Two Centuries of Observing R Coronae Borealis: What will the Role of the AAVSO be in the Next Century? (Abstract)

Volume 42 number 1 (2014)

Geoffrey C. Clayton

Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803; gclayton@fenway.phys.lsu.edu

Abstract

(Abstract only) R Coronae Borealis was found to be variable in the year 1783, and was one of the first variable stars to be identified. Its class, the R Coronae 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. Their rarity may stem from the fact that they are in an extremely rapid phase of evolution or in an evolutionary phase that most stars do not undergo. Several evolutionary scenarios have been suggested to account for the RCB stars, including a merger of two white dwarfs (WDs) or a final helium shell flash in a PN central star. The large overabundance of the rare isotope oxygen-18 found in most of the RCB stars favors the WD merger scenario while the presence of lithium in the atmospheres of five of the RCB stars favors the FF scenario. 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 supernovae. Understanding these enigmatic stars depends to a large extent on continuous monitoring to catch their irregular but rapid variations due to dust formation, their variations due to stellar pulsations, and longterm changes that may occur over centuries. The AAVSO has been instrumental in this monitoring for over a century, but how will this change in the era of all-sky surveys?