THE RADIAL-AZIMUTHAL INSTABILITY OF ACCRETION DISKS. Ⅱ. GAS PRESSURE DOMINATION

摘要

A radial-azimuthal stability analysis of a geometrically thin, gas-pressure-dominated accretion disk is presented. In the purely radial perturbation case, the disk is pulsationally unstable to the acoustic modes but stable to the thermal and viscous modes. If the coupling of radial and azimuthal perturbations is considered, the stability properties of the disk are different. With the increase of azimuthal wavenumber, the acoustic modes tend to become stable while the viscous mode becomes unstable. The acoustic instability exists only for large viscosity and small azimuthal wavenumber, while the viscous instability exists for small viscosity and large azimuthal wavenumber. The inclusion of azimuthal perturbations, however, has nearly no effect on the stability of thermal mode, which is always stable in the disk. The influence of opacity on the stability of a gas-pressure-dominated disk has also been investigated. We find that if the free-free absorption dominates the opacity, the disk is more stable than that with electron-scattering domination.