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 ( RCB) stars, have O-16/O-18 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-18 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-16/O-18 < 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-18 is expected in the FF scenario. We have quantitatively explored the idea that HdC and RCB 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-circle dot yr(-1) producing temperatures at the base of the accreted envelope of (1.2-1.9) x 10(8) 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 O-18, leading to very low values of O-16/O-18, similar to those observed. We also find qualitative agreement with observed values of C-12/C-13 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 O-18-rich RCB and HdC stars, and that more detailed simulations and modeling are justified.