%A B Ercolano
%A R Wesson
%A Y Zhang
%A MJ Barlow
%A O De Marco
%A T Rauch
%A XW Liu
%T Observations and three-dimensional photoionization modelling of the Wolf-Rayet planetary nebula NGC 1501
%J Monthly Notices of the Royal Astronomical Society
%K atomic data, stars : Wolf-Rayet, ISM : abundances, planetary nebulae : individual : NGC 1501
%X Deep optical spectra of the high‐excitation planetary nebula NGC 1501 and its W04 central star are presented. A recombination line abundance analysis of the emission‐line spectrum of the central star yields He : C : O mass fractions of 0.36 : 0.48 : 0.16, similar to those of PG 1159 stars. A detailed empirical analysis of the nebular collisionally excited line (CEL) and optical recombination line (ORL) spectrums are presented, together with fully three‐dimensional photoionization modelling of the nebula. We found very large ORL–CEL abundance discrepancy factors (ADFs) for O^{2+} (32) and Ne^{2+} (33). The mean value of ∼5100 K for the T_{e} derived from He I recombination lines ratios is 6000 K, lower than the value of 11 100 K implied by the [O III] line ratio. This result indicates the existence of a second, low-temperature nebular component, which could account for the observed ORL emission. Electron temperature
fluctuations (t^{2}) cannot account for the high ADFs found from our optical spectra of this nebula. A three‐dimensional photoionization model of NGC 1501 was constructed using the photoionization code_{mocassin}, based on our new spectroscopic data and using the three‐dimensional electron density distribution determined from long‐slit echellograms of the nebula by Ragazzoni et al. The central star ionizing radiation field is approximated by a model atmosphere, calculated using the Tübingen non‐local thermodynamic equilibrium model atmosphere package, for abundances typical of the W04 nucleus of NGC 1501 and PG 1159 stars. The nebular emission‐line spectrum was best reproduced using a central star model with an effective temperature of Teff= 110 kK and a luminosity of L_{*}= 5000 L_{⊙}. The initial models showed higher degrees of ionization of heavy elements than indicated by observations. We investigated the importance of the missing low‐temperature dielectronic recombination rates for third‐row elements and have estimated upper limits to their rate coefficients.

Our single‐phase, three‐dimensional photoionization model heavily underpredicts the optical recombination line emission. We conclude that the presence of a hydrogen‐deficient, metal‐rich component is necessary to explain the observed ORL spectrum of this object. The existence of such knots could also provide a softening of the radiation field, via the removal of ionizing photons by absorption in the knots, thereby helping to alleviate the overionization of the heavy elements in our models.
%I OXFORD UNIV PRESS
%L discovery10095990
%N 2
%V 354
%O This version is the version of record. For information on re-use, please refer to the publisher's terms and conditions.
%D 2004
%P 558-574