Exciting Waves in Accretion Disk Boundary Layers

In an accretion disk, the boundary layer is the region where the disk attaches to the star. Unlike in the disk proper, the boundary layer has a rotation profile which rises with radius, since the rotation profile must transition from that of the more slowly rotating star to that of the more quickly rotating disk. The rising rotation profile makes the boundary layer (linearly) stable to the MRI instability. Therefore, a different mechanism must be responsible for transporting angular momentum in the boundary layer and decelerating the disk material. The details of this mechanism are observationally significant since up to half of the total disk luminosity comes from the boundary layer region.

Working with Roman Rafikov and James Stone, I have discovered a new instability which is capable of transporting angular momentum and driving accretion in the boundary layer via waves rather than turbulence. This instability is related to the Papaloizou-Pringle class of instabilities, in that it is sourced by a corotation resonance in the boundary layer. However, it is distinct from the traditional Papaloizou-Pringle instability; it is also robust, feeding off the large degree of shear in the boundary layer. I will present both analytical and numerical results, which explain the mechanism of the new instability (which we call the sonic or acoustic instability) and its role in angular momentum transport in accretion disk boundary layers. I will also describe why no reasonable alpha prescription for the viscosity can be reconciled with this instability. This has significant ramifications for semi-analytical models of boundary layers, which typically assume some form of alpha viscosity.

Wide-field, high cadence surveys are allowing us to catch supernovae earlier than ever, often days if not hours after they first begin. Theoretical work focusing on this early phase of stellar explosions is critical for (1) deriving constraints on supernova progenitors from observations, (2) predicting and interpreting new discoveries, and (3) helping guide the strategies of future observational efforts. I will discuss theoretical research in each of these areas, including showing how we are beginning to directly measure the properties of stars before they die and investigating how we may potentially observe the elusive events that mark black hole formation.

Measurements of the primary anisotropies in the CMB have been the backbone of modern precision cosmology. Recently, high resolution CMB measurements from experiments, such as the Atacama Cosmology Telescope, the South Pole Telescope, and the Planck satellite are probing scales where the secondary anisotropies dominate over the primary. I will focus on the secondary anisotropies caused by the thermal and kinetic Sunyaev Zel'dovich effects. Our ability to obtain cosmological information from these secondaries is limited by our theoretical understanding of the baryons in the large-scale structure between us and the primary CMB. I will present numerical simulations that model these baryons and attempt to constrain various cosmological parameters. Additionally, I will discuss the wealth of astrophysical large-scale structure information (in particular galaxy cluster astrophysics) that is interconnected with these secondaries.

Transiting exoplanets, and in particular Hot Jupiters around bright stars, are key ingredients for understanding planetary formation and migration theories, statistics of planetary properties and planetary dynamics, and are of the most favorable for observational study.

I will present a novel approach to use low-cadence photometric surveys, such as the promising Gaia Space mission for exoplanetary transit search. We find that even if transits are undetectable in the sparsely sampled data, the "Directed Follow-Up strategy" (DFU) can be used to choose preferred times for follow-up observations that will maximize the chances to detect transiting planets. Though low-cadence surveys are usually considered irrelevant for transit searches, Gaia's yield can reach a few thousands of new Hot Jupiters, if the Directed Follow-Up strategy will be applied to schedule and conduct a follow-up campaign. Scheduling the follow-up observations is a significant part of the strategy, therefore a global telescope network, such as LCOGT, that will be available at all times and in different locations, is ideal for optimizing the outcome of the strategy.

Gravitational collapse during the Universe's first billion years transformed a nearly homogeneous matter distribution into a network of filaments - the Cosmic Web - where galaxies form and evolve and where the majority of baryons reside as rarefied gas. Since most of this material is too diffuse to form stars, its study has been limited so far to absorption probes against background sources. In this talk, I will present the result of an ongoing, successful program that uses a new approach to directly detect and study cosmic gas in the early Universe: the key idea is to use an external "source of illumination’’, a bright quasar, to light up with fluorescent Lya emission "dark" proto-galactic clouds, dense streams around galaxies (the Circumgalactic Medium) and the Cosmic Web. In the first part of the talk, I will discuss our pilot project based on deep narrow-band imaging on VLT /FORS centered on a z=2.4 hyper-luminous quasar: how we identified and characterized the physical properties of the first 12 “dark” galaxy candidates detected in the early Universe. In the second part of the talk, I will present observations of fluorescent emission from the Circumgalactic Medium of star forming galaxies and very recent, spectacular results obtained with Keck/LRIS of the detection of hundred-kpc scale filaments surrounding bright quasars: the first direct image of the Cosmic Web.