Sonic Point Instability and Stochastic Excitation of Oscillations by Turbulence

Abstract

In this final chapter two possible excitation processes which are not mentioned yet are briefly introduced. One is a sonic point instability. In the case of relativistic disks, the accretion flows fall to central objects by passing a sonic point. In the case where the sonic point is a nodal-type critical point, accretion flows are locally unstable at the sonic point and waves propagating both inward and outward are generated there. This is an interesting excitation process of oscillations, but there is a slight doubt whether it is physical. The other is a possible excitation of oscillations by stochastic processes of turbulence. The stochastic processes are known to be the main processes of excitation of solar and stellar oscillations, but they are not examined yet in disks. This excitation process will be of importance, because many oscillations modes will be excited with small amplitudes. Examination of disk oscillations by stochastic processes will become in future one of important fields of diskoseismology. A brief outline of mathematical formulation of stochastic excitation of disk oscillations will be presented here.