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Astronomy Colloquium 2020-21

Astronomy Colloquia at Caltech for 2020-2021

Colloquia are held every Wednesday during the academic year at 4pm in the Cahill Hameetman auditorium. Wine and cheese will be served in the Cahill Foyer from 5-5:30pm.
Astronomy Colloquium Youtube channel

Date Speaker Hos
September 30, 2020 Speaker: Di Li Title: The Commensal Radio Astronomy FAST Survey (CRAFTS)
Institute: Chinese Academy of Science
Host: Vikram Ravi Abstract: The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has released its first call for proposal and will be open to the international community next year. Based on a novel technique of high-cadence CAL injection, we have realized the world's first calibrated commensal survey mode, simultaneously taking data for pulsar search, HI galaxies, HI imaging, and FRBs. I introduce here one of the major survey plans, namely, the Commensal Radio Astronomy FAST Survey (CRAFTS, Li et al. 2018), which has discovered more than 100 new pulsars, including a few dozen MSPs, 5 new FRBs, including one new repeater. I will also briefly describe recent FAST results from CRAFTS and other dedicated programs, including new insights into the characteristic energy of FRBs, the formation process of neutron stars, the evolution of interstellar medium, etc.
October, 2, 2020 Speaker: Danielle Berg Title: Extreme UV Emission: Bridging Galaxy Evolution Across Cosmic Time
Institute: OSU / UT Austin
Host: Chuck Steidel Abstract:In the last few years, our first glimpse of the spectral properties of z∼5−7 galaxies has emerged. Deep UV spectra have revealed prominent high-ionization nebular emission lines (i.e., C IV, He II, C III]) indicating that extreme radiation fields may be characteristic of reionization-era systems. While such strong high-ionization emission lines are atypical of the well-studied z∼0−3 galaxy samples, our recent UV spectral campaigns have revealed several galaxies with analogous emission-line features to reionization-era systems. I will discuss the recent detection of extremely strong UV emission in nearby galaxies and the potential sources of their very hard ionizing radiation fields. Such strong detections of high-ionization emission lines have been linked to the leakage of Lyman continuum (LyC) photons (necessary for reionization) both theoretically and observationally. These extreme UV emission-line dwarf galaxies provide a template for the extreme conditions that are important for reionization, however their features are still poorly understood. In preparation for the coming UV window onto the early universe with the advent of ELTs and JWST, I will introduce the COS Legacy Archival Spectroscopic SurveY - an upcoming large HST program designed to disentangle the stellar and nebular spectral signatures of 45 star-forming galaxies. This program will calibrate new UV diagnostics that will allow us to trace galaxy evolution to the distant universe, unveiling the properties of reionization-era galaxies.
October 14, 2020 Speaker: Keith Hawkins Title: Galactic Archaeology with Gaia and Large Spectroscopic Surveys
Institute: UT Austin One of the key objectives of modern astrophysics is to understand the formation and evolution galaxies. In this regard, the Milky Way is a critical testing ground for our theories of galaxy formation. However, dissecting the assembly history of the Galaxy, requires a detailed mapping of the structural, dynamical chemical, and age distributions of its stellar populations. Recently, we have entered an era of large spectroscopic and astrometric surveys, which has begun to pave the way for the exciting advancements in this field. Combining data from the many multi-object spectroscopic surveys, which are already underway, and the rich dataset from Gaia will undoubtedly be the way forward in order to disentangle the full chemo-dynamical history of our Galaxy. In this talk, I will discuss my current work in Galactic archaeology and how large spectroscopic surveys have and can been used to dissect the structure of our Galaxy. I will also explore the future of Galactic archaeology through chemical cartography.
Host: Jim Fuller
October 21, 2020 Speaker: Ben Oppenheimer Title: Why Measuring Black Hole Masses is Essential for Understanding the Circumgalactic Medium
Institute: University of Colorado Abstract: Ripped straight from the headlines of the award of this year's Nobel Prize in Physics, I will begin by discussing the potential connection between a galaxy's central supermassive black hole (SMBH) and its gaseous halo, the circumgalactic medium. Cosmological simulations predict a direct connection between these dramatically different scales, which causally arises from the integrated history of SMBH feedback expelling gas beyond the virial radius. While observing this connection is currently challenging, I will discuss the potential of observing hot gaseous halos in X-rays around Milky Way-mass galaxies. I plan to continue discussing Milky Way-mass halos as observed through via UV absorption line signatures, including changes with redshift from z=3 to today.
Host: Chuck Steidel
October 28, 2020 Speaker: Gwen Rudie Title: Feedback and Feeding: The Circumgalactic Medium of Star-Forming Galaxies during Cosmic Noon
Institute: Carnegie Observatories The exchange of baryons between galaxies and their surrounding intergalactic medium (IGM) is a crucial but poorly constrained aspect of galaxy formation and evolution. I will present three surveys that use classical IGM absorption line techniques to better understand gas flows and the impact of environment on the life cycle of galaxies. Most of my talk will focus on a new detailed study of metal-enriched absorbing gas in the high-z (2"<"z"<"3) circumgalactic medium (CGM) using data from the Keck Baryonic Structure Survey (KBSS). I will highlight results on the size, kinematics, and thermal properties of circumgalactic gas which provide clear evidence of unbound metal-enriched gas within the halos of "z~2" galaxies and demonstrate the CGM at cosmic noon requires constant heating or replenishment of warm gas. Collectively, these data suggest the CGM provides one of the best testing grounds for models of galaxy-scale outflows. Next, I will briefly show forthcoming results from the Cosmic Ultraviolet Baryon Survey (CUBS) which aims to understand the turn down in the cosmic star formation rate density through study of the CGM at 0.4 "<" z "<" 1.0 using HST/COS. I’ll conclude with a short discussion of a new survey underway at Magellan to map the large-scale environment of galaxies at cosmic noon, the Lyman-alpha Tomography IMACS Survey (LATIS). Collectively, these surveys constrain the nature and sphere of influence of galaxy-scale outflows and intergalactic accretion, as well as the impact of environment on galaxies at cosmic noon and beyond.
Host: Chuck Steidel
November 4, 2020 Speaker: Natasha Batalha Title: Interpreting Exoplanet Atmospheres at the Onset of Next-Generation Telescopes
Institute: NASA Ames Abstract: One of NASA's primary goals is to observationally characterize exoplanet atmospheres, understand the chemical and physical processes of exoplanets and improve the understanding of the origins of exoplanetary systems. Throughout the next decade and beyond, JWST, WFIRST, future mission concepts, and ground based telescopes will work towards achieving these goals by interpreting a diverse set of exoplanet atmosphere observations, ranging from hot gas giants to small temperate rocky worlds. Our understanding and interpretation of this full gamut of spectroscopy data will hinge on our ability to accurately link observations to theoretical models. Therefore, it is imperative that our theoretical models are equipped to tackle these problems. Leading up to this new era in exoplanetary space science, one of our goals has been to ensure that the community is equipped with robust, user-friendly, open-source, theoretical models needed to both plan and execute ground-breaking science. I will first discuss the current landscape of theoretical exoplanet model development. Then, I will discuss our recent developments in cloud, opacity, and spectroscopy models that will work together to enable effective interpretation of exoplanet spectroscopy during this next-generation of observations.
Host: Tiffany Meshkat
November 11, 2020 Speaker: Subo Dong Title: Direct Collision of White Dwarfs as a Possible Major Channel for Type Ia Supernovae
Institute: Peking University Abstract: The explosion mechanism of Type Ia supernovae (SN Ia) is unknown. The continuity in the properties of SNe Ia across the luminosity function suggests a single dominant channel to explain the population. We argue that direct collisions of white dwarfs may be promising as such a major channel. I will present results of our efforts to test the collision model.
Host: Ilaria Caiasso
November 18, 2020 Speaker: Joss Bland-Hawthorn Title: Galactic seismology: the evolution of the “phase spiral” after the Sagittarius impact
Institute: University of Sydney Abstract: Our Galaxy, the Milky Way, is a benchmark for understanding disk galaxies. It is the only galaxy whose formation history can be studied using the full distribution of stars, from white dwarfs to supergiants. The oldest components provide unique insight into how galaxies form and evolve over billions of years. This is a veritable golden age for galactic archaeology with many large surveys now under way to map both chemistry and motions for stars in the Galaxy. Detailed 6D "phase space" information combined with chemistry for millions of stars heralds a new era in how we slice up the Galactic disc. This has already enabled the most remarkable discovery to emerge from ESA's Gaia satellite — the “phase spiral”. This phenomenon, which was not foreseen, is direct evidence of giant waves crossing the disc. We discuss how the phase spiral is generated and what it may tell us about our history. We review the main science goals of Galactic seismology, and look to what the future may hold. These studies will continue to play a fundamental role far into the future because there are measurements that can only be made in the near field and contemporary astrophysics depends on such observations.
Host: Chris Martin
December 2, 2020 Speaker: Ann-Marie Madigan Title: Collective Gravity in the Outer Solar System
Institute: University of Colorado Abstract: There’s something odd going on in our solar system. While the planets move on nearly-circular orbits in a well-defined plane, the minor planets beyond Neptune appear to cluster together in a highly-inclined, eccentric, and tilted structure. Astronomers have invoked the presence of an additional planet (Planet 9) or even a primordial black hole in explanation. In this talk I will show that these theories may be unnecessary. In analogy with spiral arms and bars in galaxies, the collective gravity of individually small but collectively massive bodies can create such structures in the outer solar system.
Host: Tiffany Meshkat
December 9, 2020 Speaker: Rebecca Jensen-Clem Title: Exoplanet Imaging with Extremely Large Telescopes
Institute: UCSC Abstract: In the last thirty years, over 3000 planets have been discovered orbiting nearby stars. This flood of new worlds includes planets unlike any found in our own Solar System, from Jupiter-mass planets with years as short as our day to exotic rocky worlds twice as massive as the Earth. While our understanding of exoplanets' diversity has leapt forward in recent years, fundamental questions remain. For example, what are the dominant planet formation pathways? How do planets acquire their atmospheres? Is there life on other worlds? These questions can only be answered through observations of exoplanets’ spectra, where the characteristic imprints of atoms and molecules making up a planet’s atmosphere are revealed. The most promising method for obtaining spectra of diverse exoplanets is direct imaging: by nulling the light of the parent star with an optical device called a coronagraph, the planet itself can be seen and its light dispersed into a spectrum. So far, only extremely young, massive worlds have been directly imaged, while older, lower mass objects like the Earth remain hidden in the glare of their host stars. In this talk, I will describe two avenues for advancing the state-of-the-art in exoplanet imaging: 1) detecting low-mass exoplanets at Solar System separations with the W. M. Keck Observatory and Thirty Meter Telescope via predictive wavefront control and focal plane wavefront sensing, and 2) characterizing the atmospheres of directly imaged planets with polarimetry -- an untapped method for probing the physics of clouds in the atmospheres of other worlds.
Host: Mansi Kasliwal
January 6, 2021 Speaker: Ellen Zweibel Title: Cosmic Rays in Multiphase Gas
Institute: University of Wisconsin-Madison Abstract: Cosmic rays appear to be universal in galaxies with star formation or nuclear activity. They play a role in the dynamics, energy balance, and stability of the diffuse gas in these systems, and are now included in many state of the art models of galaxy formation and evolution. The exchange of energy and momentum between cosmic rays and thermal gas is largely collisionless, mediated by kinetic scale plasma waves. The properties of these waves depend both on local conditions and the global structure of the thermal gas, introducing complex feedback loops that are only beginning to be probed theoretically and incorporated into models. I will discuss some recent progress in this area, focusing on the role of interstellar clouds.
Host: Jim Fuller
January 20, 2021 Speaker: Stephen Smartt Title: Mergers, magnetars and multi-messengers
Institute: Queens University Belfast Abstract:
Host: Mansi Kasliwal Wide-field optical sky surveys are discovering a remarkable diversity
PLEASE NOTE: THIS TALK WILL BE HELD in how stars merge, collapse and explode. A number of unusual and
AT 12:00 PM (NOON) instead of 4:00pm fast declining transients have been found, but their true volumetric rate
appears low. The powering mechanism for
many of these requires a source beyond radioactivity, plausibly a
magnetic, rapidly spinning neutron star. In addition, the discovery of
an electromagnetic counterpart to a pair of merging neutron stars
showed that gravitational wave sources produce transients that emit photons
from gamma rays to the radio. Spectral analysis provides a method to
determine the composition, ejecta velocity and kinetic energy of the transients,
which helps constrain their nature. I will highlight our recent searches for degenerate
mergers and fast transients with ATLAS and Pan-STARRS. I will also show
how spectroscopic analysis can uncover the composition of these unusual
January 27, 2021 Speaker: Erin Kara Title: Reverberation mapping black hole accretion flows
Institute: MIT Sitting at the heart of nearly all galaxies is a massive black hole, that, while typically 1000 times less massive than the galaxy in which it resides, has the potential to release enough energy via accretion of gas onto the black hole, to completely unbind the entire galaxy. Most of the power from an Active Galactic Nucleus (AGN) is released close to the black hole, and thus studying the inner accretion flow, at the intersection of gas inflow and energy outflow, is essential for understanding how black holes grow and affect galactic evolution. In the past decade, we have had a breakthrough in how we probe the inner accretion flow, through the discovery of X-ray Reverberation Mapping, where X-rays produced close to the black hole reverberate off inflowing gas. By measuring reverberation time delays, we can quantify the effects of strongly curved space time and the black hole spin, which is key for understanding how efficiently energy can be tapped from the accretion process. In this talk, I will give an overview of this field, and show how extending these X-ray spectral-timing techniques to transient accretion episodes is helping us probe the formation of X-ray coronae, jets and other relativistic outflows.
Host: Ilaria Caiazzo
February 3, 2021 Speaker: Francois Combes Title: Circumnuclear Molecular Disks: Role in AGN Fueling and Feedback
Institute: Observatoire de Paris The gas inflows directly fueling AGN are now traceable with
Host: Chris Martin current high-resolution observations with ALMA and NOEMA.
Dynamical mechanisms are essential to exchange angular
momentum and drive the gas to the super-massive black hole.
While at 100pc scale, the gas is sometimes stalled in nuclear
rings, recent observations reaching ~10pc scale (or 50mas),
inside the sphere of influence of the black hole, may bring smoking
gun evidence of fueling, within a randomly oriented nuclear molecular disk.
Observations of AGN feedback will be presented, together
with the suspected responsible mechanisms. Molecular outflows are frequently
detected in active galaxies, and the ALMA and NOEMA resolution gives clues
to their origin, either radiative of kinetic AGN mode, or starburst.
When driven by AGN with near escape velocity, these outflows are
therefore a clear way to moderate or suppress star formation.
February 10, 2021 Speaker: Silvia Toonen Title: Stellar Interactions and Transients
Institute: University of Amsterdam Abstract: The advent and development of large-scale time domain surveys are revealing the existence of a large and diverse zoo of transients; common transients that can be used as tools to constrain GR or cosmological parameters, and rare & exotic transients that are observed for the first time. The origin of the transients is often unknown, but linked to stellar systems and interactions. In this talk I will show novel channels to induce stellar interactions and subsequent transients - in electromagnetic radiation as well as gravitational waves. Amongst others I will discuss how triple stars can evolve differently from binary stars, and show their potential as transient progenitors. While triple star systems are common, our understanding of their evolution has lagged behind compared to single and binary stars. I will conclude with revealing a new primary channel of binary evolution towards supernova type Ia events.
Host: Jim Fuller
(12:00 PM PST) INSTEAD OF 4:00 PM
February 17, 2021 Speaker: Tim Bedding Title: A Golden Age of Asteroseismology with Kepler and TESS
Institute: University of Sydney Abstract: Asteroseismology uses the natural oscillation modes of a star to study its internal structure. The wonderfully precise measurements by NASA's Kepler and TESS missions are ideal data sources for the technique. These space telescopes have been monitoring the brightness of hundreds of thousands of stars, with the main goal of discovering extra-solar planets as they transit their parent stars. At the same time, observations of stellar oscillations have led to a revolution in asteroseismology. I will discuss some of the key results, including the use of gravity modes to probe the cores of red giant stars, the characterization of stars found to host exoplanets, and the measurement of ages for young stellar associations.
Host: Jim Fuller
February 24, 2021 Speaker: Takahiro Sumi Title: NIR microlensing exoplanet search by PRIME and Roman
Institute: Osaka University Abstract:
Host: Tiffany Meshkat We report the status of the NIR microlensing exoplanet search project,
the Prime-focus Infrared Microlensing Experiment (PRIME).
We are building a new 1.8m wide field infrared telescope at the Sutherland in South Africa.
One of the largest NIR camera will be build by using four H4RG-10 detectors loaned from the Roman project.
Thanks to 1.3 deg.^2 FOV, we can conduct the first high cadence microlensing survey in H-band
towards the central region of the galactic bulge, where high dust extinction prevents optical observations.
Because the stellar density is higher at the lower galactic latitude, we expect higher event rate.
We can compare the planet abundances in high and low stellar density for the first time,
which is important for the study of the planetary formation scenarios.
The event rate map produced by PRIME can be used to optimize the Roman observing fields.
If the PRIME telescope and Roman observe the same fields simultaneously,
different light curves will be observed due to the different line of sights, so-called the space-based
microlensing parallax. This enables us to measure the mass and the distance of the lens system
and enhance the Roman’s yields.The telescope will also be used for the ToO observations for
various transients including GW, GRB and so on.
March 3, 2021 Speaker: Tim Brandt Title: Discovering, Weighing, and Characterizing Exoplanets and Brown Dwarfs
Institute: UC Santa Barbara Abstract: I will present a combination of three observational techniques--astrometry, radial velocity, and imaging--to discover, weigh, and characterize massive exoplanets and brown dwarfs. While thousands of planets are known, only a few have both measured masses (from radial velocity and astrometry) and atmospheric properties (inferred from spectra). Advances in adaptive optics and infrared instrumentation now enable us to see young exoplanets millions of times fainter than their host stars. Despite huge gains in sensitivity, however, high-contrast imaging surveys remain plagued by a lack of discoveries. I have calibrated a huge data set of stellar reflex motions; it can identify unseen planets and brown dwarfs by the gravitational tugs they exert on their host stars, and enable us to measure their masses and orbits. We have already begun to discover and weigh new substellar companions by targeting accelerating stars. With masses, orbits, and spectra of a growing sample of planets and brown dwarfs, we can finally test models of substellar formation and evolution.
Host: Tiffany Meshkat
March 10, 2021 Speaker: Risa Wechsler (Biard Lecture) Title: Dark Matter Clues from the Tiniest Galaxies
Institute: Stanford University Abstract: The Universe is teeming with tiny galaxies, the smallest of which can be up to a hundred million times less massive in stars than our own Milky Way. The few years have seen a revolution in the discovery and characterization of these systems, including the discovery of many dozens of new ultrafaint galaxies in the Milky Way system and the characterization of Milky Way-like systems and their satellites well outside the Local Group. These faint galaxies trace the smallest observationally accessible dark matter halos, and can thus provide clues to the nature of dark matter as well as to the onset of galaxy formation. I will discuss new observational results on satellite galaxies within the Milky Way and similar systems and describe what we have learned about the dark matter halos that host the smallest galaxies. I will show how these measurements help place our own Galaxy in its broader cosmological context, and how they provide essential clues to the microphysical properties of the dark matter that dominates our Universe.
Host: Mansi Kasliwal
March 17, 2021 Speaker: Ruth Murray-Clay Title: Origins of Structure in Inner Planetary Systems
Institute: UC Santa Cruz Abstract: Why do many stars host close-in chains of super-Earths? Why are eccentric gas giants found in some inner planetary systems? What determines which of these outcomes will occur around a particular star? I will present a possible framework for answering these questions that appeals to the “flow isolation mass,” a limiting mass for pebble accretion. Flow isolation occurs when small particles, coupled to the gas, are pulled around a growing planet along gas streamlines. Its consequences are highly dependent on the particle sizes present in the planet’s natal disk. In inner planetary systems, if fragmentation limits “pebble” sizes to Stokes numbers approaching one, then flow isolation yields limiting masses similar in scale to the thermal mass, comparable to the distinct “pebble isolation mass.” At larger orbital separations or if Stokes numbers are smaller, these processes diverge. I will present work showing that flow isolation can yield systems of super-Earths comparable to those observed. I will then discuss how a giant impact phase for giant planets results if multiple gas giants are instead produced in inner planetary systems, yielding a population of gas giants that well matches the data and in particular explaining why higher-mass giants are more likely to have high eccentricities. Finally, I will connect these two ideas to demonstrate that for a reasonable distribution of disk parameters, we can reproduce the relative frequency of super-Earth and gas giant systems.
Host: Tiffany Meshkat
March 31, 2021 Speaker: Anna Fabiola Marino Title: Multiple stellar populations in star clusters
Institute: University of Padova Abstract: The presence of more than one stellar population in globular clusters (GCs) is one of the most fascinating recent discoveries in the field of stellar populations. Yet, the phenomenon remains an enigma. For old Milky Way clusters, the properties of the different populations of stars are constrained by chemical abundances coupled with the nicknamed Chromosome Maps photometric diagram. By using these maps we have identified two main classes of GCs: Type I, including ∼83% of the objects, and Type II clusters. Both classes host two main groups of stars, referred to as first and second population. Different stellar populations in GCs are characterized by distinct chemical abundances in specific elements, whose knowledge keeps fundamental information on the processes occurring in the early stages of life of these ancient stellar systems.
One of the most intriguing results from studies based on chemical abundances is that, contrary to what commonly believed, some GCs, besides Omega Centauri, harbour stellar populations with different metallicity. The fraction of these clusters might be high as suggested by the Type II morphologies of the Chromosome Maps.
I will present the latest results on how to read the Chromosome Maps in terms of chemical abundances. I will focus on the most intriguing features observed on the maps, namely their variety and the features that can be interpreted as proxies of metallicity variations. I will discuss the impact of these results in the context of GC formation and the assembly of the Galaxy.
Host: Lynne Hillenbrand
April 7, 2021 Speaker: Takashi Moriya Title: Revealing massive star mass loss through supernovae
Institute: NAO Japan Abstract: Mass loss is one of the most uncertain factors in massive star evolution. Mass loss strongly affects the final fate of massive stars and proper knowledge of mass loss is crucial in understanding supernovae and their progenitors. Supernova explosions occur within the circumstellar media created by their progenitors’ mass loss. The circumstellar media affect supernova properties, and it is possible to trace the mass-loss properties of massive stars through supernovae. In this talk, I will discuss how the circumstellar media affect the observational properties of supernovae. I will discuss mysterious mass loss of massive stars triggered shortly before their explosion that is revealed by identifying the signatures of the circumstellar media in supernovae.
Host: Ilaria Caiazzo
April 14, 2021 Speaker: Maura McLaughlin Title : The NANOGrav 12.5-Year Data Set: Knocking at the Gravitational Wave Door
Institute: West Virginia University Abstract: Millisecond pulsars are rapidly rotating neutron stars with phenomenal rotational stability. The NANOGrav collaboration monitors an array of about 80 of these cosmic clocks in order to detect perturbations due to gravitational waves at nanohertz frequencies. These gravitational waves will most likely result from an ensemble of supermassive black hole binaries. Their detection and subsequent study will offer unique insights into galaxy growth and evolution over cosmic time. I will present our most recent dataset and the results of our gravitational wave analysis, which suggests the presence of a common spectral signature in the data that could be the first hints of a gravitational wave background. I will then describe the gains in sensitivity that are expected from additional data, discoveries of millisecond pulsars, more sensitive instrumentation, and international collaboration and discuss prospects for detection in the next several years.
Host: Mansi Kasliwal
April 21, 2021 Speaker: Jia Liu Title: Cosmology with Massive Neutrinos
Institute: UC Berkeley Abstract:
Host: Chris Martin Ghostly neutrino particles continue to bring surprises to fundamental physics, from their existence to the phenomenon of neutrino oscillation, which implies their nonzero masses. Their exact masses, among the most curious unknowns beyond the Standard Model of particle physics, can soon be probed by the joint analysis of upcoming cosmological surveys including the Vera Rubin Observatory LSST, Simons Observatory, DESI, SPHEREx, Euclid, CMB-S4, and Roman Space Telescope. In this talk, I will discuss ongoing works studying the effects of massive neutrinos and will draw a roadmap towards discovering the neutrino mass over the next decade.
April 28, 2021 Speaker: Ilaria Caiazzo Title: Small But Mighty: The Tiniest White Dwarf and Other Stories
Institue: Caltech Abstract: The advent of Gaia and of large photometric and spectroscopic surveys is changing the landscape of white dwarf studies. These incredible new data sets, together with improved models, have enabled tackling some unsolved mysteries concerning white dwarfs as a population, as well as discovering extremely peculiar objects that challenge our understanding of white dwarf formation and evolution. In my talk, I will show how the precise astrometry from Gaia has dramatically improved our capability of studying white dwarfs in young star clusters, and therefore probe the evolution of white dwarfs born from single progenitor stars. On the other hand, the Zwicky Transient Facility (ZTF) is shedding light on the evolution of white dwarfs in binary systems, substantially increasing the number of known eclipsing white dwarf binaries and finding the final products of such binaries. In fact, ZTF is discovering a large number of massive, rapidly rotating and highly magnetized white dwarfs whose extreme properties characterize them with high confidence as remnants of white-dwarf mergers. Finding a population of white dwarf merger remnants just below the Chandrasekhar mass can help constrain the number of mergers in the Galaxy and their contribution to the type Ia supernova rate, as well as help us understand the origin of strong magnetic fields in white dwarfs. I will present some early results of the search, including the discovery of ZTF J1901+1458, a moon-sized white dwarf that is extreme in almost every respect.
Host: Jim Fuller
May 5, 2021 Speaker: Aaron Barth Title: Black Hole Mass Measurement with ALMA
Institute: UC Irvine Abstract: Supermassive black holes are found in the centers of all massive galaxies, and the scaling relations between black hole mass and host galaxy properties provide important benchmarks for testing models of black hole-galaxy coevolution. Accurate measurements of black hole masses are essential for determining these scaling relations across the full range of galaxy properties. ALMA was long anticipated to be a powerful tool for measuring masses of black holes in galaxies containing central disks of molecular gas. During the past few years this technique has been used to obtain robust mass measurements for black holes in several early-type galaxies, and in the best cases ALMA observations have led to the highest-precision measurements of black hole mass that have been obtained for giant elliptical galaxies. I will present results from an ALMA program to observe E and S0 galaxies containing circumnuclear gas disks, describe the methods used to determine black hole masses, and discuss prospects for ALMA to improve the understanding of supermassive black hole demographics.
Host: Chris Martin
May 12, 2021 Speaker: Jay Strader Title: Don’t Fear the Spiders: New Insights from Redback Millisecond Pulsars
Institute: Michigan State University Abstract: It is well-established that fast-spinning millisecond pulsars are neutron stars recycled through accretion from binary companions. For most millisecond pulsars the accretion process has permanently ceased, and they are in binary systems with low-mass white dwarf companions. Follow-up observations of newly discovered gamma-ray sources from the Fermi Gamma-Ray Space Telescope have revealed a substantial population of "spider" millisecond pulsars with hydrogen-rich companions; these systems had mostly been missed in radio pulsar surveys. I will discuss the properties of neutron stars in these binaries and constraints on the maximum mass of neutron stars. I will also discuss new candidate members of the rare class of transitional millisecond pulsars, systems that switch between pulsar and accretion disk states on short timescales.
Host: Sterl Phinney
May 19, 2021 Speaker: Katerina Chatziioannou Title: Studying the Universe with Gravitational Waves: from neutron stars to tests of gravity
Institute: Caltech Abstract: Since the first detection of gravitational waves, the field of gravitational wave astrophysics has been progressing at an accelerating pace. With dozens of detected sources, gravitational waves promise to both contribute towards addressing existing questions about our Universe and open new avenues for investigation. In this talk I will give an overview of some of the latest progress in select areas. I will talk about how gravitational waves can and have been used in conjunction with other astronomical and terrestrial probes to study the properties on neutron stars, the most dense material compact object we know of. I will further discuss some novel explorations facilitated by gravitational waves, in particularly with regards to the properties of General Relativity.
Host: Mansi Kasliwal
May 26, 2021 Speaker: Andrea Ghez (Neugebauer Lecture) Title: Our Galactic Center: A Unique Laboratory for the Physics & Astrophysics of Black Holes
Institute: UCLA Abstract: The proximity of our Galaxy's center presents a unique opportunity to study a galactic nucleus with orders of magnitude higher spatial resolution than can be brought to bear on any other galaxy. After more than a decade of diffraction-limited imaging on large ground-based telescopes, the case for a supermassive black hole at the Galactic center has gone from a possibility to a certainty, thanks to measurements of individual stellar orbits. The rapidity with which these stars move on small-scale orbits indicates a source of tremendous gravity and provides the best evidence that supermassive black holes, which confront and challenge our knowledge of fundamental physics, do exist in the Universe. This work was made possible through the use of speckle imaging techniques, which corrects for the blurring effects of the earth's atmosphere in post-processing and allowed the first diffraction-limited images to be produced with these large ground-based telescopes.

Further progress in high-angular resolution imaging techniques on large, ground- based telescopes has resulted in the more sophisticated technology of adaptive optics, which corrects for these effects in real time. This has increased the power of imaging by an order of magnitude and permitted spectroscopic study at high resolution on these telescopes for the first time. With adaptive optics, high resolution studies of the Galactic center have shown that what happens near a supermassive back hole is quite different than what theoretical models have predicted, which changes many of our notions on how galaxies form and evolve over time. By continuing to push on the cutting-edge of high-resolution technology, we have been able to capture the orbital motions of stars with sufficient precision to test Einstein’s General theory of Relativity in a regime that has never been probed before.
Host: Mansi Kasliwal & Tom Soifer
June 2, 2021 Speaker: Anna Rosen Title: The Destructive Birth of Massive Stars and Star Clusters
Institute: CfA Harvard Abstract: Massive stars play an essential role in the Universe. They are rare, yet the energy and momentum they inject into the interstellar medium (ISM) with their intense radiation fields and fast, isotropic radiatively driven winds dwarfs the contribution by their vastly more numerous low-mass cousins. This stellar feedback dominates the energy and momentum budget in star-forming regions and galaxies leading to gas ejection, which has important implications for star and galaxy formation. Massive stars form from the gravitational collapse of magnetized, dense, and turbulent molecular gas in massive pre-stellar cores, which are located in highly embedded environments. During their formation, feedback from their intense radiation fields, collimated protostellar outflows, and stellar winds can limit their growth by accretion. In this talk, I will show a series of radiation-magnetohydrodynamic (RMHD) simulations of the collapse of massive pre-stellar cores into massive stellar systems that include these feedback processes to demonstrate how stellar feedback can limit accretion onto massive stars and disrupt their natal environments. In addition, I will also discuss how stellar feedback from massive stars, which are born in clustered environments, affects the dynamics of HII regions that surround these clusters and can drive turbulence in young, star clusters and molecular clouds.
Host: Mansi Kasliwal
June 9, 2021 Speaker: Rashid Sunyaev Title: SRG Orbital Observatory X-Ray Map of the Universe
Institute: Max Planck Institute for Astrophyics Abstract: The SRG spacecraft with German (eRosita) and Russian (ART-XC) X-Ray telescopes was launched by RosKosmos on July 13, 2019 from Baikonur. During the flight to the L2 point of the Sun-Earth system, SRG performed calibrations and long duration Performance Verification (PV) observations of a dozen of targets and deep fields. Starting in the middle of December 2019, the SRG scanned the whole sky twice and finishes soon the third scan. 

During these scans, SRG discovered more than three million point X-Ray sources, mainly AGNs and QSOs, stars with hot and bright coronae, and 30 thousand clusters of galaxies. There is a competition and synergy with the search for clusters of galaxies by Atacama Cosmology and the South Pole Telescopes sensitive in the microwave spectral band.We see X-Rays from hundreds of stars accompanied by exoplanets.

SRG provided the X-Ray map of the whole sky in hard and soft bands, the last is now the best among existing. It reveals a lot of information about the distribution of absorbing cold gas in the Milky Way and provides a beautiful image of the North Polar Spur and similar bright emitting eRosita Bubble on the Southern side from the Central Part of the Galaxy. 

I plan to describe the Observatory plans for the future and to demonstrate several impressive results from the PV phase observations as well as from the second and third all-sky survey which is ongoing. The huge samples of the X-ray selected quasars at the redshifts up to z=6.2 and clusters of galaxies will be used for well-known cosmological tests and for detailed study of the growth of the large scale structure of the Universe during and after reionization.   

During the all-sky survey, SRG/eRosita is discovering every day several extragalactic objects which increased or decreased their brightness more than 10 times during half of the year after the previous scan of the same strip on the sky. A significant part of these objects has observational properties similar to the Tidal Disruption Events.  ART-XC discovered a lot of bright galactic and extragalactic transients.
Host: Mansi Kasliwal