Journal Article

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

C. S. Jeffery, A. I. Karakas and H. Saio

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 414, issue 4, pages 3599-3616
Published in print July 2011 | ISSN: 0035-8711
Published online July 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.18667.x
Double white dwarf mergers and elemental surface abundances in extreme helium and R Coronae Borealis stars

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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 initial mass 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 α burning during a hot merger. The presence of large quantities of 18O may be consistent with both scenarios, since a significant 18O 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.

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.

Keywords: stars: abundances; stars: AGB and post-AGB; binaries: close; stars: chemically peculiar; stars: evolution; white dwarfs

Journal Article.  11961 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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