Kavli Affiliate: Lars Bildsten
| First 5 Authors: Logan J. Prust, Hila Glanz, Lars Bildsten, Hagai B. Perets, Friedrich K. Roepke
| Summary:
We carry out three-dimensional computations of the accretion rate onto an
object (of size $R_{rm sink}$ and mass $m$) as it moves through a uniform
medium at a subsonic speed $v_{infty}$. The object is treated as a
fully-absorbing boundary (e.g. a black hole). In contrast to early conjectures,
we show that when $R_{rm sink}ll R_{A}=2Gm/v^2$ the accretion rate is
independent of $v_{infty}$ 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, $Theta=R_{A}/R_{rm sink}$, and is much less than the dynamical
friction for sufficiently small accretors ($R_{rm sink}ll R_{A}$).
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