Bondi-Hoyle Accretion in an Isothermal Magnetized Plasma
  In regions of star formation, protostars and newborn stars accrete mass from
their natal clouds. These clouds are threaded by magnetic fields with a
strength characterized by the plasma beta---the ratio of thermal and magnetic
pressures. Observations show molecular clouds have beta <= 1, so magnetic
fields can play a significant role in the accretion process. We have carried
out a numerical study of the effect of large-scale magnetic fields on the rate
of accretion onto a uniformly moving point particle from a uniform,
non-self-gravitating, isothermal gas. We consider gas moving with sonic Mach
numbers of up M ~ 45, magnetic fields that are either parallel, perpendicular,
or oriented 45 degrees to the flow, and beta as low as 0.01. Our simulations
utilize AMR to obtain high spatial resolution where needed; this also allows
the simulation boundaries to be far from the accreting object. Additionally, we
show our results are independent of our exact prescription for accreting mass
in the sink particle. We give simple expressions for the steady-state accretion
rate as a function of beta, M, and field orientation. Using typical molecular
clouds values of M ~ 5 and beta ~ 0.04 from the literature, our fits suggest a
0.4 M_Sun star accretes ~ 4*10^{-9} M_Sun/year, almost a factor of two less
than accretion rates predicted by hydrodynamic models. This disparity grows to
orders of magnitude for stronger fields and lower Mach numbers. We discuss the
applicability of these accretion rates versus accretion rates expected from
gravitational collapse, and when a steady state is possible. This reduction in
Mdot increases the time required to form stars in competitive accretion models,
making such models less efficient. In numerical codes, our results should
enable accurate subgrid models of sink particles accreting from magnetized
media.
