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Astrophysics Seminars


The CASS Astrophysics Seminar features world-class astrophysicists from around the world speaking on current topics of research. Presentations are aimed at the graduate and post-graduate level, but are open to the general public. CASS seminars take place on Wednesdays from 3:00 - 4:00 p.m. in 383 SERF (Marlar Seminar Room), unless otherwise noted. You can watch a live stream of the talk or prior talks at the CASS Seminar YouTube Channel. The organizers are Prof. Quinn Konopacky and Dr. Alexei Kritsuk.

Upcoming Seminars

Spring 2019

April 17, 2019

 "The Hidden Baryons of the Milky Way"

Yong Zheng
Miller Postdoctoral Fellow
UC Berkeley

 Galaxies are not lonely islands floating in the Universe. They host large gaseous envelopes of baryons, a.k.a., the circum-galactic medium (CGM), that exist far beyond a galaxy’s visible extent. Baryonic inflows from CGM replenish star-forming fuel in galaxies, whereas outflows from galaxies enrich the CGM. In this talk, I will describe the theoretical and observed distribution and flows of baryons in the MW’s CGM, including how the MW’s disk hides up to half of its CGM from direct observation. I will then describe new techniques to generate synthetic observations of the CGM using the Enzo & FOGGIE cosmological simulations, and show how these can be used to reveal the hidden baryons in the MW’s CGM. Finally, I will briefly highlight the connections between low- and high-redshift CGM studies, including new applications that rely on fast radio bursts (FRBs).

April 24, 2019

 "Gravitational wave signal from primordial hydro-magnetic turbulence"

Tina Kahniashvili
Associate Research Professor
Carnegie Mellon University

 The first direct detections of gravitational waves from the mergers of binary black holes and binary neutron stars by the LIGO and VIRGO experiments have electrified the physics and astronomy communities. A clear next experimental step is an interferometer in space, which can detect lower frequency signals than a ground-based detector, including supermassive black hole binary coalescences from early galaxy mergers and a known stochastic background from confusion-limited white-dwarf binaries. An even more intriguing signal is the stochastic background from early-universe physics. In this talk I will present our recent work (in collaboration with Axel Brandenburg, Arthur Kosowsky, Sayan Mandal, and Alberto Roper Pol). Using direct numerical simulations of early universe hydromagnetic turbulence with energy densities of up to 10% of the radiation energy density, we show that gravitational waves with energy densities of about 10-10 times the critical energy density of the Friedmann universe today were produced. Their characteristic strain today is found to be about 10-20 and should be observable with the Laser Interferometer Space Antenna (LISA) in the mHz range. The gravitational waves have positive (negative) circular polarization if the magnetic field has positive (negative) magnetic helicity. The gravitational wave energy reaches a constant value after the turbulent energy (kinetic or magnetic) has reached its maximum. Compressive modes are found to produce about 10 times stronger gravitational waves than solenoidal ones. Finally, I will discuss the range of phase transition energy scales and properties that may be detectable with the envisioned space-based interferometer configurations such as LISA.

May 1, 2019

Jeff McMahon
Associate Professor
University of Michigan

May 8, 2019

 "Multi-object spectrographs: simulation tools for design and trade studies"

Arun Surya
Postdoctoral Scholar

 Several wide-field multi-object spectrographs are proposed for the upcoming extremely large telescopes to provide optical and near-infrared spectroscopy of large samples of astronomical objects. The size of these instruments scale with the telescope aperture size creating unique problems for optics and mechanical design. Mitigating some of these design challenges involves the development of new simulation tools that can interface with different standard optical and mechanical design software to simulate realistic observations and instrument behavior. I will discuss the development of two such tools, the flexure compensation simulation tool and the target allocation tool, developed during the conceptual design phase of TMT-WFOS spectrograph. WFOS is an optical wide field multi-object spectrograph planned for the first light of Thirty Meter Telescope. Flexure Compensation Simulation (FCS) tool provides an interface to accurately simulate the effects of instrument flexure at the WFOS detector plane using perturbation of key optical elements and also derive corrective motions to compensate for such effects. The target allocation tool was developed for the fiber-based design of WFOS to simulate the allocation of fiber units on objects at the focal plane for different target densities and science cases. I will discuss how these tools help in addressing specific challenges in the design of these instruments.

May 15, 2019

 "The Origin of Cosmic Rays: How their Composition Defines their Sources, & Sites, Mixing, Injection & Acceleration"

Richard Lingenfelter
Research Physicist Emeritus

 Galactic cosmic-ray source abundance ratios (Z/H)GCRS, measured from H to Pb and ~108 to 1014 eV, differ greatly from solar system (Z/H)SS by factors of ~20-200. Yet these two compositions are drawn from essentially the same core collapse (CCSN) and thermonuclear (SN Ia) supernova ejecta: (Z/H)SS from unbiased accumulation over ~Gyr, and (Z/H)GCRS from highly biased sampling during the brief period <30 kyr of homologous early Sedov-Taylor supernova expansion that diffusive shock acceleration (DSA) is most effective. These differences reveal how (Z/H)GCRS can result from just two self-consistent processes: ubiquitous mass mixing (Z/H)SS/(Z/H)CCSN ~4 of shocked, swept-up interstellar medium with high metallicity, core collapse supernova ejecta forming a base; and selective grain injection of elements first condensed FGC as fast grains in freely expanding ejecta, or later implanted in them from ambient gas, then finally Coulomb-sputtered FCS by H and He as suprathermal ions into supernova shocks, where DSA carries them to cosmic-ray energies. This bulk mixing selectively increases source mix abundances (Z/H)SM /(Z/H)SS by ~2-10; and injection by grain condensation and implantation fractions FGC, from meteoritic chondrules, further enhances by ~6, while elemental-charge Z2/3-Coulomb grain sputtering yields, FCS give an added enrichment of ~4-20. Applying these basic processes of mixing and injection to solar system (chondrule) abundances (Z/H)SS produces grain-injected, source-mix (Z/H)SMGI that match major cosmic-ray abundances (Z/H)GCRS to 1±35% with no free parameters. Independently confirming grain injection, (Z/H)GCRS also showsno detectable contribution of Fe from SN Ia, which although they produce ~1/2 Fe in ISM, is quite consistent with there being no dust in SN Ia remnants, while CCSN are a major source.

Lingenfelter, R. E. 2013, Discovery of Cosmic Rays, AIP Conf Proc 1516, 162'
Lingenfelter, R. E. 2018, AdSpR, 62, 2750. arXiv:1807.09726
Lingenfelter, R. E. 2019, arXiv:1903.06330

May 22, 2019

Chao-Wei Tsai
NAOC, Beijing

May 29, 2019

 "Solar Eclipses, Solar Transits by Venus and Mercury, and Pluto/KBO Occultations"

Jay Pasachoff
Professor of Astronomy
Williams College

 On August 31, 2017, a total solar eclipse's band of totality swept across the Continental United States from coast to coast for the first time in 99 years. I will show and discuss some of the the images and spectra my team has obtained at the most recent eclipses, including total eclipses in Easter Island (2010), Australia (2012), Gabon (2013), Svalbard (2015), Indonesia (2016), and the United States (2017) as well as comment on annular or partial eclipses observed elsewhere. I will discuss our observational tests underway for the comparison of models of coronal heating. I will also discuss plans for the 2019 and 2020 total eclipses that cross Chile and Argentina.

I will also report on our observations of transits not only of the Sun by the Moon (that is, a solar eclipse), but also across the Sun by Venus and by Mercury. I will discuss ground-based imaging and Total Solar Irradiance space measurements as well as observations of the 2012 transit of Venus with Hubble by reflection off Jupiter and directly with Cassini from Saturn, providing solar-system close-up analogues to exoplanet transits.

I will close with some discussion of our stellar-occultation observations by Pluto and other objects in the outer solar system, and their relation to the recent New New Horizons’ flyby of Ultima Thule, a billion miles beyond Pluto.

My work at solar eclipses has recently been mainly supported by the US National Science Foundation’s Atmospheric and Geospace Sciences Division, and the Committee for Research and Exploration of the National Geographic Society. The solar-system occultation work has been supported by NASA.

June 5, 2019

 "Reaching Beyond our Spectral Grasp of Star and Planet Formation"

Gregory Mace
Research Associate
University of Texas, Austin

 New discoveries generally come from low-resolution observations with weak signals. We are then tasked with significantly increasing the signal and resolution in order to characterize these new objects. Some of the most challenging objects to characterize are young stellar objects in the dusty environments of formation and faint exoplanets close to their host stars. High-resolution infrared spectroscopy allows us to look through the obscuring dust and separate exoplanet and host star spectra. The silicon immersion gratings developed at UT Austin maintain high throughput, while providing a broad spectral grasp, with a fraction of the instrument volume of traditional reflective gratings. I will discuss the application of immersion grating spectrographs (IGRINS, iSHELL, MagNIFIES, GMTNIRS) and the order of magnitude improvement in simultaneous wavelength coverage, throughput, and/or resolution that they enable. Specifically, I will discuss their role in confirming and then characterizing exoplanets, while also tracing the star formation process down to planetary masses. Since these immersion grating instruments are new, or still in development, there is considerable return on efforts to make early use of them to answer a variety of science questions. Additionally, future instrument collaborations for ground, airborne, and space facilities in the 1-10 micron region have the potential to reveal new characteristics at a variety of spectral resolutions.

June 12, 2019

Melville Ulmer
Northwestern University