Interactions of Massive Stars with Their Parental Clouds
  Here we discuss the interaction of massive stars with their parental molecular clouds. A summary of the dynamical evolution of HII regions and wind-driven bubbles in high-pressure cloud cores is given. Both ultracompact HII regions and ultracompact wind-driven bubbles can reach pressure equilibrium with their surrounding medium. The structures stall their expansion and become static and, as long as the ionization sources and the ambient densities remain about constant, the resulting regions are stable and long lived. For cases with negative density gradients, and depending on the density distribution, some regions never reach the static equilibrium condition. For power-law density stratifications, $\rho \propto r^{-w}$, the properties of the evolution depend on a critical exponent, $w_{crit}$, above which the ionization front cannot be slowed down by recombinations or new ionizations, and the cloud becomes fully ionized. This critical exponent is $w_{crit}=3/2$ during the expansion phase. For $w>3/2 $ the gas expands supersonically into the surrounding ionized medium, and there are two regimes separated by $w=3$. For $3/2<w\leq 3$, the slow regime, the inner region drives a weak shock moving with almost constant velocity through the cloud. For $w>3$, the fast regime, the shock becomes strong and accelerates with time. Finally, the evolution of slow winds in highly pressurized region is described briefly. 
J. Franco (UNAM), G. Garcia-Segura (UNAM) and T. Plewa (MPA)