The dusty MOCASSIN: fully self-consistent 3D photoionisation and dust radiative transfer models

We present the first 3D Monte Carlo (MC) photoionisation code to include a fully selfconsistent treatment of dust radiative transfer (RT) within a photoionised region. This is the latest development (Version 2.0) of the gas-only photoionisation code MOCASSIN (Ercolano et al., 2003a), and employs a stochastic approach to the transport of radiation, allowing both the primary and secondary components of the radiation field to be treated self-consistently, whilst accounting for the scattering of radiation by dust grains mixed with the gas, as well as the absorption and emission of radiation by both the gas and the dust components. An escape probability method is implemented for the transfer of resonance lines that may be absorbed by the grains, thus contributing to their energy balance. The energetics of the co-existing dust and gas components must also take into account the effects of dust-gas collisions and photoelectric emission from the dust grains, which are dependent on the grain charge. These are included in our code using the average grain potential approximation scheme. A set of rigorous benchmark tests have been carried out for dust-only spherically symmetric geometries and 2D disk configurations. MOCASSIN’s results are found to be in agreement with those obtained by well established dust-only RT codes that employ various approaches to the solution of the RT problem. A model of the dust and of the photoionised gas components of the planetary nebula (PN) NGC 3918 is also presented as a means of testing the correct functioning of the RT procedures in a case where both gas and dust opacities are present. The two components are coupled via the heating of dust grains by the absorption of both UV continuum photons and resonance line photons emitted by the gas. The MOCASSIN results show agreement with those of a 1D dust and gas model of this nebula published previously, showing the reliability of the new code, which can be applied to a variety of astrophysical environments.