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

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, 16 O/ 18 O ≃ 1-10, 12 C/ 13 C ≥ 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 th e 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: 16 O/ 18 O 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 ∼10 6 yr after the dynamic merger phase before the star enters the RCB phase. © 2013. The American Astronomical Society. All rights reserved.

Citation

Menon, A., Herwig, F., Denissenkov, P. A., Clayton, G. C., Staff, J., Pignatari, M., & 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. The Astrophysical journal, 772(1), 59. https://doi.org/10.1088/0004-637x/772/1/59

• Journal Article Type: Article
• Acceptance Date: May 17, 2013
• Online Publication Date: Jul 5, 2013
• Publication Date: Jul 20, 2013
• Deposit Date: Jul 16, 2018
• Publicly Available Date: Jul 26, 2018
• Journal: Astrophysical Journal
• Print ISSN: 0004-637X
• Electronic ISSN: 1538-4357
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 772
• Issue: 1
• Pages: 59
• DOI: https://doi.org/10.1088/0004-637x/772/1/59
• Keywords: Hydrodynamics; Methods: numerical; Nuclear reactions, nucleosynthesis, abundances; Stars: abundances; Stars: AGB and post-AGB; Stars: evolution
• Public URL: https://hull-repository.worktribe.com/output/561764
• Publisher URL: http://iopscience.iop.org/article/10.1088/0004-637X/772/1/59/meta