List of Past Astronomy Colloquia : 01-Jan-2020 to 01-Jun-2020

Date:   Wednesday 29-Jan-2020
Speaker:   Dr. M. Coleman Miller (University of Maryland)
Title:  A NICER look at a neutron star

The cores of neutron stars contain matter in a state that cannot be probed in terrestrial laboratories. Thus observations of neutron stars are required to reduce our large uncertainty about the properties of such high-density cold matter. Knowledge of neutron star radii would be especially helpful, but most published radius estimates have potentially large systematic errors. Fortunately, studies in a number of papers over the last several years have shown that radius estimates from energy-dependent X-ray waveforms, i.e., the focus of NASA's Neutron star Interior Composition Explorer (NICER), appear to be free of such systematic errors and can achieve precisions that contribute significantly to our understanding of this important physical realm. I will describe the first mass-radius results from NICER and the implications for the state of cold matter above nuclear saturation density.

Date:   Wednesday 05-Feb-2020
Speaker:   Dr. Vadim Semenov (Harvard-Smithsonian Center for Astrophysics)
Title:  The Origin of Inefficient Star Formation in Galaxies

Observed star-forming galaxies convert their gas into stars surprisingly inefficiently. Available gas in galaxies is depleted on the timescale of several Gyr which is orders of magnitude longer than any physical timescale relevant for star formation. The origin of long gas depletion times is a long-standing puzzle. Many galaxy simulations can reproduce observed long depletion times but the physical mechanism controlling their values is not well understood. In addition, some of the simulations show a rather counter-intuitive behavior: global depletion times appear to be almost insensitive to the assumptions about local star formation in individual star-forming regions, a phenomenon described as "self-regulation." Yet another part of the puzzle is the observed tight and near-linear correlation between star formation rates and the amount of molecular gas on kiloparsec and larger scales. A linear correlation implies that depletion time of molecular gas is almost independent of molecular gas density on >kiloparsec scales, while popular models of star formation in galaxies predict a strong dependence. I will present results from a suite of isolated disk galaxy simulations in which we systematically explored the behavior of depletion times. Using insights from these simulations we formulated a physical model that explains both the origin of long gas depletion times in observed galaxies and the results of galaxy formation simulations. This model and our simulation results also provide major insights into the origin of the observed linear correlation between star formation rates and molecular gas.

Date:   Wednesday 12-Feb-2020
Speaker:   Dr. David Rupke (Rhodes College)
Title:  How big are galactic winds?

Intense star formation and rapid black hole accretion in the centers of galaxies produce energy that propels gas outward. These galactic winds affect the evolution of their host galaxies, and may self-regulate the future growth of stars and black holes. Galaxies are also known to be surrounded by enormous reservoirs of gas. Galactic winds are a likely mechanism by which these reservoirs are created. However, we don't know how far most galactic winds extend into their surroundings. I will discuss observations of galactic winds driven by star formation and black holes that probe their extent and illuminate the connection between galaxies and their surroundings.

Date:   Wednesday 19-Feb-2020
Speaker:   Dr. Cornelia Lang (The University of Iowa)
Title:  The Central Molecular Zone of the Galaxy: Dense Clouds, Massive Stars and Magnetic Fields

In addition to harboring a supermassive black hole at its very core, the Galactic center is one of the most physically extreme environments in the Galaxy. Dense and massive molecular clouds on non-circular orbits are abundant in this region, yet star formation is not as active and frequent as expected. In addition, radio observations have revealed a population of synchrotron-emitting filaments that provide insight on the magnetic field strength and configuration in this unique region of the Galaxy. I will review observational results from several recent studies undertaken by my research group: we have examined the properties and kinematics of a group of unusual molecular clouds that appear to be part of an orbital “stream” of material around the Galactic center. In addition, we have been studying the detailed structure of the synchrotron-emitting radio filaments and their connection to larger-scale energetic outflows from the Galactic center. Our relative proximity to the Galactic center provides an unprecedented view of a galactic core and studies of this region can be used as an astrophysical analog to understanding the nuclei of more distant galaxies.

Date:   Tuesday 25-Feb-2020
Speaker:   Dr. Dmitri A. Udensky (Unversity of Colorado Boulder)
Title:  Extreme Plasma Astrophysics: a New Frontier”

Bright, rapid high-energy flares and other spectacular phenomena are powered by collective plasma processes near neutron stars (NSs) and black holes (BHs), including in multi-messenger sources like gravitational-wave-emitting NS mergers and PeV-neutrino-producing blazars. While our understanding of these processes has been informed by traditional (e.g., space or fusion) plasma research, the physical conditions in plasmas around BHs and NSs are so extreme that traditional intuition often fails and a richer physical framework is required. Extreme astrophysical plasmas are relativistic, interact with radiation, and may be subject to QED pair-production effects. Studying collective plasma processes with these “exotic” effects constitutes the realm of Extreme Plasma Astrophysics. Rapid progress in exploring this new frontier is now being made thanks to concerted, vigorous theoretical efforts and computational breakthroughs due to the advent of novel first-principles relativistic kinetic plasma codes incorporating radiation and QED pair creation. Laser plasma experiments may soon also contribute to this revolution. I will describe these exciting developments, illustrating them with our recent kinetic studies of relativistic magnetic reconnection in NS magnetospheres and accreting BH coronae with radiation and pair-creation effects, and of radiative relativistic turbulence. I will end by outlining the future prospects of this vibrant Extreme Plasma Astrophysics research program.

Date:   Wednesday 26-Feb-2020
Speaker:   Dr. Ilse Cleeves (University of Virginia)
Title:  Illuminating the Chemistry of Planet Formation with X-ray Flares

The chemistry of protoplanetary disks sets the initial composition of newly formed planets and may also regulate the efficiency by which planets form. Disk chemical abundances typically evolve over timescales spanning thousands if not millions of years. Consequently, it was a surprise when ALMA observations taken over the course of a single year showed significantly variable molecular emission in H13CO+ relative to the otherwise constant thermal dust emission in the IM Lup protoplanetary disk. HCO+ is a known X-ray sensitive molecule, and one possible explanation is that stellar activity is perturbing the chemical "steady state" of the disk. If confirmed, simultaneous observations may provide a new tool to measure (and potentially map) fundamental disk parameters, such as electron density, as the light from X-ray flares propagates across the disk.

Date:   Wednesday 04-Mar-2020
Speaker:   Dr. Ray Jayawardhana (Cornell University)
Title:  Remote Sensing of Alien Worlds: Characterizing Exoplanets from the Ground and from Space

Searches for exoplanets have revealed a remarkable diversity of worlds, and comparative studies of their properties have begun in earnest. I will discuss recent results using Kepler data as well as ground-based observations, with a particular focus on high-resolution spectroscopy, which, combined with the Doppler cross-correlation technique, is emerging as a powerful probe of exoplanet atmospheres. I will report on our wide-ranging observational program targeting Jovian-mass worlds, sub-Saturns and super-Earths using a suite of frontline instruments in the optical as well as the near-infrared, and preview planned observations using the James Webb Space Telescope.

Date:   Wednesday 18-Mar-2020

Date:   Wednesday 25-Mar-2020
Speaker:   Dr. Xu (Chelsea) Huang (M.I.T.)
Title:  Exoplanets in the Solar Neighborhood in the Era of TESS

The Transiting Exoplanet Survey Satellite (TESS) has offered a new opportunity to study a large number of exoplanets in the solar neighborhood. The brightness of these exoplanet host stars will enable detailed examination of planetary systems in the next decade. In this talk, I will present examples of exciting planet populations emerging from the discoveries of TESS, all bearing new clues on planet formation and evolution. As a detailed case study, I will demonstrate how we can advance our knowledge on planetary system architecture using multiplanetary systems hosting giant planets. I will conclude with discussions on possibilities brought by future generation of wide field exoplanet surveys.

Date:   Wednesday 01-Apr-2020
Speaker:   Dr. George Zhou (Harvard University)
Title:  Mapping planets through space and time

The Solar System is peculiar. Unlike ~50% of other Sun-like stars that host planets in orbits shorter than that of Mercury, the inner most regions of the Solar System is devoid of material. How did these short period planets form, and why is the Solar System different? I am approaching this intrigue by characterizing planets around a variety of stars, at different ages, to better understand how the parent star influences the planets they bear.

We have been utilizing the TESS mission to search for planets around a variety of stellar populations untapped by previous surveys. I will discuss our efforts to characterize the occurrence rates and properties of planets around main sequence A stars, and how these observations have influenced the way we understand hot Jupiter migration mechanisms.

The TESS mission is also enabling us to search for and characterize planets around young, bright stars. The search for planets around young stars is our best proxy to watching planet formation and evolution in action. In the next decade, we will reconstruct the physical and dynamical properties of small planets as a function of age, painting a pathway that led to the dominant planet demographic around Sun-like stars.

Date:   Wednesday 08-Apr-2020
Speaker:   Dr. Marta Bryan (UC Berkeley)
Title:  Exploring the Formation and Evolution of Planetary Systems

Over the past two decades thousands of planets with an extraordinary diversity of properties have been discovered orbiting nearby stars. Many of these exoplanetary systems challenge our narrative for how planets form and evolve, motivating the search for observational clues to the underlying mechanisms that led to this diversity. In this talk

I will describe my work using a wide range of observational techniques to uncover these underlying mechanisms. I will constrain the physics of gas giant formation and evolution by discerning population statistics, system architectures, rotation rates, and atmospheric compositions of gas giant planets. I will then discuss the effect that outer gas giants have on the inner architectures of planetary systems by exploring differences in inner planet masses, separations, multiplicities, and orbital properties. Finally, I will highlight the key role that next generation instruments and telescopes will play by extending these novel observations to entirely new classes of planets.

Date:   Wednesday 15-Apr-2020
Speaker:   Dr. Thaddeus Komacek (University of Chicago)
Title:  Interpreting astronomical observations to characterize the atmospheric circulation, climate, and habitability of exoplanets

Over the past two decades, space-based observations with the Spitzer and Hubble space telescopes have allowed for detailed characterization of the atmospheric circulation of hot gas giant planets. The upcoming launch of JWST promises spectral characterization of the atmospheres of gas giant planets, along with a first glimpse at the atmospheres of terrestrial exoplanets in the habitable zones of M dwarf stars. In this talk, I will describe my research on theory and numerical simulations of the atmospheric circulation and climate of both hot gas giant exoplanets and temperate terrestrial exoplanets. First, I will show how light curves of transiting exoplanets taken over a full orbital phase can be used to understand their atmospheric circulation. I will discuss the observational differences between ultra-hot Jupiters and cooler gas giants, and describe theory for the mechanisms that impact the circulation of ultra-hot Jupiters. I will also present results from 3D general circulation modeling experiments that include the effects of hydrogen dissociation and recombination and compare these simulations to observations of KELT-9b, the hottest exoplanet discovered to date. Second, I will describe the effects of clouds on the climate and habitability of terrestrial exoplanets. I will show the effect of cloud coverage on the inner edge of the habitable zone for planets with varying radius and gravity orbiting late-type M dwarf stars. Additionally, I will show the effect of clouds on the detectability of water transmission features on terrestrial exoplanets orbiting late-type M dwarf stars with JWST. Finally, I will describe how LUVOIR/HabEx/OST promise to transform our understanding of the atmospheres of temperate terrestrial exoplanets.

Date:   Wednesday 22-Apr-2020
Speaker:   Dr. Joseph Rodriguez (Harvard University)
Title:  Understanding Planetary Evolution with Eclipsing Disks & Transiting Planets

The success of ground-based transit and RV surveys, and the Kepler/K2 and TESS missions, have shifted the exoplanet field from pure discovery to a combination of discovery, demographic analysis, and detailed characterization, especially for exoplanet atmospheres. Unfortunately, most known transiting exoplanet hosts are too faint to permit detailed characterization. We are using data from the TESS, Kepler/K2, and ground-based transi specific questions about planet formation and evolution. We are also studying the birthplaces of planets by searching for occultations of newly formed stars by their protoplanetary disks with our Disk Eclipse Search with KELT (DESK) survey. These systems provide insight into the conditions required for planet formation. I will describe our recent results from both projects and discuss how we will study these types of objects in future surveys such as LSST.

Date:   Wednesday 06-May-2020
Speaker:   Dr. Peter Shawhan (UMD)
Title:  A Tale of Two GW Events: First Published Results from the LIGO-Virgo O3 Run

The LIGO and Virgo gravitational-wave observatories have collected nearly a year of data with better sensitivity than ever before during the recent O3 observing run. Dozens of binary merger candidates were identified in near-real-time and shared with astronomers promptly, although no clear counterparts have been identified in this run. LIGO-Virgo collaboration members are currently re-analyzing the data to confirm and fully characterize most of those candidates and to search for other gravitational-wave signals in the data. So far, final analysis results are available for two events: GW190425, a binary neutron star merger with a total mass of about 3.4 solar masses; and GW190412, the first binary black hole merger confirmed to have distinctly different masses, estimated at 30 and 8 solar masses. I will discuss what we have learned from these two very interesting events.

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