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A 3D photoionization model of the extreme planetary nebula NGC 6302




We present a 3D photoionization model of the planetary nebula NGC 6302, one of the most complex and enigmatic objects of its kind. Its highly bipolar geometry and dense massive disc, coupled with the very wide range of ions present, from neutral species up to Si8 +, make it one of the ultimate challenges to nebular photoionization modelling.

Our mocassin model is composed of an extremely dense geometrically thin circumstellar disc and a large pair of diffuse bipolar lobes, a combination which was necessary to reproduce the observed emission-line spectrum. The masses of these components, 2.2 M and 2.5 M, respectively, give a total nebular mass of 4.7 M, of which 1.8 M (39 per cent) is ionized. Discrepancies between our model fit and the observations are attributed to complex density inhomogeneities in the nebula. The potential to resolve such discrepancies with more complex models is confirmed by exploring a range of models introducing small-scale structures. Compared to solar abundances helium is enhanced by 50 per cent, carbon is slightly subsolar, oxygen is solar, and nitrogen is enhanced by a factor of 6. These all imply a significant third dredge-up coupled with hot-bottom burning CN-cycle conversion of dredged-up carbon to nitrogen. Aluminium is also depleted by a factor of 100, consistent with depletion by dust grains.

The central star of NGC 6302 is partly obscured by the opaque circumstellar disc, which is seen almost edge-on, and as such its properties are not well constrained. However, emission from a number of high-ionization ‘coronal’ lines provides a strong constraint on the form of the high-energy ionizing flux. We model emission from the central star using a series of stellar model atmospheres, the properties of which are constrained from fits to the high-ionization nebular emission lines. Using a solar abundance stellar atmosphere we are unable to fit all of the observed line fluxes, but a substantially better fit was obtained using a 220 000 K hydrogen-deficient stellar atmosphere with log g= 7.0 and L= 14 300 L. The H-deficient nature of the central star atmosphere suggests that it has undergone some sort of late thermal pulse, and fits to evolutionary tracks imply a central star mass of 0.73–0.82 M. Time-scales for these evolutionary tracks suggest the object left the top of the asymptotic giant branch ∼2100 years ago, in good agreement with studies of the recent mass-loss event that formed one pair of the bipolar lobes. Based on the modelled nebular mass and central star mass we estimate the initial mass of the central star to be 5.5 M, in approximate agreement with that derived from evolutionary tracks.