Pulsational instability of accretion disks to axially symmetric oscillations

The pulsational instability of accretion disks to axially symmetric oscillations is examined in the approximation that conditions are quasiadiabatic and quasi-inviscid. The essential difference from the usual stellar pulsation is that the shear motion in the unperturbed state has two (thermal and dynamical) effects on the stability of oscillations. Both effects act so as to excite oscillations if the coefficient of viscosity increases by a certain degree in the compressed phase of oscillations, in comparison with the expanded phase. The numerical condition of the growth is examined, in particular for the nearly radial oscillations whose radial wavelength is shorter than the radius of the disk (i.e. local oscillations) but longer than the thickness of the disk. The examination is made mainly for optically thin disks, and secondarily for an optically thick disk. In the particular disk examined in the optically thick case, the radiative diffusion of thermal energy in the vertical direction contributes positively to the excitation of oscillations by the very fact that the flows of oscillations are nearly radial. The instability of accretion disks to local oscillations will be important as a possible cause of turbulence leading to viscosity.