Thesis Defenses – Week Of April 24th 2023

Monday April 24, 2023 10:00 am

Calvin Leung – April 24th @ 10:00amMarlar Lounge, 37-252/37-272
Committee: Kiyoshi Masui (Chair), Jackie Hewitt, David Kaiser

Sylvia BiscoveanuApril 25th @ 10:00am, Marlar Lounge, 37-252/37-272
Committee: Salvo Vitale (LIGO Chair)

Nicholas MehrleApril 25th @ 9:00am, #54-915
Committee: Sara Seager (Chair), Erin Kara, Andrew Vanderburg

Kaley BrauerApril 26th @ 3:00pm, Marlar Lounge, 37-252/37-272
Committee: Anna Frebel (Chair), Mark Vogelsberger, Michael McDonald

David BerardoApril 28th @ 10:00am, Marlar Lounge, 37-252/37-272
Committee: Julien de Wit (Chair), Michael McDonald, Andrew Vanderburg, Raymond Ashoori

Calvin Leung presents: Localization and Lensing of Fast Radio Bursts using CHIME/FRB and its VLBI Outriggers

Every two minutes, a luminous, millisecond-duration flash of radio light arrives at Earth from outside the Milky Way. These fast radio bursts (FRBs) are elusive: they last just a millisecond, and the vast majority are never detected again. FRBs have drawn significant interest because of their potential as a probe of black holes and cosmic structure, as well as their connection to magnetars: a rare class of neutron stars which produce the strongest magnetic fields in the Universe. Observationally, however, because FRBs are so fleeting, the field is grappling with much simpler questions: How do magnetars emit FRBs? From what galaxies (and redshifts) do FRBs originate? Pinpointing FRBs to their host galaxies using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) is perhaps the single most promising path towards uncovering the mystery of FRBs. CHIME’s wide field of view allows FRBs to be detected at an unprecedented rate of about 1000 per year: 10× more than other telescopes combined. However, CHIME lacks the resolution to pinpoint its bursts. Very-long baseline interferometry (VLBI) is a solution which uses widely-separated telescopes to achieve high angular resolution, but this technique traditionally has a narrow field of view, which until now has limited it to following up the small fraction of sources which repeat. In this thesis, I develop key technologies to overcoming a major obstacle in the field: combining wide-field observations for FRB detection with high angular resolution for localization in one instrument. As a demonstration, I successfully pinpoint a one-off FRB with sub-arcsecond precision at the time of detection, setting the stage for CHIME Outriggers: three dedicated telescopes which will enhance CHIME’s angular resolution to sub-arcsecond scales over CHIME’s entire field of view. In addition, I develop a new way to use FRBs as probes of primordial black hole dark matter on sub-solar mass scales. By exploiting multi-path interference in gravitational lensing, I develop a novel method to search for interferometrically-lensed FRBs. We find that some FRBs exhibit plasma lensing (scintillation), which we attribute to the Milky Way’s interstellar medium, and use our null search to place new constraints on extragalactic primordial black holes as dark matter.

via Zoom

Sylvia Biscoveanu presents: From black holes to the Big Bang: astrophysics and cosmology with gravitational waves and their electromagnetic counterparts

The growing catalog of gravitational-wave signals from compact object mergers has allowed us to study the properties of black holes and neutron stars more precisely than ever before and has opened a new window through which to probe the earliest moments in our universe’s history. Population-level measurements of the masses and spins of compact objects can reveal how these systems form and evolve. Multi-messenger observations of compact object mergers can shed light on the properties of the electromagnetic counterparts of these systems, such as short gamma-ray bursts and kilonovae. Finally, observations of the stochastic gravitational-wave background can constrain early-universe physics inaccessible with other means. In this talk, I will present highlights from my thesis work demonstrating how we can leverage such observations of gravitational waves and their electromagnetic counterparts to learn about astrophysics and cosmology.

via Zoom

Nicholas Mehrle presents: Resolving the Mysteries of Highly Irradiated Planets

Modern exoplanet science has an observational bias towards short-period planets. Among other things, these planets tend to be highly irradiated, either thermally resulting in high equilibrium temperatures and/or through high energy FUV/Xray radiation. The resulting planets exhibit a diverse array of physical characteristics unlike those seen on Earth. I present a collection of works broadly encompassed by the theme of understanding highly irradiated planets and a set of new techniques I develop to further analysis of these strange worlds. First I discuss observations of Upsilon Andromedae b, a non-transiting planet I have observed the atmosphere of for the first time, and Venus, Earths twin sister that turned out so different. Each of these observations is enabled by a new method I introduce for that class of analyses. I then present my work on radiation-hydrodynamics simulations of atmospheres subject to intense high energy radiation, for which I have developed a new simulation code with a unique purpose.

Kaley Brauer presents: The Legacy of the First Galaxies: Exploring Ancient Stars in the Milky Way

In the first several hundred million years after the Big Bang, the first stars and galaxies transformed the Universe. These ancient systems launched the creation of every galaxy we see today, including the Milky Way. Ever since, for the last 13 billion years, the Milky Way has grown through galaxy mergers. Several of these mergers were with other similarly-sized galaxies and possibly a hundred of these mergers were with small dwarf galaxies. The smallest dwarf galaxies accreted by the Milky Way, the ultra-faint dwarfs (UFDs), are relics of the first galaxies in the Universe and provide important insight into early galaxy formation and chemical enrichment. Currently, though, accreted UFDs are poorly understood and we lack ways to identify stars that accreted from UFDs. By utilizing a simulation suite of 35 Milky Way-mass galaxies forming, I find that chemical tagging with r-process elements and clustering in kinematic phase space can help us identify stars that accreted together from these dwarf galaxies. Kinematic clustering only identifies recently accreted UFDs (z<0.5), so we recommend chemical tagging as the more robust method to identify these stars. I also present an analytic model of collapsar enrichment that can self-consistently explain the observed scatter in r-process chemical elements of old stars. I am expanding on these studies with highly-resolved hydrodynamic simulations of the earliest dwarf galaxies, the Aeos simulations. The methodology and initial results for these simulations are also presented.

David Berardo presents: Exoplanetary Systems in Technicolor

As the number of confirmed extra-solar planets surpasses 5000, our ability to characterize exoplanets has equally advanced in the era of high precision photometry afforded by instruments such as the James Webb Space Telescope. As the quantity and quality of observations improves however, we must ensure that the techniques used to analyze extra-solar planets remain robust and accurate, such that we can extract that maximal amount of information from these rich data sets. I first demonstrate the opportunities afforded by the bright, multi-planet system HIP41378 with an observing baseline of over four years across multiple instruments, allowing us to study inter-planetary interactions, including the potential signs of a previously unknown sixth planet. Following this I will present work I have done towards understanding the observational biases that may arise if planets are not considered to be fully 3-dimensional objects, in particular if a tidally or rotationally distorted planet is considered to be perfectly spherical. Finally, I turn my attention to the stars themselves around which planets orbit. Given that any observation of an exoplanet system is typically dominated by the host star, non-homogeneities in the stellar surface have been found to be potential bottlenecks when analyzing high-precision data acquired by space telescopes, and I discuss ways of mitigating these bottlenecks in order to extract photon-noise limited planetary emission spectra.

 (pwd: space2023)  via Zoom

Best of luck to Calvin, Sylvia, Nicholas, Kaley, & David!


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