An analytic study of Bondi-Hoyle-Lyttleton accretion - II. Local stabilityanalysis

The adiabatic shock produced by a compact object moving supersonically rela tive to a gas with uniform entropy and no vorticity is a source of entropy gradients and vorticity. We investigate these analytically. The non-axisymm etric Rayleigh-Taylor and axisymmetric Kelvin-Helmholtz linear instabilitie s are potential sources of destabilization of the subsonic accretion flow a fter the shock. A local Lagrangian approach is used in order to evaluate th e efficiency of these linear instabilities. However, the conditions require d for such a WKB type approximation are fulfilled only marginally: a quanti tative estimate of their local growth rate integrated along a flow line sho ws that their growth time is at best comparable to the time needed for adve ction onto the accretor, even at high Mach number and for a small accretor size. Despite this apparently low efficiency, several features of these mec hanisms qualitatively match those observed in numerical simulations: in a g as with uniform entropy, the instability occurs only for supersonic accreto rs. It is nonaxisymmetric, and begins close to the accretor in the equatori al region perpendicular to the symmetry axis. The mechanism is more efficie nt for a small, highly supersonic accretor, and also if the shock is detach ed. We also show by a 3-D numerical simulation an example of unstable accretion of a subsonic flow with non-uniform entropy at infinity. This instability is qualitatively similar to the one observed in 3-D simulations of the Bond i-Hoyle-Lyttleton flow, although it involves neither a bow shock nor an acc retion line.