Three-dimensional photoionization modelling of the planetary nebula NGC 3918

The three-dimensional Monte Carlo photoionization code MOCASSIN has been applied to construct a realistic model of the planetary nebula NGC 3918. Three different geometric models were tried, the first being the biconical density distribution already used some years ago by Clegg et al. In this model the nebula is approximated by a biconical structure of enhanced density, embedded in a lower-density spherical region. Spindle-like density distributions were used for the other two models (models A and B). Model A used a mass distribution slightly modified from one of Mellema's hydrodynamical models that had already been adopted by Corradi et al. for their observational analysis of NGC 3918. Our spindle-like model B instead used an analytical expression to describe the shape of the inner shell of this object as consisting of an ellipsoid embedded in a sphere. The effects of the interaction of the diffuse fields coming from two adjacent regions of different densities were investigated. These are found to be non-negligible, even for the relatively uncomplicated case of a biconical geometry. We found that the ionization structure of low-ionization species near the boundaries is particularly affected. It is found that all three models provided acceptable matches to the integrated nebular optical and ultraviolet spectrum. Large discrepancies were found between all of the model predictions of infrared fine-structure line fluxes and ISO SWS measurements. This was found to be largely due to an offset of approximate to14 arcsec from the centre of the nebula that affected all of the ISO observations of NGC 3918. For each model, we also produced projected emission-line maps and position-velocity diagrams from synthetic long-slit spectra, which could be compared to recent HST images and ground-based long-slit echelle spectra. This comparison showed that spindle-like model B provided the best match to the observations. Although the integrated emission-line spectrum of NGC 3918 can be reproduced by all three of the three-dimensional models investigated in this work, the capability of creating projected emission-line maps and position-velocity diagrams from synthetic long-slit spectra was found to be crucial in allowing us to constrain the structure of this object.