The Formation of Massive Stars by Accretion through Trapped Hypercompact HII Regions

The formation of massive stars may take place at relatively low accretion rates over a long period of time if the accretion can continue past the onset of core hydrogen ignition. The accretion may continue despite the formation of an ionized HII region around the star if the HII region is small enough that the gravitational attraction of the star dominates the thermal pressure of the HII region. The accretion may continue despite radiation pressure acting against dust grains in the molecular gas if the momentum of the accretion flow is sufficient to push the dust grains through a narrow zone of high dust opacity at the ionization boundary and into the HII region where the dust is sublimated. This model of massive star formation by continuing accretion predicts a new class of gravitationally-trapped, long-lived hypercompact HII regions. The observational characteristics of the trapped hypercompact HII regions can be predicted for comparison with observations. Subject headings: accretion, ISM: HII regions