The Galactic R Coronae Borealis Stars: The C2 Swan Bands, The Carbon Problem, and The 12C/13C Ratio

Abstract

Observed spectra of R Coronae Borealis (RCB) and hydrogen-deficient carbon (HdC) stars are analyzed by synthesizing the C 2 Swan bands (1, 0), (0, 0), and (0, 1) using our detailed line list and the Uppsala model atmospheres. The (0, 1) and (0, 0) C 2 bands are used to derive the 12 C abundance, and the (1, 0) 12 C 13 C band to determine the 12 C/ 13 C ratios. The carbon abundance derived from the C 2 Swan bands is 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%). Carbon abundances derived from C i lines are about a factor of four lower than the carbon abundance of the adopted model atmosphere over the same C/He interval, as reported by Asplund et al., who dubbed the mismatch between adopted and derived C abundance as the "carbon problem." In principle, the carbon abundances obtained from C 2 Swan bands and that assumed for the model atmosphere can be equated for a particular choice of C/He that varies from star to star. Then, the carbon problem for C 2 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. A more likely solution to the C 2 carbon problem may lie in a modification of the model atmosphere's temperature structure. The derived carbon abundances and the 12 C/ 13 C ratios are discussed in light of the double degenerate and the final flash scenarios.

Line strengths and updated molecular constants for the C2 Swan system

“…C 2 is an important molecule in the fields of astronomy, combustion science and materials science. It has often been observed in comets [1,2,3,4,5,6] and in other astronomical environments such as interstellar clouds [7,8,9,10,11,12,13], late-type stars [14,15,16,17] and the Sun [18,19]. Its reactions are believed to be involved in the formation of hydrocarbons and other organic compounds in interstellar clouds [20].…”

“…The old rotational line list reported by Phillips and Davis in 1968 [32] has recently been used in deriving the carbon abundance and 12 C/ 13 C ratio in R Coronae Borealis and hydrogen-deficient carbon stars [17], and in comets [43]. Line positions and intensities must be known to derive C 2 abundance from observed spectra, and it would be beneficial to have a new rotational line list, based on recent measurements and calculations.…”