Confronting MHD Theories of Accretion Disks with Observations

Coordinators: Omer Blaes, Joan Najita, Jim Stone, and Neal Turner

Scientific Advisors: Christian Knigge and Jean-Pierre Lasota-Hirszowicz

Accretion flows are important in a wide variety of astrophysical phenomena including protostellar disks around young stars, accreting white dwarfs, neutron stars and black holes in binary systems, and accreting supermassive black holes in quasars, active galactic nuclei, and our own Galactic center. Fundamental to the physics of accretion disks is the process whereby material in the disk loses angular momentum and drifts inward, and how the resulting gravitational binding energy that is released is converted to observable forms. The dominant mechanism in sufficiently ionized disks is almost certainly magnetohydrodynamic (MHD) in nature, including magnetorotational turbulence and MHD winds, and these mechanisms also play a role in largely neutral, weakly electrically conducting disks.

Tremendous progress has been made in recent years in large-scale computer simulations of these MHD processes, including not only the dynamics, but also the thermodynamics (turbulent dissipation and radiative cooling). Simulations have therefore reached the level of sophistication where they should be able to explain observations and make new predictions. Outbursting accretion disks in binary systems (dwarf novae and soft X-ray transients) contain some of the richest observational data and are now amenable to simulation. Moreover, the quiescent states of these systems share many of the non-ideal MHD properties of protoplanetary disks, where exciting new spatially resolved data is coming from the Atacama Large Millimeter/Submillimeter Array (ALMA). The program will therefore bring together theorists and observers to work on dwarf novae, X-ray binaries, and protoplanetary disks, although active galactic nuclei and the Galactic center will also receive attention.