  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.
