
Abstract
As the lowest-mass objects created by star formation processes, brown dwarfs are essential to a complete understanding of star formation in our galaxy. The Initial Mass Function (IMF) for brown dwarfs remains poorly constrained due to incomplete samples of these faint objects in the Solar neighborhood and nearby star-forming regions. I will describe two of my projects that take different approaches to probe the low-mass IMF. One, we are using deep HST image maps of five benchmark Milky Way star-forming regions to comprehensively identify members down to planetary masses and explore variations of the IMF with star-forming environment. Two, we have built a parallax-defined volume-limited sample of brown dwarfs within 25 pc of the Sun, and we are using population synthesis to constrain the IMF and birth history that gave rise to the local brown dwarf population
Hope Chen, The University of Texas at Austin
Core Evolution through a Coherent Phase: An Updated Core Evolution Theory Told by Machine Learning
Abstract
The conventional core formation theory usually focuses on the gravitational boundedness of a core. A core is assumed to form out of a gravitationally bound density structure in a molecular cloud. In this talk, I will present the latest effort to examine the evolution of density structures found in a MHD simulation, in order to better understand the formation and evolution of star forming cores. By applying a principal component analysis (PCA) on the density and velocity distributions, we find that there exists a prolonged coherent phase in the evolution of density structures preceding the gravitational collapse and star formation in dense cores. We find that the coherent phase lasts ~ 3x10^5 to 4x10^5 years and is essential in the formation of star forming cores. By tracking the density structures in the simulation and comparing them to structures identified in observations, we characterize an evolutionary track in the parameter space and try to provide an updated theory of core formation and evolution.
Chenxu Liu, The University of Texas at Austin
Searching for “Naked” Supermassive Black Hole Candidates with HETDEX
Abstract
As the largest blind spectroscopic survey ever undertaken, HETDEX offers the possibility to explore unique, rare and important objects in the universe. I will focus on discovery of (nearly) naked black holes. There is a supermassive black hole (SMBH) in the nucleus of every galaxy, with their properties often correlated with the bulges of their hosts. The hierarchical merging of small galaxies thereby cause their black holes to merger. If a merger is tidally disrupted before the event is completed, one or both BHs could recoil and remain without a galaxy (i.e., naked) for a few gigayears. These naked SMBHs will be optically faint, making them unlikely to be targeted by almost all of the sky surveys. I will start from the introduction of the current status of the HETDEX survey, through our efforts to identify the real lines, to the selection of the naked BH candidates with HDR1.
Yifan Zhou, The University of Texas at Austin
Time-Resolved Observations of Directly-imaged Planetary-Mass Companions and Exoplanets
Abstract
Condensate clouds pose a central challenge in the atmospheric characterization of brown dwarfs and exoplanets. For variable brown dwarfs, it has been demonstrated that time-resolved observations place unique and strong constraints on their clouds. I will present my research on extending this technique to high-contrast observations of directly-imaged planetary-mass companions and exoplanets. We have achieved sub-percent level precisions in high-contrast HST/WFC3/IR light curves. With such precisions, we find the likelihood of detecting variability on planetary-mass companions is high. We also find tentative evidence for similar cloud structures shared between planetary-mass companions and brown dwarfs. There is substantial synergy between my studies and ongoing research at UT. I am looking forward to exciting collaborations on these topics.
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| Tuesday, September 10, 2019 |
03:30pm |
Chat Hull, National Astronomical Observatory of Japan |
Magnetized star formation in the ALMA era
Abstract
New ALMA polarization observations continue to both expand and confound our understanding of the role played by the magnetic field in low-mass star formation. The sample of very young, Class 0 protostellar sources observed with high resolution and high sensitivity with ALMA is now large enough that we are beginning to see the same surprising features in many different sources. The first of these are magnetic field morphologies that beautifully trace the outflow cavity walls in several objects, indicating that the outflow has shaped the magnetic field. The polarization along the cavities is strongly enhanced, and in some cases is co-located with emission from UV-tracing molecules, suggesting that the origin of the enhanced polarization is the strong irradiation of the outflow cavities. The second, more puzzling set of features are thin structures with well organized magnetic fields that are not associated with outflow cavity walls, and yet have high polarization fractions in spite of being deeply embedded and far from any obvious source of the photons necessary to align the grains. While on one hand these results challenge our understanding of both magnetic grain-alignment and grain growth, they also have the potential to open up new windows into the dust-grain properties and radiation environments in young star-forming sources.
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| Tuesday, September 17, 2019 |
03:30pm |
Stephanie Douglas, Harvard-Smithsonian Center for Astrophysics |
Calibrating Stellar Activity, Rotation, and Multiplicity With Open Clusters
Abstract
Open clusters, such as the Pleiades, Praesepe, the Hyades, and NGC 6811, are critical benchmarks for calibrating stellar properties such as rotation and magnetic activity. I will discuss recent rotation measurements in open clusters, including the first periods measured for fully convective Hyads. Discrepancies between open cluster data and models of angular momentum evolution imply that we still do not fully understand how magnetic fields affect stellar spin-down. I will show how we can use these clusters to test theories of stellar magnetic fields. Finally, I will discuss the potential impact of binary companions and protoplanetary disks on stellar angular momentum evolution.
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| Tuesday, September 24, 2019 |
03:30pm |
Rob Kennicutt, University of Arizona and Texas A&M University |
The Schmidt Law at Sixty
Abstract
Sixty years have passed since Maarten Schmidt's conjecture that star formation in galaxies was closely coupled to gas density, and since that time the Schmidt law has become an indispensable tool for interpreting, modeling, and simulating large-scale star formation in galaxies. Despite its success as a sub-grid "recipe" for the star formation rate, however, we remain far away from an ab initio theory of star formation, or even a clear understanding of the observed scaling laws themselves. This talk will review the current state of our observational understanding of star formation in galaxies, and the complexity which lies beneath the surface of the observed SFR scaling relations. We are witnessing an observational and theoretical renaissance in the subject, as multi-wavelength observations reveal the multi-scale nature of the star formation process and the complex interactions which are taking place between cosmological, gravitational, interstellar, and stellar feedback processes on these different scales. The picture which emerges is one in which the superficially simple star formation scaling laws are manifestations of a highly dynamic, complex, and self-regulating ecosystem in galactic disks.
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| Tuesday, October 01, 2019 |
03:30pm |
Charlie Conroy, Harvard-Smithsonian Center for Astrophysics |
Assembling our Galaxy
Abstract
I will review ongoing work aimed at understanding when and how the major structural components of our Galaxy came into place. The combination of Gaia and ground-based spectroscopic data has revealed that the stellar halo contains a remarkable degree of structure, and appears to have formed partially by dynamical processes within the disk, and partially from accreted dwarf galaxies. Our simulations of the stellar disk predict that the clustered nature of star formation imprints a high degree of structure in phase+chemistry space. This structure is now being measured in the data, and promises to deliver new insights into the nature of star formation and the dynamical history of the disk.
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| Tuesday, October 08, 2019 |
03:30pm |
Jonathan Fortney, UC Santa Cruz |
Population-Level Planetary Physics
Abstract
Comparative planetary science, as typically practiced over the past few decades, has looked for connections in physical processes between very small numbers of planets. However, due to the exoplanet revolution, and in particular for transiting planets, we can use the astronomical perspective to look for trends in planetary structure and physical processes that can only be seen with a large sample size. This work complements the pursuit of more detailed science questions that can be asked in the solar system. I will discuss modeling work that we have done to address several exoplanet topics, including the "evaporation valley" for sub-Neptunes and super Earths, the giant planet mass-metallicity relation, connections with atmospheric physics for these planets, and the long-standing issue of the radius anomaly of hot Jupiters.
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| Tuesday, October 15, 2019 |
03:30pm |
Heather Knutson, California Institute of Technology |
Exploring the Mysterious Origins of Super-Earths and Mini-Neptunes
Abstract
Nearly a decade has passed since the discovery that planets with sizes intermediate between that of the Earth and Neptune (“super-Earths” or “mini-Neptunes”, depending on their densities) dominate the observed population of close-in exoplanets. These planets have no solar system analogue, yet 30% of Sun-like stars appear to have at least one (and often more) interior to Mercury’s orbit. Did these planets form in situ, or did they migrate inward from a more distant formation location? Either way, the implications for our understanding of planet formation are bound to be significant. In my talk I will describe current efforts to address this question by characterizing the bulk densities and compositions of these planets and searching for outer gas giant companions.
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| Tuesday, October 22, 2019 |
03:30pm |
BashFest 2019 - No colloquium |
BashFest 2019 - No colloquium
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| Tuesday, October 29, 2019 |
03:30pm |
Michelle Ntampaka, Harvard-Smithsonian Center for Astrophysics |
A Deep Learning Approach to Galaxy Cluster X-ray Masses
Abstract
I will present a machine-learning approach for estimating galaxy cluster masses from Chandra x-ray mock observations. I will describe how a Convolutional Neural Network (CNN) -- a deep machine learning tool commonly used in image recognition tasks -- can be used to infer cluster masses from these images, reducing scatter in the mass estimates by up to 50%. I will also show an interpretation tool, inspired by Google DeepDream, that can be used to gain some physical insight into what the CNN sees.
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| Tuesday, November 05, 2019 |
03:30pm |
Laura Sales, UC Riverside |
Dwarf Galaxies and their Dark Matter Content
Abstract
Dwarf galaxies are extremely diverse in their morphology, from rotationally-supported star-forming disks to gas-free spheroidal stellar systems with no star-formation and negligible rotation. They are also believed to be the most dark matter dominated objects within the Lambda Cold Dark Matter (LCDM) model and, as such, they pose the most significant challenges to our cosmological scenario. LCDM galaxy formation models make two clear predictions: i) galaxy formation should become increasingly inefficient in lower mass halos, implying that dwarfs are only able to collect a few percent of their baryonic content, and (ii) dwarfs, like any galaxy, should be surrounded by a wealth of dark-matter substructure, implying that faint satellites of dwarfs should be common. I will present our current efforts using hydrodynamical simulations to address these predictions and to compare them with available observational constraints including: stellar halos, the inventory of dwarfs in the Local Volume and dwarf galaxies in dense cluster environments such as Virgo and Coma.
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