Accretion Disks in Interacting Binary Stars

John Cannizzo

A paradigm has developed over the past 20 years to explain the time dependent accretion disk behavior in interacting binaries showing semiperiodic outbursts -- dwarf novae (DNe) and X-ray novae (XNe). In these systems, gas accumulates for long intervals in a relatively inviscid accretion disk about the central star (a WD in the DNe, and either a NS or BH in the XNe). When a critical surface density is reached somewhere in the disk, a thermal instability is launched. This initiates a transition to a state of enhanced transport during which significant accretion occurs, thereby producing outbursts. The "accretion disk limit cycle model" accounts for the outbursts seen in systems for which the mass feeding rate to the outer edge is below a certain critical rate. The manifestation of the mechanism is quite different between DNe and XNe, thereby producing very different outbursts. Two effects which appear to be dominant in the XNe, for instance, are irradiation of the disk and evaporation of disk gas.

I will review different aspects of the limit cycle model with emphasis on what we have learned about the physics of accretion disks in DNe and XNe. I will also address various problems and inconsistencies between theory and observation.

For movies showing the time dependent evolution of the accretion disk limit cycle process, see movie3.mov and movie4.mov.