Dusty Cloud Acceleration with Multiband Radiation

Abstract

We perform two-dimensional and three-dimensional simulations of cold, dense clouds, which are accelerated by radiation pressure on dust relative to a hot, diffuse background gas. We examine the relative effectiveness of acceleration by ultraviolet (UV) and infrared (IR) radiation fields, both independently and acting simultaneously on the same cloud. We study clouds that are optically thin to IR emission but with varying UV optical depths. Consistent with previous work, we find relatively efficient acceleration and long cloud survival times when the IR band flux dominates over the UV flux. However, when the UV flux is dominant or even a modest percentage (∼5%–10%) of the IR irradiating flux, it can act to compress the cloud, first crushing it and then disrupting the outer layers. This drives mixing of the outer regions of the dusty gas with the hot diffuse background to the point where most dust is not likely to survive or stay coupled to the gas. Hence, the cold cloud is unable to survive for a long enough timescale to experience significant acceleration before disruption even though efficient IR cooling keeps the majority of the gas close to the radiative equilibrium temperature ( T ≲ 100 K). We discuss the implications for observed systems, concluding that radiation pressure driving is most effective when the light from star-forming regions is efficiently reprocessed into the IR.