On the Relative Surface Density Change of Thermally Unstable Accretion Disks

Abstract

The relations among the relative changes of surface density, temperature, disk height and vertical integrated pressure in three kinds of thermally unstable accretion disks were quantitatively investigated by assuming local perturbations. The surface density change was found to be very small in the long perturbation wavelength case but can not be ignored in the short wavelength case. It becomes significant in an optically thin, radiative cooling dominated disk when the perturbation wavelength is shorter than 15H (H is the scale height of disk) and in a geometrically thin, optically thick and radiation pressure dominated disk when the perturbation wavelength is shorter than 50H. In an optically thick, advection-dominated disk, which is thermally unstable against short wavelength perturbations, the relative surface density change is much larger. We proved the positive correlation between the changes of surface density and temperature in an optically thick, advection- dominated disk which was previously claimed to be the essential point of its thermal instability. Moreover, we found an anticorrelation between the changes of disk height and temperature in an optically thick, advection-dominated disk. This is the natural result of the absence of appreciable vertical integrated pressure change.