Observations on the Formation of Massive Stars by Accretion

Observations of the H66a recombination line from the ionized gas in thecluster of newly formed massive stars, G10.6-0.4, show that most of thecontinuum emission derives from the dense gas in an ionized accretion flow thatforms an ionized disk or torus around a group of stars in the center of thecluster. The inward motion observed in the accretion flow suggests that despitethe equivalent luminosity and ionizing radiation of several O stars, neitherradiation pressure nor thermal pressure has reversed the accretion flow. Theobservations indicate why the radiation pressure of the stars and the thermalpressure of the HII region are not effective in reversing the accretion flow.The observed rate of the accretion flow, 0.001 solar masses/yr, is sufficientto form massive stars within the time scale imposed by their short mainsequence lifetimes. A simple model of disk accretion relates quenched HIIregions, trapped hypercompact HII regions, and photo-evaporating disks in anevolutionary sequence