The Dynamics of Ultracompact HII Regions
  Many ultracompact HII regions exhibit a cometary morphology in radio
continuum emission. In such regions, a young massive star is probably ablating,
through its ultraviolet radiation, the molecular cloud clump that spawned it.
On one side of the star, the radiation drives an ionization front that stalls
in dense molecular gas. On the other side, ionized gas streams outward into the
more rarefied environment. This wind is underpressured with respect to the
neutral gas. The difference in pressure draws in more cloud material, feeding
the wind until the densest molecular gas is dissipated.
  Recent, time-dependent simulations of massive stars turning on within
molecular gas show the system evolving in a direction similar to that just
described. Here, we explore a semi-analytic model in which the wind is
axisymmetric and has already achieved a steady state. Adoption of this
simplified picture allows us to study the dependence of both the wind and its
bounding ionization front on the stellar luminosity, the peak molecular
density, and the displacement of the star from the center of the clump. For
typical parameter values, the wind accelerates transonically to a speed of
about 15 km / s, and transports mass outward at a rate of 10^-4 solar masses
per year. Stellar radiation pressure acts to steepen the density gradient of
the wind.
