Reproducing the Observed Abundances in RCB and HdC Stars with Post-double-degenerate Merger Models-Constraints on Merger and Post-merger Simulations and Physics Processes

Menon, A. and Herwig, F. and Denissenkov, P. A. and Clayton, G. C. and Staff, J. and Pignatari, Marco and Paxton, B. (2013) Reproducing the Observed Abundances in RCB and HdC Stars with Post-double-degenerate Merger Models-Constraints on Merger and Post-merger Simulations and Physics Processes. ASTROPHYSICAL JOURNAL, 772 (1). ISSN 1538-4357

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Abstract

The R Coronae Borealis (RCB) stars are hydrogen-deficient, variable stars that are most likely the result of He-CO WD mergers. They display extremely low oxygen isotopic ratios, 16O/18O ≃ 1 − 10, 12C/ 13C ≥ 100, and enhancements up to 2.6dex in F and in s-process elements from Zn to La, compared to solar. These abundances provide stringent constraints on the physical processes during and after the double-degenerate merger. As shown before O-isotopic ratios observed in RCB stars cannot result from the dynamic double-degenerate merger phase, and we investigate now the role of the long-term 1D spherical post-merger evolution and nucleosynthesis based on realistic hydrodynamic merger progenitor models. We adopt a model for extra envelope mixing to represent processes driven by rotation originating in the dynamical merger. Comprehensive nucleosynthesis post-processing simulations for these stellar evolution models reproduce, for the first time, the full range of the observed abundances for almost all the elements measured in RCB stars: 16O/18O ratios between 9 and 15, C-isotopic ratios above 100, and ∼ 1.4 – 2.35 dex F enhancements, along with enrichments in s-process elements. The nucleosynthesis processes in our models constrain the length and temperature in the dynamic merger shell-of-fire feature as well as the envelope mixing in the post-merger phase. s-process elements originate either in the shellof-fire merger feature or during the post-merger evolution, but the contribution from the AGB progenitors is negligible. The post-merger envelope mixing must eventually cease ∼ 106yr after the dynamic merger phase, before the star enters the RCB phase.

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