  The hypothesis that massive stars form by accretion can be investigated by
simple analytical calculations that describe the effect that the formation of a
massive star has on its own accretion flow. Within a simple accretion model
that includes angular momentum, that of gas flow on ballistic trajectories
around a star, the increasing ionization of a massive star growing by accretion
produces a three-stage evolutionary sequence. The ionization first forms a
small quasi-spherical HII region gravitationally trapped within the accretion
flow. At this stage the flow of ionized gas is entirely inward. As the
ionization increases, the HII region transitions to a bipolar morphology in
which the inflow is replaced by outflow within a narrow range of angle with
about the bipolar axis. At higher rates of ionization, the opening angle of the
outflow region progressively increases. Eventually, in the third stage, the
accretion is confined to a thin region about an equatorial disk. Throughout
this early evolution, the HII region is of hypercompact to ultracompact size
depending on the mass of the enclosed star or stars. These small HII regions
whose dynamics are dominated by stellar gravitation and accretion are different
than compact and larger HII regions whose dynamics are dominated by the thermal
pressure of the ionized gas.
