Observations on the formation of massive stars by accretion

Observations of the H66 alpha recombination line from the ionized gas in the cluster of newly formed massive stars, G10.6-0.4, show that most of the continuum emission derives from the dense gas in an ionized accretion flow that forms an ionized disk or torus around a group of stars in the center of the cluster. The inward motion observed in the accretion flow suggests that despite the equivalent luminosity and ionizing radiation of several O stars, neither radiation pressure nor thermal pressure has reversed the accretion flow. The observations indicate why the radiation pressure of the stars and the thermal pressure of the H II region are not effective in reversing the accretion flow. The observed rate of the accretion flow, 10(-3) M-circle dot yr(-1), is sufficient to form massive stars within the timescale imposed by their short main-sequence lifetimes. A simple model of disk accretion relates quenched H II regions, trapped hyper-compact H II regions, and photoevaporating disks in an evolutionary sequence.