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 are discussed. 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 12C13C bandhead indicating that 13C is abundant: spectrum synthesis shows that 12C/13C is about 3–4. 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 C2 molecules with a rotational temperature of about 1200 K, a temperature suggestive of gas in which carbon is condensing into soot. The presence of rapidly outflowing gas is shown by blueshifted absorption components of the Na i D and K i 7698 Å resonance lines.