The Dynamics of Ultracompact HII Regions

Nathaniel Roth
Steven W. Stahler
Eric Keto

Dept. of Physics, U. of California, Berkeley CA 94720 USA
Dept. of Astronomy, U. of California, Berkeley, CA 94720 USA
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 USA

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 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} msun/yr. Stellar radiation pressure acts to steepen the density gradient of the wind.

Reference: MNRAS
Status: Manuscript has been accepted