A Solution to the Pre-Main-Sequence Accretion Problem

Accretion rates of order 10^-8 M_\odot/yr are observed in young protostars ofapproximately a solar mass with evidence of circumstellar disks. The accretionrate is significantly lower for protostars of smaller mass, approximatelyproportional to the second power of the stellar mass, \dot{M}_accr\propto M^2.The traditional view is that the observed accretion is the consequence of theangular momentum transport in isolated protostellar disks, controlled by diskturbulence or self--gravity. However, these processes are not well understoodand the observed protostellar accretion, a fundamental aspect of starformation, remains an unsolved problem. In this letter we propose theprotostellar accretion rate is controlled by accretion from the large scale gasdistribution in the parent cloud, not by the isolated disk evolution.Describing this process as Bondi--Hoyle accretion, we obtain accretion ratescomparable to the observed ones. We also reproduce the observed dependence ofthe accretion rate on the protostellar mass. These results are based onrealistic values of the ambient gas density and velocity, as inferred fromnumerical simulations of star formation in self--gravitating turbulent clouds.Comment: 4 pages, 2 figures, ApJ Letters, in pres