AN EXTREMELY LARGE EXCESS OF ~(18)O IN THE HYDROGEN-DEFICIENT CARBON STAR HD 137613

摘要

We report the discovery of a uniquely large excess of ~(18)O 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 μm in which three strong absorption bands of ~(12)C~(18)O are clearly present. Bands of ~(12)C~(16)O also are present, but no bands of ~(13)C~(16)O or ~(12)C~(17)O are seen. We estimate an isotopic ratio ~(16)O/~(18)O approx< 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-asymptotic giant branch (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 HdC stars resemble those of the R Coronae Borealis (RCB) stars, but HdC stars do not produce clouds of dust that produce declines in brightness. None of the other RCB or HdC stars observed show significant ~(18)O.