Hydrodynamic Instability of Ionization Front in HII Regions: From Linear to Nonlinear Evolution

We investigate hydrodynamic instabilities at an ionization front in a radiatively driven molecular cloud, using two-dimensional hydrodynamic simulations, including absorption of UV radiation from OB stars, recombination in HII gas, and radiative molecular cooling. We find a strong stabilization mechanism, if the initial perturbation is small, whereas column like structures can emerge, if the perturbation is large. Recombination plays a key role in the observed stabilization mechanism. When recombination is turned off in the simulations, the stabilization disappears, and the perturbation grows at a growth rate consistent with the classical Rayleigh-Taylor instability.