Two Centuries of Observing R Coronæ Borealis

Abstract

AbstractR Coronæ Borealis was found to be variable in the year 1783, and was one of the first variable stars to be so identified. Its class, the R Coronæ Borealis (RCB) stars, are rare hydrogen-deficient carbon-rich supergiants. RCB stars undergo massive declines of up to 8 mag due to the formation of carbon dust at irregular intervals. The mechanism of dust formation around RCB stars is not well understood, but the dust is thought to form in or near the atmosphere of the star. Their rarity may stem from the fact that they are in an extremely rapid phase of the evolution, or are in an evolutionary phase that most stars do not undergo. Several evolutionary models have been suggested to account for the RCB stars, including a merger of two white dwarfs (WDs) or a final helium-shell flash (FF) in a PN central star. The large overabundance of 18O found in most of the RCB stars favours the WD merger model, while the presence of Li in the atmospheres of five RCB stars favours the FF one. In particular, the measured isotopic abundances imply that many, if not most, RCB stars are produced by WD mergers, which may be the low-mass counterparts of the more massive mergers thought to produce type Ia supernovæ. Understanding these enigmatic stars depends to a large extent on continuous monitoring to catch their irregular but rapid variations caused by dust formation, their variations due to stellar pulsations, and long-term changes that may occur over centuries. I will use observations of R Coronæ Borealis obtained over 200 years to demonstrate what kinds of monitoring are necessary for these and similar classes of variables.