Three-dimensional photoionization modelling of the hydrogen-deficient knots in the planetary nebula Abell 30
We have constructed a photoionization model, using the three-dimensional Monte Carlo code MOCASSIN, for one of the hydrogen-deficient knots (J3) of the born-again planetary nebula Abell 30. The model consists of spherical knots, comprising a cold, dense, hydrogen-deficient core with very high metal abundances. The inner core, occupying 9.1 per cent of the total volume of the knot, is surrounded by a less dense hydrogen-deficient and metal-enriched gas envelope, with less extreme abundances. The envelope of the knot might have been formed by the mixing of the knot material with the surrounding nebular gas.This bi-chemistry, bi-density model did not produce enough heating to match the fluxes of the collisionally excited emission lines (CELs) and of the optical recombination lines (ORLs) observed in the spectrum of the knot. We therefore included heating by photoelectric emission from dust grains in the thermal equilibrium calculations, and found that dust-to-gas ratios of 0.077 and 0.107 by mass for the central core and the envelope of the knot, respectively, are sufficient to fit the spectrum. Surprisingly, photoelectric emission from grains is the dominant source of heating in the hot envelope of the knot, while heating by photoionization of helium and heavy elements dominates in the cold core. We obtain a good fit with the observations for most of the significant emission lines treated in our model. The two major discrepancies occurred for the [O II] 3727,29-Å doublet and the [N II] 6548,6584-Å lines, which are severely underestimated in our model. Recombination contributions could be significant and we included them for the O II transitions. However, this was not sufficient to resolve the discrepancy, due to the high collisional de-excitation rates in the dense core, where most of the recombination lines would be produced. This possibly highlights a weakness in using a discontinuous density distribution like ours, where in reality one might expect an intermediate phase to exist. The chemical abundances inferred from our modelling of the central core region and of the envelope of the knot are, at least qualitatively, in agreement with the abundances derived by the empirical analysis of Wesson, Liu & Barlow, although the discrepancies between the core and the envelope abundances that we find are less dramatic than those implied by the ORL and CEL empirical analysis. Our models also indicate, in agreement with the empirical analysis of Wesson et al., that the C/O ratio in the two regions of the knot is less than unity, contrary to theoretical predictions for born-again nebulae.
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- October 2003
- ISM: abundances;
- planetary nebulae: individual: Abell 30;
- 11 pages, 4 figures. Accepted for publication by MNRAS