High-resolution spectroscopy of the R Coronae Borealis Star V Coronae Australis

Abstract

Optical high-resolution spectra of the R Coronae Borealis star V CrA at light maximum and during minimum light arediscussed. Abundance analysis confirms previous results showing that V CrA has the composition of the small subclass of R Coronae Borealis (RCB) stars know as `minority' RCBs, i.e., the Si/Fe and S/Fe ratios are 100 times their solar values. A notable novel result for RCBs is the detection of the 1-0 Swan system ${}^{12}$C${}^{13}$C bandhead indicating that ${}^{13}$C is abundant: spectrum synthesis shows that ${}^{12}$C/${}^{13}$C is about 3 to 4. Absorption line profiles are variable at maximum light with some lines showing evidence of splitting by about 10 km s${}^{-1}$. A spectrum obtained as the star was recovering from a deep minimum shows the presence of cool C${}_2$ molecules with a rotational temperature of about 1200K, a temperature suggestive of gas in which carbon is condensing into soot. The presence of rapidly outflowing gas is shown by blue-shifted absorption components of the Na {\sc i} D and K {\sc i} 7698 Åresonance lines.

14 pages, 18 figures, 3 tables, accepted to MNRAS

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