From Ultracompact to Extended HII Regions. II: Cloud Gravity and Stellar   Motion
  The dynamical evolution of HII regions with and without stellar motion in dense, structured molecular clouds is studied. Clouds are modeled in hydrostatic equilibrium, with gaussian central cores and external halos that obey r**-2 and r**-3 density power laws. The cloud gravity is included as a time-independent, external force. Stellar velocities of 0, 2, 8, and 12 km/s are considered. When stellar motion is included, stars move from the central core to the edge of the cloud, producing transitions from ultracompact to extended HII regions as the stars move into lower density regions. The opposite behavior occurs when stars move toward the cloud cores. The main conclusion of our study is that ultracompact HII regions are pressure-confined entities while they remain embedded within dense cores. The confinement comes from ram and/or ambient pressures. The survival of ultracompact regions depends on the position of the star with respect to the core, the stellar life-time, and the core crossing time. Stars with velocities less than the cloud dispersion velocity can produce cometary shapes smaller than 0.1 pc at times of 20,000 yr or more. The sequence Ultracompact to Compact to Extended HII region shows a variety of unpredictable structures due to ionization-shock front instability. Some ultracompact HII regions with a core-halo morphology might be explained by self-blocking effects, when stars overtake and ionize leading, piled-up clumps of neutral gas. We use thermal energy to support the cloud against gravity; the results remain the same if other types of isotropic cloud support are used. 
Jose Franco, Guillermo Garcia-Segura, and Stan Kurtz