Thanatology in Protoplanetary Disks

The magnetorotational instability (MRI) is the most promising mechanism by which angular momentum is efficiently transported outwards in astrophysical disks. However, its application to protoplanetary disks remains problematic. These disks are so poorly ionized that they may not support magnetorotational turbulence in regions referred to as "dead zones". While purported dead zones have captured the attention of disk theorists for nearly 20 years, there is generally no consensus regarding whether they are, in fact, dead. The problem is complicated by two non-ideal MHD processes -- ambipolar diffusion and the Hall effect -- the latter having been largely ignored in numerical studies of the MRI until recently. In this seminar, I will present several surprising results on how these effects are likely to modify the magnetic and turbulent behavior of protoplanetary disks. In disks dominated by a vertical magnetic flux, the MRI can saturate by producing large-scale, long-lived, axisymmetric structures in the magnetic field that cause a steep reduction in turbulent transport and instigate dust-corralling zonal flows. In disks dominated by an azimuthal flux, the dead zone can be eliminated by large-scale magnetic stresses and a strong outflow produced. These results call into question contemporary models of layered accretion.

Circumcluster and circumgalactic medium in the Virgo Cluster

I present the first systematic QSO absorption line survey for a galaxy cluster. Using COS onboard HST, we find 43 Lyα absorbers toward 23 background QSOs in and around the Virgo Cluster. We find Lyα absorbers are predominantly in the cluster outskirts where the infalling substructures of the Virgo Cluster are. The cluster is surrounded by this diffuse, warm gas in contrast with field environment. Our finding is consistent with cosmological simulations. We also examine the CGM by connecting the Lyα absorbers to their closest galaxies. The CGM of galaxies in the cluster is suppressed compared with circumcluster and field environments.

Origins of Gas Giant Planets

Several giant planets have now been directly imaged, offering the first view of extrasolar planets at wide separations from their host stars. Formation of these objects by either leading theory--core accretion or gravitational instability--presents substantial difficulties. These challenges may be reinterpreted as opportunities. In this talk, I will demonstrate how to use upcoming constraints from direct imaging to distinguish between theories of giant planet formation. Along the way, I will discuss whether gravitational instability could have formed the iconic directly-imaged planetary system HR 8799, present a new theory of planetary core growth in the presence of gas that extends the reach of core accretion to large stellocentric distances, and show how the atmospheric compositions of giant planets record signatures of their formation locations.

XXI century reverberation mapping: inferring black hole mass, geometry and dynamics of the broad line region in active galaxies

Constraining the inner regions of active galactic nuclei (AGN) is difficult due to the very small scales involved. By using reverberation mapping, we can substitute time resolution for spatial resolution and begin to probe AGN on the scales of the broad line region. However traditional reverberation mapping analysis uses only a fraction of the available information. We have been developing advanced analysis methods that give us new insights into the properties of AGN, including the absolute black hole mass and the geometry and dynamics of the broad line region. Applying this technique to five AGNs from the Lick AGN Monitoring Project 2008 sample, we find that the BLR geometry is consistent with a thick disk and the dynamics are a combination of elliptical orbits and inflow.

Two Possible Types of Tidal Disruption Events and Their Fancy for E+A Galaxies

A Tidal Disruption Event (TDE) is what happens to a star which gets too close to a supermassive black-hole and is consequently torn apart by gravitational tidal forces. Optical (and possibly also UV and X-ray) flares are thought to accompany such events, but the expected observational spectroscopic signature is still a topic of debate. In 2012, a TDE candidate published in Nature (Gezari et al), surprisingly showed no hydrogen, but only helium, in its spectrum, contrary to previous expectations. I will present the results of an archival search for TDEs in the Palomar Transient Factory database, where we find three such events - one similar to the Gezari et al. (He-rich) TDE, and two which do display hydrogen. Also, we find that these events seem to show a surprising preference for rare post-starburst (E+A) galaxies.