The H-deficient non-LTE model atmosphere (Todt et al., 2010), which was calculated using the PoWR models for expanding atmospheres (Gräfener et al., 2002; Hamann & Gräfener, 2004), was used as an ionizing source in our photoionization models. The PoWR models were constructed by solving the non-LTE radiative transfer equation of an expanding stellar atmosphere under the assumptions of spherical symmetry and chemical homogeneity. The PoWR model used was calculated for the stellar surface abundances H:He:C:N:O = 40:55:1.3:2:1.3 (by mass), the stellar temperature =52kK, the stellar luminosity , the transformed radius R and the wind terminal velocity kms, which well matches the dereddened stellar spectra from FUSE, IUE and MIKE, as well as 2MASS JHK bands (Todt et al., 2010). We see that the nebular OIII 5007 line flux relative to the H flux is well reproduced with an effective temperature of kK in our photoionization models. The stellar luminosity of adopted by Todt et al. (2010) is related to a remnant core with a typical mass of (e.g. Miller Bertolami & Althaus, 2007; Schönberner et al., 2005a). The distance was also varied in order to reproduce the nebular emission-line fluxes, under the constraints of our adopted stellar parameters and spherical density distribution. The best results for the photoionization models were obtained at a distance of 4.9 kpc.
Figure 3 compares the non-LTE model atmosphere flux of PB 8 with a blackbody flux at the same temperature. At energies higher than 54 eV (HeII ground state), there is a significant difference between the non-LTE model atmosphere and blackbody flux. As discussed by Rauch (2003), a blackbody is not an accurate representation of the ionizing flux. The H-deficient non-LTE model atmosphere has a major departure from the solar model atmosphere at higher energies due to the small opacity from hydrogen. In our photoionization models, we theretofore used an non-LTE model atmosphere as the ionizing source to provide the best fit to the nebular spectrum. However, the difference may not be largely noticeable in our model as high-excitation lines (e.g. HeII) are not observed.