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The Galactic R Coronae Borealis Stars and the Final He-shell Flash Object V4334 Sgr (Sakurai's Object): A Comparison

The high resolution optical spectra of H-deficient stars, R Coronae Borealis stars and H-deficient carbon stars are analyzed by synthesizing the C2 Swan bands (0,1), (0,0), and (1,0) using our detailed line-list and Uppsala model atmosphere, to determine the C-abundances and the 12C/13C ratios which are potential clues to the formation process of these stars. The C-abundances derived from C2 bands are about the same for the adopted models constructed with different carbon abundances over the range 8.5 (C/He = 0.1%) to 10.5 (C/He = 10%). The carbon abundances derived from C I lines are a factor of four lower than that adopted for the model atmosphere over the same C/He interval, as reported by Asplund et al.: 'the carbon problem'. In principle, the carbon abundances obtained from C2 Swan bands and that adopted for the model atmosphere can be equated for a particular choice of C/He that varies from star to star (unlike C I lines). Then, the carbon problem for C2 bands is eliminated. However, such C/He ratios are in general less than those of the extreme helium stars, the seemingly natural relatives to the RCB and HdC stars. The derived carbon abundances and the 12C/13C ratios are discussed in light of the double degenerate (DD) and the final flash (FF) scenarios. The carbon abundance and the 12C/13C ratios for the FF product, Sakurai's Object is derived. The carbon abundance in the Sakurai's object is 10 times higher than in the RCB star VZ Sgr. On an average, the carbon abundance in the Sakurai's Object is about 10 to 100 times higher than in RCB stars. The 12C/13C ratio in Sakurai's Object is 3.4, the equilibrium value, as expected for FF products.