New Light on Young Stars:
Spitzer's View of Circumstellar Disks

Accretion of Gas onto Young Circumstellar Disks: Implications for Disks and Planets

First Author:
Henry Throop
Email: throop AT boulder.swri.edu
SWRI
1050 Walnut St Ste 300
Boulder, CO 80304
Coauthors:
Bally, John, U. Colorado / CASA

Abstract
Young stars orbiting within young 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. We present simulations of accretion by stars orbiting in a variety of young clusters, from N=30 to N=3000 stars. The simulations include the gravitational potential of the molecular gas which smoothly disperses over time. We find that the disks surrounding solar-mass stars accrete roughly 10^-8 M_sol/yr (1 MMSN per Myr). The accretion rate scales as M² for stars of mass M. The accretion rate is a few times lower for N=3000 cluster, due to its higher stellar velocities and higher temperature. The Bondi-Hoyle accretion rates onto the disks are comparable to the accretion rates observed directly onto young stars (e.g., Muzerolle et al 2005): these two accretion rates follow the same M² 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.
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