Entropic-acoustic instability in shocked accretion flows

A new instability mechanism is described in accretion flows where the gas i s accelerated from a stationary shock to a sonic surface. The instability i s based on a cycle of acoustic and entropic waves in this subsonic region o f the flow. When advected adiabatically inward, entropy perturbations trigg er acoustic waves propagating outward. If a shock is present at the outer b oundary, acoustic waves reaching the shock produce new entropy perturbation s, thus creating an entropic-acoustic cycle between the shock and the sonic surface. The interplay of acoustic and entropy perturbations is estimated analytically using a simplified model based on the compact nozzle approxima tion. According to this model, the entropic-acoustic cycle is unstable if t he sound speed at the sonic surface significantly exceeds the sound speed i mmediately after the shock. The growth rate scales like the inverse of the advection time from the outer shock to the sonic point. The frequency of th e most unstable perturbations is comparable to the refraction cutoff, defin ed as the frequency below which acoustic waves propagating inward are signi ficantly refracted outward. This generic mechanism should occur in Bondi-Ho yle-Lyttleton accretion, and also in shocked accretion discs.