Detailed Far‐Ultraviolet 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 +30 3639, 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-temperature (RT × T*) and H-R diagrams. NGC 5315 showed itself as an odd object among the previously analyzed central stars. Its temperature (~76 kK) is considerably lower than other early-type [WR] stars (~120-150 kK). 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 +30 3639, NGC 40, and NGC 5315 through the doublet transition P V λλ1118, 1128 (3p2Po-3s2S). 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 we determined a solar Si abundance for BD +30 3639 and NGC 40. Regarding P, an oversolar abundance in the NGC 5315 model was preferred. Upper limits for the S abundance were estimated. We found that Ne is overabundant in BD +30 3639. In the other stars, Ne is weak or undetectable. Our results are in agreement with theoretical predictions and show the usefulness of [WR] stars as test beds for nucleosynthesis calculations in the AGB and post-AGB phases.