Effects of opacity temperature dependence on radiatively accelerated clouds

Abstract

ABSTRACT We study how different opacity–temperature scalings affect the dynamical evolution of irradiated gas clouds using time-dependent radiation-hydrodynamics 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 $\sim\! 35{{\ \rm per\ cent}}$ faster than clouds that become optically thin. An enhancement of $\sim\! 85{{\ \rm per\ cent}}$ in the acceleration can be achieved by having a broken power-law 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 $\sim\! 2{{\ \rm per\ cent}}$ 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 reradiation. 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. Our work shows that, for some parameters, the rocket effect due to radiation-ablated matter leaving the back of the cloud is important for cloud acceleration. We suggest that this rocket effect can be at work in active galactic nuclei outflows.