The R Coronae Borealis stars - atmospheres and abundances

An abundance analysis of the H-deficient and He- and C-rich R Coronae Borea lis (R CrB) stars has been undertaken to examine the ancestry of the stars. The investigation is based on high-resolution spectra and line-blanketed H -deficient model atmospheres. The models successfully reproduce the flux di stributions and all spectral features, both molecular bands and high-excita tion transitions, with one important exception, the CI lines. Since photoio nization of CI dominates the continuous opacity, the line strengths of CI a re essentially independent of the adopted carbon abundance and stellar para meters. All predicted Cr lines are, however, much too strong compared with observations, with a discrepancy in abundance corresponding to 0.6 dex with little star-to-star scatter. Various solutions of this "carbon problem" ha ve been investigated. A possible solution is that classical model atmospher es are far from adequate descriptions of supergiants such as the RCrB stars . We can also not exclude completely, however, the possibility that the gf- values for the CI lines are in error. This is supported by the fact that th e CII, [CI] and Ca lines are reproduced by the models with no apparent comp lications. In spite of the carbon problem, various tests suggest that abundance ratios are little affected by the uncertainties. Judging by chemical composition, the RCrB stars can be divided into a homogeneous majority group and a dive rse minority, which is characterized by extreme abundance ratios, in partic ular as regards Si/Fe and S/Fe. All stars show evidence of H- and He-burnin g in different episodes as well as mild s-process enhancements. Four of the majority members are Li-rich, while overabundances of Na, Al, Si and S are attributes of all stars. An anti-correlation found between the H and Fe ab undances of H-deficient stars remains unexplained. These enigmatic stars ar e believed to be born-again giants, formed either through a final He-shell flash in a post-AGE star or through a merger of two white dwarfs. Owing to a lack of theoretical predictions of the resulting chemical compositions, i dentification of the majority and minority groups with the two scenarios is unfortunately only preliminary. Furthermore, Sakurai's object and V854 Cen exhibit aspects of both majority and minority groups, which may suggest th at the division into two groups is too simplistic.