Oxygen isotopic abundances in evolved stars. II. Eight MS and S stars

Oxygen isotope ratios have been measured in five MS stars, two S stars, and the slightly s-process--rich M giant 30 Her. Both the 16O/17O and 16O/18O ratios are found to be much larger than expected from stellar evolution theories. Both 16O/17O and 16O/18O are found to correlate well with the abundance of 12C and with the degree of neutron exposure necessary to explain the s-process element abundances. These correlations cannot be explained by current theoretical models of AGB stars, nor by simple modifications of these models. Explosive nucleosynthesis in the envelope at extremely high temperatures during the helium core flash is tentatively proposed to explain them