Coronae Borealis Stars: Evidence for White Dwarf Mergers

We have found that at least seven hydrogen-deficient carbon (HdC) and R Coronae Borealis (RCB) stars, have O/O ratios close to and in some cases less than unity, values that are orders of magnitude lower than measured in other stars (the Solar value is 500). Greatly enhanced O is evident in every HdC and RCB we have measured that is cool enough to have detectable CO bands. The three HdC stars measured have O/O < 1, lower values than any of the RCB stars. These discoveries are important clues in determining the evolutionary pathways of HdC and RCB stars, for which two models have been proposed: the double degenerate (white dwarf (WD) merger), and the final helium-shell flash (FF). No overproduction of O is expected in the FF scenario. We have quantitatively explored the idea that HdC and RCB stars originate in the mergers of COand He-WDs. The merger process is estimated to take only a few days, with accretion rates of 150M⊙ yr −1 producing temperatures at the base of the accreted envelope of 1.2−1.9×10K. Analysis of a simplified one-zone calculation shows that nucleosynthesis in the dynamically accreting material may provide a suitable environment for a significant production of O, leading to very low values of O/O, similar to those observed. We also find qualitative agreement with observed values of C/C and with the CNO elemental ratios. H-admixture during the accretion process from the small H-rich C/O WD envelope may play an important role in producing the observed abundances. Overall our analysis Department of Physics & Astronomy, Louisiana State University, Baton Rouge, LA 70803; [email protected] Gemini Observatory, 670 N. A’ohoku Place, Hilo, HI 96720; [email protected] Keele Astrophysics Group, School of Physical and Geographical Sciences, Keele University, Staffordshire ST5 5BG, UK; [email protected] Los Alamos National Laboratory, Los Alamos, NM 87545; [email protected] Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston, ACT 2611, Australia; [email protected]