3D photoionisation and dust RT modelling with MOCASSIN: geometry effects on the emission line spectra of star-forming regions

Emission line spectra from H II regions are often used to study properties of the gas in star-forming regions, as well as temperatures and luminosities of the ionising sources. Empirical diagnostics for the interpretation of observational data must often be calibrated with the aid of photoionisation models. Most studies so far have been carried out by assuming spherical or plane-parallel geometries, with major limitations on allowed gas and dust density distributions and with the spatial distribution of multiple, non-centrally-located ionising sources not being accounted for. We present the first results of our theoretical study of geometric effects, via the construction of a number of 3D photoionisation models using the MOCASSIN code for a variety of spatial configurations and ionisation sources. We compare integrated emission line spectra from such configurations and show evidence of systematic errors caused by the simplifying assumption of a single, central location for all ionising sources. Effects of 3D geometry on the emission line spectra of ionised regions The MOCASSIN code: Spatially-resolved studies of star-forming regions indicate that the assumption of spherical geometry is not realistic in most cases, further complication is added by the gas being ionised by multiple non-centrally located sources. The 3D photoionisation code MOCASSIN was designed to treat all of the above, allowing for the transfer of both primary and secondary radiation to be treated self-consistently, without the need of approximations. The code was thoroughly benchmarked (Ercolano et al. 2003, 2005) and has been applied to the study of several ionised regions. The current version includes a fully self-consistent treatment of the radiative transfer of dust grains mixed within the gas, taking into account the microphysics of dust-gas interactions within the geometry-independent Monte Carlo transfer. 3D gas and star distribution on the emission line spectra of ionised regions: We are investigating geometric effects theoretically, via the construction of a number of 3D photoionisation models for a variety of spatial configurations and ionisation sources. We compare integrated emission line spectra from such configurations in search of systematic errors, which may be caused by the simplifying assumption of a single central location for all ionising sources. Our study is initially tailored at H II region spectra, but will successively be extended to star clusters and cluster complexes. Our preliminary models are based on homogeneous or porous spherical density distributions ionised by 240 sources, comprising of 37 M⊙ (35 kK) stars and 56 M⊙ (56 kK) stars. The stars are either located at the centre or distributed in the halfor full-spherical volume. The ionising fluxes were calculated using STARBURST99 (Leitherer et al. 1999), assuming solar metallicities and an age of 1Myr.