Can R Coronae Borealis stars form from the merger of two helium white dwarfs?
Article first published online: 7 SEP 2012
© 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS
Monthly Notices of the Royal Astronomical Society: Letters
Volume 426, Issue 1, pages L81–L85, October 2012
How to Cite
Zhang, X. and Jeffery, C. S. (2012), Can R Coronae Borealis stars form from the merger of two helium white dwarfs?. Monthly Notices of the Royal Astronomical Society: Letters, 426: L81–L85. doi: 10.1111/j.1745-3933.2012.01330.x
- Issue published online: 1 OCT 2012
- Article first published online: 7 SEP 2012
- Accepted 2012 August 7. Received 2012 August 5; in original form 2012 July 1
- stars: abundances;
- binaries: close;
- stars: evolution;
- stars: peculiar;
- white dwarfs
Due to orbital decay by gravitational wave radiation, some close binary helium white dwarfs are expected to merge within a Hubble time. The immediate merger products are believed to be He-rich subdwarf O (sdO) stars, essentially helium main-sequence stars. We present new evolution calculations for these post-merger stars beyond the core He-burning phase. The most massive He-rich sdO stars develop a strong He-burning shell and evolve to become He-rich giants. We include nucleosynthesis calculations following the merger of 0.4 M⊙ He white dwarf pairs with metallicities Z= 0.0001, 0.004, 0.008 and 0.02. The surface chemistries of the resulting giants are in partial agreement with the observed abundances of R Coronae Borealis (R CrB) and extreme He stars. Such stars might represent a third, albeit rare, evolution channel for the latter, in addition to the CO+He white dwarf merger and the very late thermal pulse channels proposed previously. We confirm a recent suggestion that Li seen in R CrB stars could form naturally during the hot phase of a merger in the presence of 3He from the donor white dwarf.