List of Past Astronomy Colloquia : 01-Jan-2021 to 01-Jun-2021

Date:   Wednesday 10-Feb-2021
Speaker:   Dr. Erez Michaely (UMD)
Title:  Collisional dynamics in the field:Ultra-wide BH systems as sources of GW signals

Dynamical interactions are known to play an important role in the evolution of dense stellar systems such as globular clusters and galactic nuclei. In contrast, the galactic field (where most of the stellar systems are) is a low-density environment, where strong stellar encounters were thought to be too rare to play any role in collisional dynamics. However, the low density in the field also allows for the existence of long-lived ultra-wide systems which would have been quickly destroyed in a dense environment. The large separation of such binaries can therefore compensate for the low densities and still allow for relatively high rate of interactions with field stars. Such interactions change the orbital properties of the binaries or outer binaries in case of triples. Therefore, when considering the evolution of ultra-wide systems one needs to account for random gravitational interaction with passing stars, flybys, even in low density environments like the field of the host galaxy. The outcome of the interactions between ultra-wide black-holes (BH) systems with random field stars may excite the systems eccentricity sufficiently to radiate gravitational waves (GW) and drive the system to a merger within Hubble time. This channel may explain a fraction of the current observed binary BH merger rate and provides predictions to future observational runs, e.g. eccentric mergers. The underlying dynamics of this channel is not unique to BH or GW sources, and have implications to other stellar exotica such: low-mass X-ray binaries, Type Ia SNe, main sequence - white dwarf collisions and more.

Date:   Wednesday 17-Feb-2021
Speaker:   Dr. Bing Zhang (UNLV)
Title:  The Physical Mechanisms of Fast Radio Bursts

Fast radio bursts (FRBs) are cosmological millisecond-duration bursts in the radio band. Recent observations of the Galactic FRB 200428 suggest that magnetars can produce FRBs. In this talk, I will first review the observational facts of FRBs and then turn to discuss their physical origins in reference of two related astrophysical phenomena, radio pulsars and gamma-ray bursts. I will discuss observational evidence and theoretical modeling of of pulsar-like radiation mechanisms for FRBs, as well as a possible type of “slow radio bursts” from Galactic magnetars which may arise from FRBs viewed off-beam. Several open questions in the field regarding repeaters vs. non-repeaters, as well as whether there are engines other than magnetars will be discussed in the end.

Date:   Wednesday 24-Feb-2021
Speaker:   Dr. Kathyrn Kreckel (Heidelberg University)
Title:  "Mapping the ionized ISM in nearby galaxies"

The ionized interstellar medium (ISM) provides crucial insights into understanding baryon cycling within disk galaxies and tracing radiative and mechanical feedback from young massive stars. With new VLT/MUSE optical integral field spectroscopy, the PHANGS team now has a wealth of emission line maps that trace different ionization sources and physical conditions across nearby disk galaxies at the 50pc spatial scales needed to isolate individual ionized regions (e.g. HII regions, supernova remnants, planetary nebulae) from surrounding diffuse ionized gas. I will present our most recent results connecting the molecular gas with observed sites of massive star formation, and measuring the gas phase oxygen abundances across thousands of HII regions. Within the context of the large scale galactic environment, these studies have implications for our understanding of how spiral structure acts to organize and mix the ISM, and regulate star formation.

Date:   Wednesday 03-Mar-2021
Speaker:   Dr. Kevin Walsh (SWRI Boulder)
Title:  Solar System History via Near-Earth Asteroids (...and NASA's OSIRIS-REx Space Mission)

We report on the current status and scientific results of NASA's OSIRIS-REx sample return mission that is visiting the B-type near-Earth asteroid (101955) Bennu. The spacecraft launched in September 2016, arrived at Bennu in December 2018, surveyed and studied the asteroid until SUCCESSFULLY COLLECTING A SAMPLE in 2020 and will return it to Earth in 2023. What will be learned from studying and sampling one near-Earth Asteroid?

Near-Earth Asteroids (NEAs) are transients that have escaped the Main Asteroid Belt and spend a paltry 10 Myr on planet-crossing orbits before hitting the Sun, a planet or getting ejected from the Solar System. All of the various taxonomic types of asteroids are represented amongst NEAs, but due to their chaotic orbits it is not possible to precisely retrace their history and determine where in the Main Asteroid Belt they came from. Furthermore, km-sized NEAs are unlikely to have survived Solar System history intact and are expected to be reaccumulated remnants from a larger disrupted asteroid - they are often referred to as "rubble piles". In sum, any given small NEA comes from an unknown place and has an unknown parent asteroid and history. However, with Bennu, ultimately, returned samples from Bennu should clarify its history and evolution, and in the meantime its geology can reveal much of its history and shed light on its history in the Main Asteroid Belt. Here, we place Bennu in the context of the Solar System and then place the expectations of the sample in the context of the observed geology of Bennu.

Date:   Wednesday 10-Mar-2021
Speaker:   Dr. Hagai Perets (Technion)
Title:  The origins of type Ia supernovae

ype Ia supernovae (SNe) are thought to originate from the thermonuclear explosions of carbon-oxygen (CO) white dwarf (WD) stars. They produce most of the iron-peak elements in the universe, and bright type Ia SNe serve as important "standard candle" cosmological distance indicators. The proposed progenitors of standard type Ia SNe have been studied for decades, and can be, generally, divided into explosions of CO WDs accreting material from stellar non-degenerate companions (single-degenerate; SD models), and those arising from the explosive interaction of two CO WDs (double-degenerate; DD models). However, current models for the progenitors of such SNe fail to reproduce the diverse properties of the observed explosions, nor do they explain the inferred rates and the characteristics of the observed populations of type Ia SNe and their expected progenitors. In the talk, I'll discuss new results from our studies on the little-explored mergers of CO-WDs with hybrid Helium-CO (He-CO) WDs. We find that such He-enriched mergers give rise to double detonations, first catalyzed by Helium and the second in the carbon-oxygen core of the CO-WD. We find that the observable properties of the explosions produced from such mergers resemble those of observed type Ia SNe, and in particular, they can produce a wide range of peak-luminosities, consistent with those observed for normal type Ia SNe. Moreover, our population synthesis models show that, together with the contribution from mergers of massive double CO-WDs (producing the more luminous SNe), they can potentially reproduce the full range of type Ia SNe, their rate, and delay-time distribution. Furthermore, mergers with low-mass CO WDs give rise to a wide variety of partial explosions that might reproduce the properties of subluminous types of type Ia SNe. We, therefore, suggest the mergers of hybrid WDs could play a key role in explaining the origins of both normal and peculiar type Ia supernovae.

Date:   Wednesday 17-Mar-2021
Speaker:   ** Spring Break - No Colloquium **
Title:  None

Date:   Wednesday 24-Mar-2021
Speaker:   Dr. Meredith Hughes (Wesleyan University)
Title:  Planet Formation through Radio Eyes

Circumstellar disks provide the raw material and initial conditions for planet formation. Millimeter-wavelength interferometry is a powerful tool for studying gas and dust in planet-forming regions, and has recently undergone an immense leap in sophistication with the advent of the ALMA interferometer. I will discuss some ways in which millimeter-wavelength interferometry is being used to study the process of planet formation in circumstellar disks, with particular emphasis on the composition of the enigmatic gas found in some debris disk systems, and the degree to which debris disk structure reflects the dynamics of embedded planetary systems.

Date:   Wednesday 31-Mar-2021
Speaker:   Dr. Lisa Randall(Harvard University)
Title:  What (Else) Can We Learn About Black Hole Binaries?

Now that we are in a new era of gravitational wave detection it is worth asking how far we can take these measurements in searching for new physics. But to do so we also want to better understand the signals we do see. One outstanding question is the origin of binary black holes. We show how measurements at current and future gravitational wave detectors can shed light on the environment in which the black holes were created. We also argue for other interesting measures of the black hole environment. We also summarize recent work on an efficient signal to noise approximation for eccentric black holes.

Date:   Wednesday 07-Apr-2021
Speaker:   Dr. Andrew Baker (Rutgers)
Title:  The LADUMA Deep HI Survey

To understand how galaxies evolve across cosmic time, we must understand not only their dark matter and their stars, but also the properties of their interstellar gas, from which new stars form and into which old stars release the products of their nucleosynthesis. Neutral atomic hydrogen (HI), which is best traced at radio wavelengths using the 21cm spin-flip transition, represents a significant fraction of normal galaxies' gas reservoirs. I will describe what we can learn from HI observations of galaxies about their evolutionary states, and how this work will soon be extended to a redshift of 1.4 (i.e., a lookback time of 9 Gyr) in the context of the LADUMA (Looking At the Distant Universe with the MeerKAT Array) deep HI survey. LADUMA will use South Africa's "MeerKAT" SKA precursor to observe a single position on the sky for over three thousand hours, complementing other MeerKAT surveys that will probe HI across a set of wider/shallower fields, in individual nearby galaxies, and in absorption against bright background quasars. I will also discuss prospects for using detections of OH megamasers in the LADUMA field to study the cosmic history of gas-rich galaxy mergers.

Date:   Wednesday 14-Apr-2021
Speaker:   Dr. Christine Clark (UNLV)
Title:  : Dancing with Pink Elephants: Facing and Taking Action to Dismantle Barriers to Equity, Diversity, Inclusion, and Justice Work at the Departmental Level

This department colloquium will examine barriers to moving EDIJ work forward in higher education at the academic department level. While conversations about EDIJ in higher education have been taking place in various manners for many decades, many academic departments in higher education are still comprised of majority white male faculty. While gains have been made in student bodies, those gains follow predictable limited patterns—white women and international Students of Color are doing well, while U.S. Students of Color from historically and persistently marginalized and minoritized groups continue to be underrepresented. What factors protect these persistent of status quo demographic patterns? What can department faculty, staff, and students do to dismantle these factors? What are the educational benefits of EDIJ to all students, to academic departments, to industry, and to national laboratories in the Astronomy arena?

Date:   Wednesday 21-Apr-2021
Speaker:   Dr. Justin Spilker (University of Texas)
Title:  How to Regulate Galaxy Growth Through Cosmic History

One of the most important realizations of the last fifteen years is the vital role that galactic feedback must play in the evolution of galaxies. An umbrella term for a wide array of physical processes that already began to affect galaxies in the first billion years after the Big Bang, feedback allows galaxies to self-regulate their growth, links the buildup of stellar mass and black holes, and connects galaxies to the dark matter haloes they live in. Because of the vast range of physical and temporal scales involved and a lack of direct observational tracers, studying feedback in action has remained difficult. I will discuss a few of my ongoing efforts to understand the causes and effects of galactic feedback, focusing on populations of galaxies that show the effects of feedback most strikingly. Key results include the first-ever sample of objects with detected galaxy-scale cold gas outflows at z = 4 - 7, and a qualitatively new way to quench star formation in distant objects from observations of lower-redshift post-starburst galaxies. While a complete statistical understanding of feedback and galactic outflows will require the powerful observatories of the next decade, I will argue that significant progress can be made now by carefully picking and choosing galaxies where the effects of feedback are most prominent.

Date:   Wednesday 28-Apr-2021
Speaker:   Dr. Eve Lee (McGill University)
Title:  Towards the origin of exoplanet diversity: from super-Earths to Jupiters

The discovery of thousands of exoplanets revealed a huge variety in the sizes, masses, and orbital characteristics of planets outside of our solar system. How can we understand the origin of such diversity? I will discuss how the theories of gas accretion and disk processes can be combined with the measured distributions of radii and orbital periods to reveal the formation conditions of exoplanets, from close-in super-Earths to far-out Jupiters.

Date:   Wednesday 05-May-2021
Speaker:   Dr. Andrew Winter (Heidelberg University)
Title:  Links between star and planet formation: Disentangling the known exoplanet population

The exotic range of known planetary systems has provoked an equally exotic range of physical explanations for their diverse architectures. However, numerous empirical and theoretical studies are converging on the idea that the observed planet population is not a directly inherited from formation processes in a protoplanetary disc of dust and gas. Both internal and external mechanisms can perturb a planetary system from its 'natural' formation configuration. In this talk, I first review evidence for the role of environment in the formation and evolution of planetary systems. I then discuss how stellar host kinematics link to planetary system configurations. Finally, I show that factoring in this inhomogeneity in the origins of diverse planetary systems may help to unravel the problems in explaining their formation. In light of the possibility that a considerable fraction of planetary systems may be (internally or externally) perturbed, I conclude that simple models for their initial formation are worth revisiting.

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