Morphology and Mach Number Dependence of Subsonic Bondi-Hoyle Accretion

We carry out three-dimensional computations of the accretion rate onto an object (of size R_ sink and mass m) as it moves through a uniform medium at a subsonic speed v_∞. The object is treated as a fully-absorbing boundary (e.g. a black hole). In contrast to early conjectures, we show that when R_ sink≪ R_A=2Gm/v^2 the accretion rate is independent of v_∞ and only depends on the entropy of the ambient medium, its adiabatic index, and m. Our numerical simulations are conducted using two different numerical schemes via the Athena++ and Arepo hydrodynamics solvers, which reach nearly identical steady-state solutions. We find that pressure gradients generated by the isentropic compression of the flow near the accretor are sufficient to suspend much of the surrounding gas in a near-hydrostatic equilibrium, just as predicted from the spherical Bondi-Hoyle calculation. Indeed, the accretion rates for steady flow match the Bondi-Hoyle rate, and are indicative of isentropic flow for subsonic motion where no shocks occur. We also find that the accretion drag may be predicted using the Safronov number, Θ=R_A/R_ sink, and is much less than the dynamical friction for sufficiently small accretors (R_ sink≪ R_A).