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

Athira Menon, Falk Herwig, Pavel A. Denissenkov, Geoffrey C. Clayton, Jan Staff, Marco Pignatari, Bill Paxton

Research output: Contribution to journalArticlepeer-review

22 Citations (Scopus)

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 2.6 dex 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 previously, O-isotopic ratios observed in RCB stars cannot result from the dynamic double-degenerate merger phase, and we now investigate the role of the long-term one-dimensional 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 shell-of-fire merger feature or during the post-merger evolution, but the contribution from the asymptotic giant branch progenitors is negligible. The post-merger envelope mixing must eventually cease ~10⁶ yr after the dynamic merger phase before the star enters the RCB phase.
Original languageEnglish
Pages (from-to)1-16
Number of pages16
JournalAstrophysical Journal
Volume772
Issue number1
DOIs
Publication statusPublished - 2013
Externally publishedYes

Keywords

  • hydrodynamics
  • methods: numerical
  • nuclear reactions, nucleosynthesis, abundances;
  • stars: abundances
  • stars: AGB and post-AGB
  • stars: evolution

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