The Formation of Massive Stars: Accretion, Disks, and the Development of Hypercompact HiiRegions

The hypothesis that massive stars form by accretion can be investigated bysimple analytical calculations that describe the effect that the formation of amassive star has on its own accretion flow. Within a simple accretion modelthat includes angular momentum, that of gas flow on ballistic trajectoriesaround a star, the increasing ionization of a massive star growing by accretionproduces a three-stage evolutionary sequence. The ionization first forms asmall quasi-spherical HII region gravitationally trapped within the accretionflow. At this stage the flow of ionized gas is entirely inward. As theionization increases, the HII region transitions to a bipolar morphology inwhich the inflow is replaced by outflow within a narrow range of angle withabout the bipolar axis. At higher rates of ionization, the opening angle of theoutflow region progressively increases. Eventually, in the third stage, theaccretion is confined to a thin region about an equatorial disk. Throughoutthis early evolution, the HII region is of hypercompact to ultracompact sizedepending on the mass of the enclosed star or stars. These small HII regionswhose dynamics are dominated by stellar gravitation and accretion are differentthan compact and larger HII regions whose dynamics are dominated by the thermalpressure of the ionized gas.Comment: accepted by Ap