Detailed Far‐Ultraviolet to Optical Analysis of Four [WR] Stars

We present far-UV to optical analyses of four hydrogen deficient centralstars of planetary nebulae: BD+303639, NGC 40, NGC 5315 and NGC 6905. Using theradiative transfer code CMFGEN, we determined new physical parameters andchemical abundances for these stars. The results were analyzed in the contextof the [WR] => PG 1159 evolution via the transformed radius(Rt)-temperature andHR diagrams. NGC 5315 showed itself as an odd object among the previouslyanalyzed central stars. Its temperature (~76kK) is considerably lower thanother early-type [WR] stars (~120-150kK). From our models for NGC 5315 and NGC6905, it is unclear if early-type [WR] stars have smaller C/He mass ratios thanother spectral classes, as claimed in the literature. A ratio of ~0.8 is foundfor NGC 6905. We analyzed FUSE spectra of these stars for the first time, andidentified phosphorus in the spectra of BD+303639, NGC 40 and NGC 5315 throughthe transitions P V 1118,1128. The Fe, Si, P, S and Ne abundances were analyzedin the context of the nucleosynthesis occurring in previous evolutionaryphases. We found evidence for Fe deficiency in BD +30 3639 and NGC 5315, andfrom fits to the Si IV lines we determined a solar Si abundance for BD+303639and NGC 40. For phosphorus, an oversolar abundance in the NGC 5315 model waspreferred, while in the other stars a solar phosphorus abundance cannot bediscarded. Regarding sulfur, we estimated upper limits for its abundance, sinceno conspicuous lines can be seen in the observed spectra. We found that Ne isoverabundant in BD +30 3639. In the other stars, Ne is weak or undetectable andupper limits for its abundance were estimated. Our results are in agreementwith theoretical predictions and show the usefulness of [WR] stars as testbedsfor nucleosynthesis calculations in the AGB and post-AGB phases.(abridged)