The R Coronae Borealis stars - atmospheres and abundances

Abstract

An abundance analysis of the H-deficient and He- and C-rich R Coronae Borealis (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 distributions and all spectral features, both molecular bands and high-excitation transitions, with one important exception, the C i lines. Since photoionization of C i dominates the continuous opacity, the line strengths of C i are essentially independent of the adopted carbon abundance and stellar parameters. All predicted C i 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'' have been investigated. A possible solution is that classical model atmospheres are far from adequate descriptions of supergiants such as the R CrB stars. We can also not exclude completely, however, the possibility that the gf-values for the C i lines are in error. This is supported by the fact that the C ii, [C i] and C_2 lines are reproduced by the models with no apparent complications. In spite of the carbon problem, various tests suggest that abundance ratios are little affected by the uncertainties. Judging by chemical composition, the R CrB stars can be divided into a homogeneous majority group and a diverse minority, which is characterized by extreme abundance ratios, in particular as regards Si/Fe and S/Fe. All stars show evidence of H- and He-burning 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 abundances of H-deficient stars remains unexplained. These enigmatic stars are believed to be born-again giants, formed either through a final He-shell flash in a post-AGB star or through a merger of two white dwarfs. Owing to a lack of theoretical predictions of the resulting chemical compositions, identification 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 that the division into two groups is too simplistic.