Program: GS-2009A-DD-3

The R Coronae Borealis (RCB) stars are rare hydrogen-deficient carbon-rich supergiants, all apparently single stars which are consistent with being post-AGB stars. Their rarity may stem from the fact that they are in an extremely rapid phase of their 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 (FF) in a PN central star. The large overabundance of 18O found in most of the RCB stars favors the WD merger scenario while the presence of Li in the atmospheres of four of the RCB stars favors the FF scenario. Therefore, in the RCB stars, the presence or absence of circumstellar material and the morphology of this material provides a fossil record of previous evolutionary stages. In particular, we expect to see evidence for an old PN shell in the FF stars but not in the WD merger products. Such a shell has already been detected in one of the Li-rich RCB stars. We are proposing to take advantage of an extremely deep decline in R CrB, another Li-rich RCB star, to obtain images with Gemini/GMOS to look for the presence of an old PN shell. We will also use Monte Carlo radiative transfer models to investigate the dust scattering in the fossil shell. Understanding the RCB stars is a key test for any theory which aims to explain the evolution of post-AGB stars and hydrogen deficiency including WR stars and non-DA WD.