  We present the first 3D Monte Carlo (MC) photoionisation code to include a
fully self-consistent 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. 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.
