Dwarf Novae — A Hot Dam Instability

Abstract

When mass input rates are low enough, accretion disks cease to be fully ionized. The strong and oppositely directed temperature dependences of opacity (e.g, in hydrogen ionization regions) drive a thermal instability which modulates mass transfer throughout the disk.

The conventional thin-disk treatment breaks down in cool regions; instead, the surface boundary condition dominates the structure of these largely radiative zones. Convection, when it occurs, is inefficient and essentially irrelevant. We modify the conventional treatment to produce useful approximations to local vertical energy losses.

Our local cooling modification is then incorporated into global disk computations. Such disks are indeed thermally unstable. Large areas can be cool and optically thin between outbursts. Non-steady mass transfer within the disk allows the instability to be triggered at different radial positions. True hysteresis occurs, a new outburst depending on an unrelaxed memory of the previous outburst. Outbursts of alternating character, as in SS Cyg, are found quite naturally.