Radiation Forces on Dust Envelopes

TL;DR

This paper analyzes radiation forces on spherical dust envelopes around luminous stars, providing analytical formulas, numerical code calibration, and insights into different optical regimes, with implications for complex radiation transfer modeling.

Contribution

It introduces a unified approach to quantify radiation forces on dust envelopes, including analytical formulas and numerical code calibration, across various optical regimes.

Findings

Analytical formulas for radiation force parameters in different regimes.

Monte Carlo codes may underestimate radiation forces if not properly resolved.

Calibration of Hyperion and DUSTY codes improves accuracy of radiation force estimates.

Abstract

We address in detail the radiation forces on spherical dust envelopes around luminous stars, and numerical solutions for these forces, as a first step toward more general dust geometries. Two physical quantities, a normalized force and a force-averaged radius, suffice to capture the overall effects of radiation forces; these combine to yield the radiation term in the virial theorem. In addition to the optically thin and thick regimes, the wavelength dependence of dust opacity allows for an intermediate case in which starlight is easily trapped but infrared radiation readily escapes. Scattering adds a non-negligible force in this intermediate regime. We address all three regimes analytically and provide approximate formulae for the force parameters, for arbitrary optical depth and inner dust temperature. Turning to numerical codes, we examine the convergence properties of the Monte Carlo code Hyperion run in Cartesian geometry. We calibrate both Hyperion and our analytical estimate using the DUSTY code, run in spherical geometry. We find that Monte Carlo codes tend to underestimate the radiation force when the mean free path of starlight is not well resolved, as this causes the inner dust temperature, and therefore the inner Rosseland opacity, to be too low. We briefly discuss implications for more complicated radiation transfer problems.