A fresh look at the unstable simulations of Bondi-Hoyle-Lyttleton accretion

The instability of Bondi-Hoyle-Lyttleton accretion, observed in numerical simulations, is analyzed through known physical mechanisms and possible numerical artefacts. The mechanisms of the longitudinal and transverse instabiliti es, estab- lished within the accretion line model, are clarified. They c annot account for the instability of BHL accretion at moderate Mach number when the pressure forces within the shock cone are taken into account. The advective-acoustic instability is con sidered in the context of BHL accretion when the shock is detached from the accretor. This mechanism naturally explains the stability of the flow when the shock is weak, and the instability when the accretor is small. In particular, it is a robust proof of the instability of 3D accretion when = 5/3 if the accretor is small enough, even for moderate shock strength (M ∼ 3). The numerical artefacts that may be present in existing numerical simulations are reviewed, with particular attention pai d to the advection of entropy/vorticity perturbations and the artificial acoustic feedba ck from the accretor boundary condition. Several numerical tests are proposed to test these mechanisms.