@article{discovery10095990,
           month = {October},
          number = {2},
            year = {2004},
           pages = {558--574},
           title = {Observations and three-dimensional photoionization modelling of the Wolf-Rayet planetary nebula NGC 1501},
          volume = {354},
            note = {This version is the version of record. For information on re-use, please refer to the publisher's terms and conditions.},
       publisher = {OXFORD UNIV PRESS},
         journal = {Monthly Notices of the Royal Astronomical Society},
        abstract = {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 {$\sim$}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 T{\"u}bingen 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\_\{{$\odot$}\}. 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.},
        keywords = {atomic data, stars : Wolf-Rayet, ISM : abundances, planetary nebulae : individual : NGC 1501},
             url = {https://doi.org/10.1111/j.1365-2966.2004.08218.x},
          author = {Ercolano, B and Wesson, R and Zhang, Y and Barlow, MJ and De Marco, O and Rauch, T and Liu, XW}
}