The Evolution of R Coronae Borealis Stars (Abstract)

Volume 37 number 2 (2009)

Geoffrey C. Clayton

Abstract

(Abstract only) Among the hydrogen-deficient post-asymptotic giant branch (post-AGB) stars are the R Coronae Borealis (RCB) stars, a small group of carbon-rich supergiants. About fifty RCB stars are known in the Galaxy and the Magellanic Clouds. Their defining characteristics are hydrogen deficiency and unusual variability—RCB stars undergo massive declines of up to 8 magnitudes due to the formation of carbon dust at irregular intervals. Apparently related to the RCB stars are the hydrogen-deficient carbon (HdC) stars. The five known HdC stars are similar to the RCB stars spectroscopically but do not show declines or IR excesses. The evidence for and against the two scenarios that have been proposed for the origin of RCB stars is discussed in the light of recent observational data. These scenarios are, the double degenerate and the final helium-shell flash models. The former involves the merger of a CO- and a He-white dwarf. In the latter, a star evolving into a planetary nebula central star is blown up to supergiant size by a final helium shell flash.