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

Zhang, Xianfei, Jeffery, C. Simon

Figure 1. Evolutionary tracks at different metallicities on a surfacegravityòÀÓeffective temperature diagram for a 0.8-M_☉ post-merger model for a He+He white dwarf binary. The dashed lines show the evolutionary track during accretion and the solid lines show the post-merger evolution. The colour from grey to dark (a colour version is available in the online journal, from pink to red) shows the different metallicities, that is, Z = 0.0001, 0.004, 0.008 and 0.02. The cross symbols show the R CrB stars from Jeffery et al. (2011), the circle symbols shows the EHe stars from Jeffery et al. (2011) and the dots show the He sdO stars from Hirsch (2009). The inset (top left-hand side) shows an enlargement around the phase of core He burning. The inset (bottom right-hand side) shows an enlargement of the R CrB star region. Three stages are marked: A for inward shell burning; B for outward shell burning; C for giant branch to WD.

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.

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Last Revised: 2012 September 26th