EXCITATION OF TRAPPED OSCILLATIONS IN DWARF-NOVA ACCRETION DISKS

As a possible mechanism for quasi-periodic oscillations (QPOs) of dwarf novae during outbursts, axially symmetric radial oscillations of accretion disks are investigated. According to the thermal-instability scenario, the dwarf-nova disk in the decay from outbursts consists of inner, vertically expanded hot regions and outer cool regions. We first show that acoustic waves with a narrow range of frequencies, once excited, can be trapped just inside the transition front between the inner hot and outer cool regions. This contrasts with the usual case of standard-disk configurations in which waves cannot be trapped. We then examine the stability of the disk by adopting the conventional alpha-viscosity, finding that trapped oscillations can grow by viscous and thermal processes, which produce quasi-periodic light variations. The QPO frequency is, however, expected to increase by a factor of approximately 4 as the disk luminosity decreases, which has not yet been observed.