Star Formation in W49A: Gravitational Collapse of the Molecular Cloud Core Toward a Ring of Massive Stars

High-resolution molecular line and continuum radio images from the Hat Creek Radio Observatory and the Very Large Array suggest that the core of the W49A star-forming region is undergoing gravitational collapse. The radio continuum shows a 2-parsec ring of at least ten distinct ultracompact H-II regions, each associated with at least one O star. The ring is a region of large-scale, organized massive star formation. Recombination line velocities and HCO(+) excitation requirements indicate that the ring is rotating around 50,000 solar masses of material. Because the HCO(+) (1-0) line shows red-shifted absorption but blue-shifted emission, the molecular cloud core is believed to be collapsing toward the center of the ring. The HCO(+) radial velocities, as well as H-I, H(2)CO, and magnetic-field measurements, fit a simple model of inside-out gravitational collapse of a once magnetically supported cloud.