Three-dimensional photoionisation modelling of the hydrogen-deficient knots in the planetary nebula Abell 30

摘要

We have constructed a photoionization model, using the 3-D Monte Carlo codeMOCASSIN for one of the hydrogen-deficient knots (J3) of the born-againplanetary nebula Abell 30. The model consists of spherical knots, comprising acold, dense, hydrogen-deficient core with very high metal abundances. The innercore is surrounded by a less dense hydrogen-deficient and metal-enriched gasenvelope, with less extreme abundances. The envelope of the knot might havebeen formed by the mixing of the knot material with the surrounding nebulargas. This bi-chemistry, bi-density model did not produce enough heating tomatch the fluxes of the collisionally excited emission lines (CELs) and of theoptical recombination lines (ORLs) observed in the spectrum of the knot. Wetherefore included heating by photoelectric emission from dust grains in thethermal equilibrium calculations, and found that dust-to-gas ratios of 0.077and 0.107 by mass for the central core and the envelope of the knot,respectively, are sufficient to fit the spectrum. Surprisingly, photoelectricemission from grains is the dominant source of heating in the hot envelope ofthe knot, while heating by photoionization of helium and heavy elementsdominates in the cold core. The chemical abundances inferred from our modellingindicate, in agreement with the empirical analysis of Wesson et al. (2003),that the C/O ratio in the two regions of the knot is less than unity, contraryto theoretical predictions for born-again nebulae.