ISOTOPIC ABUNDANCES IN CARBON-RICH CIRCUMSTELLAR ENVELOPES - A FURTHER ITERATION ON THE OXYGEN ISOTOPE PUZZLE

We have detected the (J = 1-0) and (J = 2-1) lines of the oxygen and carbon isotopomers of CO in five dusty carbon-rich envelopes (CIT6, IRC + 10216, CRL618, CRL2688, NGC7027) which represent evolution stages ranging from early AGB to planetary nebula. The (CO)C-17/(CO)-C-18 abundance ratios, assumed equal to the elemental O-17/O-18 in the gas phase, are found to be larger than 1 in the last four envelopes; a lower limit of 1 is derived for CIT6. The O-16/O-17 and O-16/O-18 ratios, calculated from measured (CO)C-13-O-16/(CO)-C-12-O-17, (CO)-C-13-O-16/(CO)-C-12-O-18 and (CS)-C-13-S-32/(CS)-C-12-S-34 ratios, range respectively from 300 to 800 and 300 to 1300. It can be noticed that in the five envelopes, we measure C-12/C-13 ratios larger than 30. The O-17/O-18 ratios measured in these five dusty C-rich envelopes appear similar to those obtained from infrared observations in less opaque C-rich and O-rich envelopes, i.e. 4 to 5 times larger than the remarkably constant value observed in the interstellar medium (Penzias 1981). We argue that AGB stars with C-rich (or translucent O-rich) envelopes cannot be the dominant sources of oxygen (and presumably, more generally, of star processed matter) in the interstellar medium.