Bondi‐Hoyle Accretion in a Turbulent Medium

The Bondi-Hoyle formula gives the approximate accretion rate onto a pointparticle accreting from a uniform medium. However, in many situations accretiononto point particles occurs from media that are turbulent rather than uniform.In this paper, we give an approximate solution to the problem of a pointparticle accreting from an ambient medium of supersonically turbulent gas.Accretion in such media is bimodal, at some points resembling classicalBondi-Hoyle flow, and in other cases being closer to the vorticity-dominatedaccretion flows recently studied by Krumholz, McKee, & Klein. Based on thisobservation, we develop a theoretical prediction for the accretion rate, andconfirm that our predictions are highly consistent with the results ofnumerical simulations. The distribution of accretion rates is lognormal, andthe mean accretion rate in supersonically turbulent gas can be substantiallyenhanced above the value that would be predicted by a naive application of theBondi-Hoyle formula. However, it can also be suppressed by the vorticity, justas Krumholz, McKee, & Klein found for non-supersonic vorticity-dominated flows.Magnetic fields, which we have not included in these models, may furtherinhibit accretion. Our results have significant implications for a numberastrophysical problems, ranging from star formation to the black holes ingalactic centers. In particular, there are likely to be significant errors inresults that assume that accretion from turbulent media occurs at theunmodified Bondi-Hoyle rate, or that are based on simulations that do notresolve the Bondi-Hoyle radius of accreting objects.Comment: Accepted for publication in ApJ; 13 pages, 9 figures, emulateap