2008 APS April Meeting and HEDP/HEDLA Meeting
Volume 53, Number 5
Friday–Tuesday, April 11–15, 2008;
St. Louis, Missouri
Session 2HE: Accretion and Photoionized Plasmas
11:15 AM–2:50 PM,
Friday, April 11, 2008
Hyatt Regency St. Louis Riverfront (formerly Adam's Mark Hotel),
Room: Promenade F
Sponsoring
Units:
HEDP HEDLA
Chair: Roberto Mancini, University of Nevada-Reno
Abstract ID: BAPS.2008.APR.2HE.3
Abstract: 2HE.00003 : Astrophysics of Accretion onto Compact Objects*
12:05 PM–12:30 PM
Preview Abstract
Abstract
Author:
John Hawley
(University of Virginia)
The most energetic phenomena in the universe are systems powered by gravity
through accretion. For compact stars such as white dwarfs, neutron stars,
and especially black holes, the energy released per unit mass accreted can
significantly exceed that released by nuclear reactions. Over the last half
century a growing body of observations has revealed a plethora of
environments in which accretion plays a significant or even dominant role.
Our theoretical understanding of accretion disk systems has not kept pace.
Until recently theory has been based primarily on a one-dimensional
time-steady model consisting of an optically thick, vertically-thin,
Keplerian disk with an unknown, parameterized internal stress. While these
analytic models have served us well to understand many properties of a wide
variety of accretion systems, their limitations are now well-known, and the
observation data demand moving beyond this standard. Space- and ground-based
observations are providing increasingly detailed evidence that accretion
systems are dynamic. For example, the spectral energy distribution and
luminosity for sources such as X-ray binaries and AGN are strongly variable,
often with substantial amplitudes. The timescales for variability are rapid,
often comparable to the dynamical times associated with orbits near the
central black hole. This variability must arise not from secular changes in
the accretion rate (the only process accessible to time-stationary analytic
disk models) but from processes that occur within the disk.
Numerical simulations provide a way to investigate the dynamics of accretion
flows directly with far fewer limitations compared to analytic models.
Because magnetic fields are fundamentally important for jets and disks, and
because we now know that magnetic turbulence accounts for the internal
stress, the governing equations are those of compressible
magnetohydrodynamics (MHD). Accretion disk dynamics can thus be investigated
using three-dimensional MHD simulation codes that employ both global and
local computational domains. Although it is not yet possible to do fully
global time-dependent radiation transport in disk models, the observational
implications of these simulations can be investigated using simple emission
and absorption models coupled with relativistic ray tracing. The time and
length-scales involved make such simulations challenging, but even the first
results are intriguing. They have revealed details about time-dependent
properties of disks, magnetic disk dynamos, jet launching mechanisms, and
the dynamical properties of systems other than the standard thin disk. As
the capabilities of computational hardware increase, and the development of
advanced numerical codes continues, our theoretical understanding of
accretion physics will substantially increase.
*Supported by NSF Grant PHY-0205155 and NASA grant NNG04GK77G.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2008.APR.2HE.3