ON THE RELATIVE SURFACE-DENSITY CHANGE OF THERMALLY UNSTABLE ACCRETION DISKS

The relations among the relative changes of surface density, temperatu re, disk height, and vertically integrated pressure in three kinds of thermally unstable accretion disks are quantitatively investigated, as suming local perturbations. The surface density change is found to be very small in the long perturbation wavelength case but cannot be igno red in the short-wavelength case. It becomes significant in an optical ly thin, radiative cooling-dominated disk when the perturbation wavele ngth is shorter than 15H (where 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 opti cally thick, advection-dominated disk, which is thermally unstable aga inst short-wavelength perturbations, the relative surface density chan ge is much larger. We prove the positive correlation between the chang es of surface density and temperature in an optically thick, advection -dominated disk that has previously been claimed to be the essential p oint of its thermal instability. Moreover, we find an anticorrelation between the changes of disk height and temperature in an optically thi ck, advection-dominated disk. This is the natural result of the absenc e of appreciable vertically integrated pressure change.