Do R Coronae Borealis stars form from double white dwarf mergers?

Jan E. Staff, Athira Menon, Falk Herwig, Wesley Even, Chris L. Fryer, Patrick M. Motl, Tom Geballe, Marco Pignatari, Geoffrey C. Clayton, Joel E. Tohline

Research output: Contribution to journalArticlepeer-review

27 Citations (Scopus)

Abstract

A leading formation scenario for R Coronae Borealis (RCB) stars invokes the merger of degenerate He and CO white dwarfs (WDs) in a binary. The observed ratio of ¹⁶O/¹⁸O for RCB stars is in the range of 0.3-20 much smaller than the solar value of ~500. In this paper, we investigate whether such a low ratio can be obtained in simulations of the merger of a CO and a He WD. We present the results of five three-dimensional hydrodynamic simulations of the merger of a double WD system where the total mass is 0.9 M⊙ and the initial mass ratio (q) varies between 0.5 and 0.99. We identify in simulations with q ≲ 0.7 a feature around the merged stars where the temperatures and densities are suitable for forming ¹⁸O. However, more ¹⁶O is being dredged up from the C- and O-rich accretor during the merger than the amount of ¹⁸O that is produced. Therefore, on the dynamical timescale over which our hydrodynamics simulation runs, an ¹⁶O/¹⁸O ratio of ~2000 in the "best" case is found. If the conditions found in the hydrodynamic simulations persist for 10⁶s the oxygen ratio drops to 16 in one case studied, while in a hundred years it drops to ~4 in another case studied, consistent with the observed values in RCB stars. Therefore, the merger of two WDs remains a strong candidate for the formation of these enigmatic stars.
Original languageEnglish
Pages (from-to)1-17
Number of pages17
JournalAstrophysical Journal
Volume757
Issue number1
DOIs
Publication statusPublished - 2012
Externally publishedYes

Keywords

  • binaries: close
  • stars: abundances
  • stars: AGB and post-AGB
  • white dwarfs

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