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

摘要

Optical high-resolution spectra of the R Coronae Borealis star V CrA at lightmaximum and during minimum light arediscussed. Abundance analysis confirmsprevious results showing that V CrA has the composition of the small subclassof R Coronae Borealis (RCB) stars know as `minority' RCBs, i.e., the Si/Fe andS/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: spectrumsynthesis shows that $^{12}$C/$^{13}$C is about 3 to 4. Absorption lineprofiles are variable at maximum light with some lines showing evidence ofsplitting by about 10 km s$^{-1}$. A spectrum obtained as the star wasrecovering from a deep minimum shows the presence of cool C$_2$ molecules witha rotational temperature of about 1200K, a temperature suggestive of gas inwhich carbon is condensing into soot. The presence of rapidly outflowing gas isshown by blue-shifted absorption components of the Na {\sc i} D and K {\sc i}7698 \AA resonance lines.