Capture and escape: The dependence of radiation forces on clumping in dusty envelopes

Abstract

Dust barriers effectively capture the photon momentum of a central light source, but low-density channels, along with re-emission at longer wavelengths, enhance its escape. We use Monte Carlo simulations to study the effects of inhomogeneity on radiation forces imparted to a dust envelope around a central star. We survey the strength and scale of an inhomogeneous perturbation field, as well as the optical depth of its spherical reference state. We run at moderate numerical resolution, relying on our previous resolution study for calibration of the associated error. We find that inhomogeneities matter most when their scale exceeds the characteristic mean free path. As expected, they tend to reduce the net radiation force and extend its range; however, there is significant variance among realizations. Within our models, force integrals correlate with the emergent spectral energy distribution, given a specified set of dust properties. A critical issue is the choice of integral measures of the radiation force: for strong deviations from spherical symmetry the relevant measures assess radial forces relative to the cloud centre of mass. Of these, we find the virial term due to radiation to be the least stochastic of several integral measures in the presence of inhomogeneities.