Bondi-Hoyle-Lyttleton Accretion onto Star Clusters
  An isolated star moving supersonically through a uniform gas accretes
material from its gravitationally-induced wake. The rate of accretion is set by
the accretion radius of the star and is well-described by classical
Bondi-Hoyle-Lyttleton theory. Stars, however, are not born in isolation. They
form in clusters where they accrete material that is influenced by all the
stars in the cluster. We perform three-dimensional hydrodynamic simulations of
clusters of individual accretors embedded in a uniform-density wind in order to
study how the accretion rates experienced by individual cluster members are
altered by the properties of the ambient gas and the cluster itself. We study
accretion as a function of number of cluster members, mean separation between
them, and size of their individual accretion radii. We determine the effect of
these key parameters on the aggregate and individual accretion rates, which we
compare to analytic predictions. We show that when the accretion radii of the
individual objects in the cluster substantially overlap, the surrounding gas is
effectively accreted into the collective potential of the cluster prior to
being accreted onto the individual stars. We find that individual cluster
members can accrete drastically more than they would in isolation, in
particular when the flow is able to cool efficiently. This effect could
potentially modify the luminosity of accreting compact objects in star clusters
and could lead to the rejuvenation of young star clusters as well as globular
clusters with low-inclination and low-eccentricity.
