December 3, 2015 3:30PM BPB-207. Phil Armitage. New directions in astrophysical accretion.

Accretion disks play a critical role in planet formation, and in compact object and black hole astrophysics. I will discuss the implications of recent numerical simulations of astrophysical disks for both classes of system, which challenge long-held assumptions as to how disks work. In protoplanetary disks, the non-ideal physics of the Hall effect appears to dominate disk evolution on planet-forming scales, and this may lead to a bimodal distribution of planetary system properties. In black hole disks I will argue that strongly magnetized disks provide an appealing framework for addressing a range of unexplained observations. I will discuss the obstacles and prospects for developing a theory for the long-term evolution of both planet forming and black hole accretion flows.

November 20, 2015 3:30PM BPB-207. Bharat Ratra. Dark Energy: constant or time variable? (... and other open questions).

Experiments and observations over the last decade and a half have persuaded cosmologists that (as yet undetected) dark energy is by far the main component of the energy budget of the universe. I review a few simple dark energy models and compare their predictions to observational data, to derive dark energy model-parameter constraints and to test consistency of different data sets. I conclude with a list of open cosmological questions.

November 13, 2015 3:30PM BPB-207. Kailash Sahu. Detecting Exoplanets, and Isolated, Stellar-Mass Black Holes through Microlensing.

This talk consists of two parts. In the first part, I will review how almost all the extra-solar planets discovered so far have been relatively nearby --- within ~500 parsecs from the Sun, and within about 2 AU from their parent stars. I will discuss how microlensing provides insights into the frequency of planets across the Milky Way, and up to several AU from the host stars. In the second part, I will discuss the technique of astrometric microlensing, and two HST programs underway aimed at the first detections of isolated, stellar-mass black holes through this technique.

November 13, 2015 10:30AM BPB-248. Daniel Antonio Phd. Defense. High Pressure X-Ray Absorption Spectroscopy Studies of Heavy-Fermion Cerium and Uranium Compounds.

Investigations into f- electron heavy-fermion materials have revealed a wide range of novel behavior. Hydrostatic pressure is a valuable "clean" non-thermal parameter that can be used to systematically study them by tuning their ground state properties. The rare earth compound CeCu2Ge2 shows an unusual two-domed region of unconventional superconductivity under pressure, similar to its isostructural counterpart CeCu2Si2. While the lower pressure dome at about 10 GPa is caused by a magnetic quantum critical point, the higher one at about 16 GPa is less well understood. Previous structural measurements have indicated that it may be caused by critical valence fluctuations, so in this study the valence of CeCu2Ge2 is directly measured using X-ray Absorption Near Edge Spectroscopy (XANES) under pressure in a diamond anvil cell up to 20 GPa. An expected valence discontinuity is not seen, but comparisons to CeCu2Si2 show interesting similarities. Uranium's 5f electrons are intermediate between localized and delocalized. Studying the degree of localization is vital to completely understanding properties of actinides. Using XANES and partial Florescence Yield (PFY) in a diamond anvil cell to tune the distance between uranium atoms, I have measured the energy shift in the white line of UCu2Si2, U3Ni5Al19, and UCd11 with pressure. A positive shift in energy indicated a delocalization of 5f electrons, a change in 5f configurations, or a combination of both.

November 6, 2015 3:30PM BPB-207. Duncan Lorimer. Fast Radio Bursts: The Story So Far.

I will describe a brief history of discovery and some exciting recent developments in the world of pulsars and fast radio bursts. Pulsars, rapidly rotating highly magnetized neutron stars, were discovered in 1967 and continue to surprise and delight astronomers as powerful probes of fundamental physics and astrophysics. Fast radio bursts are millisecond-duration pulses of currently unknown origin that were discovered in 2007. Both pulsars and fast radio bursts have great promise at probing the universe on large scales and in fundamental ways. I will describe the science opportunities these phenomena present, and discuss the challenges and opportunities presented in their discovery.

October 26, 2015 12:00PM BPB-207. Yan-Fei Jiang. Iron Opacity Bump Changes the Structure and Stability of Black Hole Accretion Disks in AGNs.

In the standard thin accretion disk model, electron scattering is usually thought to be the dominant opacity in the radiation pressure dominated inner region of black hole accretion disks. I will show that this is not the case for most AGNs, where the density and temperature ranges in the accretion disks are similar as in the envelopes of massive stars and opacity bump caused by Irons are much more important. Through a series of self-consistent radiation MHD simulations, I will show that the iron opacity bump can significant decrease the growth rate of thermal instability in the standard thin accretion disk model compared to case with dominated electron scattering opacity. It can even stabilize the disk. Viscous instability also does not exist for AGN disks with iron opacity bump. I will also discuss implications of the iron opacity bump on resolving various puzzles between standard thin disk models and observations.

October 16, 2015 3:30PM BPB-207. Jason Steffen. Laser Probes of the Dark Sector.

The majority of the universe is composed of exotic matter and energy of which we know very little. This "dark sector" comprising dark matter, dark energy, and gravity, is likely to be one of the primary scientific challenges of the 21st century. I report on a series of three laboratory experiments at Fermilab to probe this dark sector. These experiments include searches for dark matter axions, dark energy chameleons, and tests of the very nature of space time.