Bulletin of the American Physical Society
2009 APS April Meeting
Volume 54, Number 4
Saturday–Tuesday, May 2–5, 2009; Denver, Colorado
Session Q5: Computational Astrophysics of Disks: From Black Holes to Planets |
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Sponsoring Units: DCOMP Chair: Richard Klein, Lawrence Livermore National Laboratory Room: Governor's Square 15 |
Monday, May 4, 2009 10:45AM - 11:21AM |
Q5.00001: Radiative Models of Black Hole Accretion Flows Invited Speaker: I will discuss models of black hole accretion based on self-consistent general relativistic magnetohydrodynamic (MHD) simulations. We have developed techniques for calculating the emergent radiation from the MHD simulations in both image and spectral forms. I will describe the techniques, capabilities, and implications of our models. The models are motivated in part by recent observational developments in low luminosity galactic nuclei that raise the prospect of direct imaging of black hole accretion flows at the event horizon scale within the next decade. [Preview Abstract] |
Monday, May 4, 2009 11:21AM - 11:57AM |
Q5.00002: Radiative Hydrodynamics and the Formation of Gas Giant Planets Invited Speaker: Gas giant planets undoubtedly form from the orbiting gas and dust disks commonly observed around young stars, and there are two principal mechanisms proposed for how this may occur. The core accretion plus gas capture model argues that a solid core forms first and then accretes gas from the surrounding disk once the core becomes massive enough (about 10 Earth masses). The gas accumulation process is comparatively slow but becomes hydrodynamic at later times. The disk instability model alternatively suggests that gas giant planet formation is initiated by gas-phase gravitational instabilities (GIs) that fragment protoplanetary disks into bound gaseous protoplanets rapidly, on disk orbit period time scales. Solid cores then form more slowly by accretion of solid planetesimals and settling. The overall formation time scales for these two mechanisms can differ by orders of magnitude. Both involve multidimensional hydrodynamic flows at some phase, late in the process for core accretion and early on for disk instability. The ability of cores to accrete gas and the ability of GIs to produce bound clumps depend on how rapidly gas can lose energy by radiation. This regulatory process, while important for controlling the time scale for core accretion plus gas capture, turns out to be absolutely critical for disk instability to work at all. For this reason, I will focus in my talk on the use of radiation hydrodynamics simulations to determine whether and where disk instability can actually form gas giant planets in disks. Results remain controversial, but simulations by several different research groups support analytic arguments that disk instability leading to fragmentation probably cannot occur in disks around Sun-like stars at orbit radii of 10's of Earth-Sun distances or less. On the other hand, very recent simulations suggest that very young, rapidly accreting disks with much larger radii (100's of times the Sun-Earth distance) can indeed readily fragment by disk instability into super-Jupiters and brown dwarfs. It is possible that there are two distinct modes of gas giant planet formation in Nature which operate at different times and in different regions of disks around young stars. The application of more radiative hydrodynamics codes with better numerical techniques could play an important role in future theoretical developments. [Preview Abstract] |
Monday, May 4, 2009 11:57AM - 12:33PM |
Q5.00003: MHD Simulations of Disk-Star Interaction Invited Speaker: Many disk-accreting stars have a dynamically important magnetic field which strongly influences matter flow in their vicinity and determines the observational properties of these stars. Examples include young Solar-type stars at the stage of planet formation, neutron stars and white dwarfs in binary systems, and other stars. Recent 3D simulations have shown that accretion to such stars may be in the stable or unstable regime. In the stable regime, matter is lifted above the equatorial plane and accretes to the star in two ordered funnel streams which form two ordered hot spots on the star, and the magnetospheric gap in the equatorial plane has a low matter density. In the unstable regime, matter penetrates through the magnetosphere due to the 3D interchange instability, where the low-m perturbation modes dominate, due to which matter accretes to the star through a few chaotic tongues which form chaotic hot spots on the star. I will discuss the observational appearance of both regimes of accretion, and also the possible importance of the magnetospheric gap for the survival of close-in planets. [Preview Abstract] |
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