High-mass Star Forming Regions
The general progression of low-mass star formation is pretty well understood. A starless core undergoes gravitational collapse to form a protostar surrounded by an accretion disk and embedded in an envelope. As it ages it accretes more material and blows away its envelope with outflows and winds. Eventually the accretion disk is also cleared, and a pre-main sequence star is born, perhaps with a protoplanetary disk.
High-mass star formation is less well understood. High-mass star forming regions are comparatively rare, and are therefore further from the Sun and harder to study. High-mass stars form in clusters, so it can be difficult to study the formation of a single object without contamination from its neighbors. It is therefore not known what the timescales and processes are that convert dense gas in a molecular cloud into massive stars. Although in some ways high-mass star formation can be seen simply as scaled-up low-mass star formation, a fundamental difference is the substantial effects of stellar raditiation from the massive protostar that creates an HII region.
In one scenario of massive star formation, a high-mass protostellar object (HMPO) initially has an accretion disk around it which launches jets and outflows. As the HMPO becomes more massive and luminous, it begins to form an HII region around it. This HII region can be seen from the free-free emission at centimeter wavelengths, and it begins to disrupt the accretion disk around it. Eventually the HII region entirely destroys the disk. Working with John Carpenter, I have surveyed 15 HMPO's with the CARMA at 1 arcsecond resolution to test this timeline. Our observations show that there is 3mm continuum flux (from compact envelopes and/or disks) associated with sources that also have outflows and masers (and therefore accretion disks) whether or not there is centimeter emission (and therefore an HII region). We never see 3mm continuum emission from those sources with radio emission but no outflows, indicating the the HII region has indeed disrupted the disk. Our observations are therefore consistent with this model. The paper describing these results can be found here.