Effects of Opacity Temperature Dependence on Radiatively Accelerated Clouds

We study how different opacity-temperature scalings affect the dynamical evolution of irradiated gas clouds using time-dependent, radiation-hydrodynamics (rad-HD) simulations. When clouds are optically thick, the bright side heats up and expands, accelerating the cloud via the rocket effect. Clouds that become more optically thick as they heat accelerate $\sim 35\%$ faster than clouds that become optically thin. An enhancement of $\sim 85\%$ in the acceleration can be achieved by having a broken powerlaw opacity profile, which allows the evaporating gas driving the cloud to become optically thin and not attenuate the driving radiation flux. We find that up to $\sim 2\%$ of incident radiation is re-emitted by accelerating clouds, which we estimate as the contribution of a single accelerating cloud to an emission or absorption line. Re-emission is suppressed by "bumps" in the opacity-temperature relation since these decrease the opacity of the hot, evaporating gas, primarily responsible for the re-radiation. If clouds are optically thin, they heat nearly uniformly, expand and form shocks. This triggers the Richtmyer-Meshkov instability, leading to cloud disruption and dissipation on thermal time-scales.