Dynamical instability of standing shock waves in adiabatic accretion flows and wind flows

We analytically investigate an instability of adiabatic accretion flows on to (or wind flows from) a compact object with an axisymmetric standing shock wave. If the instability occurs, the shock front deviates from its equilibrium position, keeping axisymmetry, and the post-shock flow is destroyed. We prove that this instability occurs if the post-shock flow is accelerated and if the shock is not extremely strong. Also, we show that the growth occurs on a time-scale of the order of the dynamical one. These results suggest that the nature of the shock instability of adiabatic flows is fundamentally unchanged from that of isothermal flows, although the former includes the generation of entropy waves at the shock front, whereas the latter does not. It is probable that the present instability is a universal phenomenon, occurring independently of the detailed properties of gas, and is predictable via a simple rule: Post-shock acceleration causes the shock instability.