Abstract
The dynamical evolution of HII regions with and without stellar motion in dense, structured molecular clouds is studied. The cloud gravity is taken into account and is simulated by a time-independent, external force. Several cases, with different stellar velocities and gas density distribution are considered. When stellar motion is considered, stars are allowed to move from the central core to the edge of the cloud and vice versa. There are transitions from ultracompact to extended HII regions as the stars moves into lower density regions. However, the opposite behavior is also feasible when stars fall into the cloud core. The main conclusion of our study is that ultracompact HII regions are pressure-confined entities as long as they remain embedded within dense cores. The confinement in this study comes from either ram or thermal pressures, or a combination of both. Their survival as ultracompact regions only depends on the stellar life-time and on the crossing time of the cloud core.
