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

TL;DR

This study presents high-resolution spectra of the R Coronae Borealis star V CrA, revealing its unique chemical composition, isotopic abundances, and dynamic atmospheric phenomena during different brightness phases.

Contribution

It provides the first detection of abundant $^{13}$C in V CrA and analyzes spectral line variability and molecular features during different light states.

Findings

V CrA has a minority RCB star composition with high Si/Fe and S/Fe ratios.

Detection of $^{13}$C with a $^{12}$C/$^{13}$C ratio of about 3-4.

Observation of cool C$_2$ molecules and outflowing gas during recovery from minimum.

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 \AA resonance lines.