  Young stars orbiting in the gravitational potential well of forming star
clusters pass through the cluster's dense molecular gas and can experience
Bondi-Hoyle accretion from reservoirs outside their individual protostellar
cloud cores. Accretion can occur for several million years after the stars
form, but before the cluster disperses. This accretion is predominantly onto
the disk and not the star. N-body simulations of stars orbiting in three young
model clusters containing 30, 300, and 3000 stars are presented. The
simulations include the gravitational potential of the molecular gas which
smoothly disperses over time. The clusters have a star formation efficiency of
33% and a radius of 0.22 pc. We find that the disks surrounding solar-mass
stars in the N=30 cluster accretes ~0.01 M_sol (~1 minimum-mass solar nebula,
MMSN) per Myr. The accretion rate scales as M^2.1 for stars of mass M. The
accretion rate is ~5 times lower for N=3000 cluster, due to its higher stellar
velocities and higher temperature. The Bondi-Hoyle accretion rates onto the
disks are several times lower than accretion rates observed directly onto young
stars (e.g., Muzerolle et al 2005): these two accretion rates follow the same
M^2 behavior and may be related. The accreted disk mass is large enough that it
may have a substantial and unappreciated effect on disk structure and the
formation of planetary systems. We discuss a variety of implications of this
process, including its effect on metallicity differences between cluster stars,
compositional differences between a star and its disk, the formation of
terrestrial and gas-giant planets, and isotopic anomalies observed in our Solar
System.
