From Ultracompact to Extended HiiRegions. II. Cloud Gravity and Stellar Motion

The dynamical evolution of HII regions with and without stellar motion indense, structured molecular clouds is studied. Clouds are modeled inhydrostatic equilibrium, with gaussian central cores and external halos thatobey r**-2 and r**-3 density power laws. The cloud gravity is included as atime-independent, external force. Stellar velocities of 0, 2, 8, and 12 km/sare considered. When stellar motion is included, stars move from the centralcore to the edge of the cloud, producing transitions from ultracompact toextended HII regions as the stars move into lower density regions. The oppositebehavior occurs when stars move toward the cloud cores. The main conclusion ofour study is that ultracompact HII regions are pressure-confined entities whilethey remain embedded within dense cores. The confinement comes from ram and/orambient pressures. The survival of ultracompact regions depends on the positionof the star with respect to the core, the stellar life-time, and the corecrossing time. Stars with velocities less than the cloud dispersion velocitycan 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 ofunpredictable structures due to ionization-shock front instability. Someultracompact HII regions with a core-halo morphology might be explained byself-blocking effects, when stars overtake and ionize leading, piled-up clumpsof neutral gas. We use thermal energy to support the cloud against gravity; theresults remain the same if other types of isotropic cloud support are used.Comment: 27 pages, 7 ps figures, 5 png figures; submitted to Astrophysical Journa