Seminar Schedule

Astrophysics Seminar meets in room 309 VAN on Tuesdays from 12:30 am - 1:30 pm. 

The format is either Astrophysics Journal club or a research presentation.  Journal club papers for the current week are listed  at VoxCharta (select University of Iowa). Everyone is encouraged to briefly review the selected papers before seminar. Moderators are expected to provide cookies.

Date Speaker/Moderator Title
Aug 23 Prof. Kazumasa Imai, Kochi National College, Japan Modulation lanes in Jupiter's Decametric Radio Spectrum
Aug 29 Prof. Ken Gayley Entanglement and the demise of local realism
Sep 5 Dr. DooSoo Yoon, Shanghai Astronomical Observatory Interplay between ejecta from black holes and their surrounding media
Sep 12 Dr. Rui Xue The Diversity ofhe Diffuse Lyman alpha Nebulae around Star-forming galaxies at high redshift
Sep 19 Prof. Jack Scudder A Post-Spitzer Heat Law for Astrophysical Plasmas
Sep 26 Ryan Urquhart, Curtin University, Australia Discovery of the first eclipsing ULX
Oct 3 Dr. Yuki Harada TBA
Friday Oct 13 11am Dr. Robert Jedicke, IfA, Univ. Hawaii Super catastrophic disruption of asteroids at small perihelion distances
Thursday Oct 19 Dr. Mark Gorski, Univ. New Mexico Molecular Tracers of Star Formation Feedback in Nearby Galaxies
Oct 24 Prof. Shea Brown Astrophysical Machine Learning
Oct 31 Gian Andeone, Dan McGinnis TBA
Nov 7 Prof. Robert Mutel Thermal gyro-synchrotron emission as a probe of dense hot magnetized plasma
Nov 14 Prof. Jasper Halekis, UI The Mars Hydrogen Cycle: Precipitation, Escape, and Aurora
Nov 21 Tom Turkey Thanksgiving Break
Nov 28 Dr. Joran Moen Introduction to the physics of the Grand Challenge Initiative - Cusp Rocket Program
Dec 5 Allie Costa, Stephanie Howard TBA


Abstracts

 K. Gayley: Entrangelment and the demise of local realism

Abstract: Most physical theories take a bottom-up perspective, wherein we drill down to the fundamental building blocks of some system we are studying, and attempt to reconstruct the whole as a sum of its parts and their interactions. At a philosophical level, the approach to reality as a sum of parts is called local realism, whereby everything that is real is physically transported from place to place and time to time by particles and fields that involve, at the simplest level, definite locations or values at every location, or at a more sophisticated level, definite amplitudes at every local event with which they could possibly interact. This view of reality is still normally accepted without much critical thought, even today, despite a host of significant refutations over the past century. This seminar will survey a wide array of leaks that spout from local realism in the physics we all learn, culminating in, but not restricted to, the body blow delivered to it by entanglement and Bell's theorem.

 Rui Xue: The Diversity of Diffuse Lyα Nebulae around Star-forming Galaxies at High Redshift

Abstract: We report diverse sizes of Lyman Alpha Halos (LAHs) measured for normal star-forming galaxies at high redshift. Based on ~1400 galaxies residing in field and dense protocluster environments, we investigate how the LAH characteristics vary with host galaxy large-scale environment and physical properties such as UV luminosity, Lyα luminosity and equivalent widths. The average LAH scalelength, measured for galaxies selected as Lyman Alpha Emitters, is modest at 5-6 kpc (physical); the most UV-luminous galaxies have more extended halos (8-9 kpc). Our analysis largely rules out the presence of very large LAHs around UV-luminous protocluster member galaxies as previously reported based on a similarly significant high-redshift protocluster system. Based on the observed trends of LAH sizes with galaxy parameters, combined with the measured average radial profile of our sample galaxies, we speculate that i) LAHs are primarily powered by central star formation: namely, Lyα photons produced therein escape the interstellar medium and subsequently scatter through the circumgalactic medium (CGM); and ii) the physical conditions of the CGM change with halo mass and/or star formation rates.

 Robert Jedicke: Super catastrophic disruption of asteroids at small perihelion distances.

Abstract: Most near-Earth objects (NEOs) originate in the main asteroid belt as small objects that drift via non-gravitational thermal forces into resonant escape routes that then push them onto planet-crossing orbits. The dynamics of how these objects evolve in the inner solar system is well understood and has been used to predict the orbital nature and size-frequency distribution of the NEO population. However, problems have emerged with existing NEO models as NEO discoveries have increased. For example, many NEOs should be on orbits that closely approach the Sun yet few have been discovered. In addition, even though the NEO population is roughly an even mix of low-albedo (<10%) and high-albedo (>10%) asteroids, the characterized NEOs with small perihelion distances show a strong preference for high albedos. Our new NEO model shows that the deficit is likely produced by the super-catastrophic disruption of a substantial fraction of small and mid-sized NEOs when they reach perihelion distances of a few tens of solar radii.

 Ryan Urquhart: Discovery of the first eclipsing ULX

Abstract: Ultraluminous X-ray sources (ULXs) are some of the most energetic stellar objects in the Universe. The nature of ULXs is still heavily debated; they have been interpreted as stellar-mass black holes/neutron stars accreting at super-Eddington rates, massive stellar-mass black holes accreting around Eddington and even intermediate-mass black holes accreting at sub-Eddington rates. In order to differentiate between these potential models, a constraint on the mass of the compact object is required. Understanding the accretion/ejection coupling and spectral properties of these extremely luminous sources are also important open questions in ULX research. Our recent discovery of the first eclipsing ULX will allow us to make progress on these unsolved problems. The presence of eclipses allows us to place constraints on the physical parameters of the binary system, such as the mass function and orbital period. Eclipses tell us that the source is seen almost edge-on; therefore, we can test the relationship between geometry and spectral appearance of ULXs. The source also shows the presence of a radio jet and coincident optical nebula. Recent spectroscopic data have allowed us to determine the dynamics of the gas and the mechanism behind the nebular inflation. All of this enables us to explore the inflow/outflow coupling and power distribution in the most luminous compact stellar remnants.

 Dr. DooSoo Yoon: Interplay between ejecta from black holes and their surrounding media

Abstract: It is widely believed that black holes are not merely gas consumers but release substantial amount of gas and energy back to their surrounding media in the form of jets, outflows, and radiation. Interaction of such ejecta with interstellar media provides insights for better understanding black hole physics and the evolution of galaxies. In the seminar, I will present theoretical studies of the interaction in two different scales of black holes: stellar mass black holes in X-ray binaries (XRBs) and supermassive black holes in active galactic nuclei (AGN). For the former topic, I will present the ways in which jets from the stellar mass black holes evolve in the presence of strong stellar wind in high-mass XRBs: jet recollimation, bending and disruption. For the latter topic, I will present the effects of AGN feedback on the co-evolution of supermassive black holes and their host galaxies: black hole mass growth, star formation, and more. From the studies presented, I will discuss the importance of such interactions (i.e., feedbacks) to explaining the observed signatures around microquasars and quasars.

 Prof. Jack Scudder: A Post-Spitzer Heat Law for Astrophysical Plasmas

Abstract: Many interesting problems in astrophysics center on understanding the energetics. As most space plasmas are fully ionized, they have significant kinetic temperature; with high temperatures a significant role for heat conduction is commonly required to make a quantitative model of the energetics. Unfortunately there is no heat law at present that supersedes the Spitzer-Harm 1953 heat law that is inappropriate for almost all space plasmas. This talk will discuss a new approach to the heat law for an astrophysical plasma, that draws its new insight from the role of the "Thermal Force" in a plasma, the role and variation of non-thermal velocity distributions, and the importance of addressing strong gradients (finite Knudsen numbers). A new fluid closure formula for the heat law is derived with a much different dependence on other variables than Spitzer's simple Fourier law.

 Prof. Robert Mutel: A Search for Thermal Gyrosynchrotron Emission from Hot Stellar Coronae

Abstract: We have conducted a search for thermal gyro-synchrotron (GS) radio emission from a sample of eight radio-loud stars whose X-ray coronae contain a hot (T > 107 K) component. We used the JVLA to measure Stokes I and V spectral energy distributions (SEDs) over the frequency range 15 - 45 GHz, fitting for best-fit models parameters using both power-law and thermal gyrosynchrotron emission models. For five of the eight stars (Algol, UX Arietis, HR1099, AD Leo), the SEDs were well-fit by a power-law GS model, with no evidence for a thermal component. However, for three stars (V410 Tau, HDE 283572, and UV Ceti), the SEDs had a circularly polarized enhancement above 30 GHz that was inconsistent with a pure power-law electron distribution. These spectra were well-fit by summing the emission from an extended coronal region with power-law GS emission and a smaller region with hotter thermal plasma and much stronger magnetic fields emitting thermal GS emission. The inferred magnetic fields range from 1000 - 2500 gauss in these hot coronal regions.

 Prof. Jasper Halekis: The Mars Hydrogen Cycle: Precipitation, Escape, and Aurora

Abstract: I will discuss the life and times of hydrogen atoms at Mars, based on the latest greatest observations from MAVEN, Mars Express, and the Hubble Space Telescope. Hydrogen at Mars comes from both above and below, from the solar wind and from the atmosphere. Hydrogen from below is derived from the lower atmosphere and transported to high altitudes to form an extended corona around Mars. Surprising recent observations indicate that the hydrogen abundance in this corona varies by at least an order of magnitude with Martian season. Thanks to Mars' weak gravity, the H corona extends well beyond the bow shock, where it interacts with hydrogen from above in the form of solar wind protons (a.k.a. hydrogen ions). Charge exchange reactions between solar wind and coronal hydrogen create energetic neutral atoms that precipitate into the upper atmosphere. MAVEN has directly observed this hydrogen precipitation for the first time. Better yet, two spacecraft have just reported detections of "proton aurora" UV emissions produced by the interaction of precipitating hydrogen with neutral atoms in the upper atmosphere.

Copyright University of Iowa 2016