An algorithm for Monte-Carlo time-dependent radiation transfer

TL;DR

This paper introduces a novel Monte-Carlo algorithm for time-dependent radiative transfer that handles multiple regimes and includes scattering, validated against benchmarks and applied to model circumstellar disc variability.

Contribution

The paper presents a new Monte-Carlo algorithm for LTE radiative transfer that is polychromatic, includes scattering, and works across different optical regimes.

Findings

The algorithm agrees well with benchmark solutions.

Time lag between optical and infrared variability is linear with wavelength.

The method effectively models spectral energy distribution in circumstellar discs.

Abstract

A new Monte-Carlo algorithm for calculating time-dependent radiative-transfer under the assumption of LTE is presented. Unlike flux-limited diffusion the method is polychromatic, includes scattering, and is able to treat the optically thick and free-streaming regimes simultaneously. The algorithm is tested on a variety of 1-d and 2-d problems, and good agreement with benchmark solutions is found. The method is used to calculate the time-varying spectral energy distribution from a circumstellar disc illuminated by a protostar whose accretion luminosity is varying. It is shown that the time lag between the optical variability and the infrared variability results from a combination of the photon travel time and the thermal response in the disc, and that the lag is an approximately linear function of wavelength.