Speaker: James Stone (Princeton University)　　  

   Time: Thursday, 3:00pm, September 25th　

　    Location: Lecture Hall, 3rd floor　　 　　　  

   For black holes accreting at anything more than a small fraction of the Eddington limit, the inner regions of the flow will be dominated by radiation rather than gas pressure.  I will present results from a new study of the magnetohydrodynamics of accretion disks in this regime, using new numerical methods based on a formal solution of the radiation transfer　　  

　equation using short characteristics, rather than the flux-limited diffusion approximation.  For gas pressure dominated disks, long-lived stable vertical structures in which turbulent heating is balanced by radiative cooling are observed.  However, when radiation pressure greatly exceeds gas pressure, we find the disk always undergoes a thermal runaway, either collapsing or expanding over several thermal times.  The physics of this runaway is quite different from the classical linear instability in radiation dominated alpha disks predicted by Shakura and Sunyaev. Saturation of the runaway will require global models of radiation pressure dominated accretion flows, and we discuss future work on this problem.