Athira Menon, Falk Herwig, Pavel A. Denissenkov, Geoffrey C. Clayton, Jan Staff, Marco Pignatari, Bill Paxton
The R Coronae Borealis (RCB) stars along with the H-deficient Carbon stars (HdCs) and Extreme Helium stars (EHes), are believed to be the results of He-CO WD mergers. Their H-poor, C-rich atmospheres are characterized by very unusual isotopic ratios such as extremely low O16/O18 ~ 1 - 10, C12/C13 >= 100, and enhancements up to 2.6 dex in F and in s-process elements from Zn to La, compared to solar. As shown before such O-isotopic ratios cannot result from the dynamic double-degenerate merger phase. We therefore investigate now the role of the long-term post-merger evolution and nucleosynthesis. We construct post-merger 1D spherical models based on realistic merger progenitor models and the outcome of our previous hydrodynamic simulations and follow their evolution into the RCB domain of the HR diagram. Along with surface convection zones, we adopt a model for extra mixing that is meant to represent processes driven by rotation originating in the dynamical merger. We perform complete multi-zone post-processing nucleosynthesis simulations for these stellar evolution models, using a nuclear network that includes over 1000 isotopes. Our models reproduce, for the first time, the full range of the observed abundances for almost all the elements measured in RCB stars: O16/O18 ratios between 9 and 15, C12/C13 > 100, and ~ 1.4-2.35 F enhancements, along with enrichments in s-process elements. The nucleosynthesis processes in our models constrain the conditions of the dynamic merger shell-of-fire feature as well as the envelope mixing in the post-merger phase. The post-merger envelope mixing must eventually become inefficient at 10^6 yr after the dynamic merging phase but before the star enters the RCB phase.
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http://arxiv.org/abs/1211.3392
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