Double white dwarf mergers and elemental surface abundances in extreme helium and RCoronae Borealis stars

The surface abundances of extreme helium (EHe) and R Coronae Borealis (RCB) stars are discussed in terms of a model for their origin in the merger of a carbon-oxygen white dwarf with a helium white dwarf. The model is expressed as a linear mixture of the individual layers of both constituent white dwarfs, taking account of the specific evolution of each star. In developing this recipe from previous versions, particular attention has been given to the intershell abundances of the asymptotic giant branch (AGB) star which evolved to become the carbon-oxygen white dwarf. Thus the surface composition of the merged star is estimated as a function of the initialmass and metallicity of its progenitor. The question of whether additional nucleosynthesis occurs during the white dwarf merger has been examined by including the results of recent hydrodynamical merger calculations which incorporate the major nuclear networks. The high observed abundances of carbon and oxygen must either originate by dredge-up from the core of the carbon-oxygen white dwarf during a cold merger or be generated directly by a burning during a hot merger. The presence of large quantities of O-18 may be consistent with both scenarios, since a significant O-18 pocket develops at the carbon/helium boundary in a number of our post-AGB models. The production of fluorine, neon and phosphorus in the AGB intershell propagates through to an overabundance at the surface of the merged stars, but generally not in sufficient quantity to match the observed abundances. However, the evidence for an AGB origin for these elements, together with near-normal abundances of magnesium, points to progenitor stars with initial masses in the range 1.9-3 M-circle dot. There is not yet sufficient understanding of the chemical structure of CO white dwarfs, or of nucleosynthesis during a double white dwarf merger, to discriminate the origin (fossil or prompt) of all the abundance anomalies observed in EHe and RCB stars. Further work is required to quantify the expected yields of argon and s-process elements in the AGB intershell, and to improve the predicted yields of all elements from a hot merger.