Abstract:
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.
