3D photoionisation and dust RT modelling with MOCASSIN: geometry effects   on the emission line spectra of star-forming regions
  Emission line spectra from HII 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. 
Barbara Ercolano & Nate Bastian (Department of Physics and Astronomy   University College London)