GENERALIZED THERMAL-INSTABILITY CRITERION OF BLACK-HOLE ACCRETION DISKS

The conventional thermal instability criterion cannot be applied to th e advection-dominated accretion disks around black holes where the rad iative cooling is insufficient to balance the viscous heating. The sur face density change associated with the temperature perturbations, whi ch was usually neglected in deriving the conventional criterion, was r ecently shown to be much significant in the advection-dominated disks. Considering both advection and surface density change, a generalized thermal instability criterion is suggested. By applying it to the opti cally thin and optically thick advection-dominated disks, it is found that the former one is thermally stable and the latter one is thermall y unstable against short wavelength perturbations, which agrees well t o those found recently by both analytic and quantitative stability ana lyses.