Program: GS-2005B-Q-20

We recently discovered a uniquely large excess of 18O in the hydrogen-deficient carbon (HdC) star, HD 137613, based on a spectrum of the first overtone bands of CO at 2.3-2.4 microns in which three strong absorption bands of 12C18O are clearly present. Bands of 12C16O also are present in HD 137613, but no bands of 13C16O or 12C17O are seen. We estimate an isotopic ratio 16O/18O ~ 1. The Solar value of this ratio is ~500. Neither He-core burning nor He-shell flash burning can produce the isotopic ratios of oxygen and carbon observed in HD 137613. However, a remarkable similarity exists between the observed abundances and those found in the outer layers of the broad He-shell of early-AGB stars, soon after the end of He-core burning. It is not known how the outer envelope down to the He-shell could be lost but some mechanism of enhanced mass loss must be involved. HD 137613 may be a post-early-AGB star with the outer layers of the former He-burning shell as its photosphere. The unusual elemental abundances of the extremely rare Hydrogen Deficient Carbon (HdC) stars are known to resemble those of the R Coronae Borealis (RCB) stars (although HdC stars do not produce clouds of dust that produce deep declines in brightness) and thus it is believed that the two classes of stars are closely related. We thus propose to obtain high S/N spectra of a small sample of HdC and RCB stars with Gemini/GNIRS to investigate the range of 18O and other oxygen and carbon isotopic abundances in these stars. Understanding the RCB and HdC stars is a key test for any theory that aims to explain hydrogen deficiency in post-AGB stars.