Detailed Far-UV to Optical Analysis of Four [WR] Stars

Abstract

We present far-UV to optical analyses of four hydrogen deficient central stars of planetary nebulae: BD+303639, NGC 40, NGC 5315 and NGC 6905. Using the radiative transfer code CMFGEN, we determined new physical parameters and chemical abundances for these stars. The results were analyzed in the context of the [WR] => PG 1159 evolution via the transformed radius(Rt)-temperature and HR diagrams. NGC 5315 showed itself as an odd object among the previously analyzed central stars. Its temperature (~76kK) is considerably lower than other early-type [WR] stars (~120-150kK). From our models for NGC 5315 and NGC 6905, it is unclear if early-type [WR] stars have smaller C/He mass ratios than other spectral classes, as claimed in the literature. A ratio of ~0.8 is found for NGC 6905. We analyzed FUSE spectra of these stars for the first time, and identified phosphorus in the spectra of BD+303639, NGC 40 and NGC 5315 through the transitions P V 1118,1128. The Fe, Si, P, S and Ne abundances were analyzed in the context of the nucleosynthesis occurring in previous evolutionary phases. We found evidence for Fe deficiency in BD +30 3639 and NGC 5315, and from fits to the Si IV lines we determined a solar Si abundance for BD+303639 and NGC 40. For phosphorus, an oversolar abundance in the NGC 5315 model was preferred, while in the other stars a solar phosphorus abundance cannot be discarded. Regarding sulfur, we estimated upper limits for its abundance, since no conspicuous lines can be seen in the observed spectra. We found that Ne is overabundant in BD +30 3639. In the other stars, Ne is weak or undetectable and upper limits for its abundance were estimated. Our results are in agreement with theoretical predictions and show the usefulness of [WR] stars as testbeds for nucleosynthesis calculations in the AGB and post-AGB phases.(abridged)