A solution to the pre-main-sequence accretion problem

Accretion rates of order 10(-8) yr-1 are observed in young pre-main-sequence (PMS) stars of approximately a solar mass with evidence of circumstellar disks. The accretion rate is significantly lower for PMS stars of smaller mass, approximately proportional to the second power of the stellar mass (M)over dot(accr) proportional to M-2. The traditional view is that the observed accretion is the consequence of the angular momentum transport in isolated circumstellar disks, controlled by disk turbulence or self-gravity. However, these processes are not well understood and the observed accretion, a fundamental aspect of star formation, remains an unsolved problem. In this Letter, we propose the stellar accretion rate is controlled by accretion from the large-scale gas distribution in the parent cloud, not by the isolated disk evolution. Approximating this process as Bondi-Hoyle accretion onto the star-disk system, we obtain accretion rates comparable to the observed ones. We also reproduce the observed dependence of the accretion rate on the stellar mass. These results are based on realistic values of the ambient gas density and velocity, as inferred from numerical simulations of star formation in self-gravitating turbulent clouds.