Can R Coronae Borealis stars form from the merger of two helium white dwarfs?

Abstract

Abstract 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.