Very large excesses of ¹⁸O in hydrogen-deficient carbon and R Coronae Borealis stars: Evidence for white dwarf mergers

We have found that at least seven hydrogen-deficient carbon (HdC) and R Coronae Borealis (RGB) stars, have ¹⁶O/¹⁸O 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 ¹⁸O is evident in every HdC and RGB we have measured that is cool enough to have detectable CO bands. The three HdC stars measured have ¹⁶O/¹⁸O < 1, lower values than any of the RGB stars. These discoveries are important clues in determining the evolutionary pathways of HdC and RGB 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 ¹⁸O is expected in the FF scenario. We have quantitatively explored the idea that HdC and RGB stars originate in the mergers of CO- and He-WDs. The merger process is estimated to take only a few days, with accretion rates of 150 M⊙ yr^-1 producing temperatures at the base of the accreted envelope of (1.2-1.9) × 10⁸ K. 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 ¹⁸O, leading to very low values of ¹⁶O/¹⁸O, similar to those observed. We also find qualitative agreement with observed values of ¹²C/ ¹³C 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 shows that WD mergers may very well be the progenitors of O¹⁸-rich RGB and HdC stars, and that more detailed simulations and modeling are justified.