The Chemical Composition of Red Giants. II. Helium Burning and the s-Process in the MS and S Stars

Carbon, nitrogen, oxygen, Fe-peak, and heavy s-process elemental abundances have been determined for an additional 12 stars of type M, MS, or S. When combined with results obtained previously for nine stars of type M or MS and the K5 giant α Tau, abundances are available for 22 cool red giants. Mg-isotopic ratios and technetium abundances are estimated for selected stars. Of the 22 stars studied, eight are of type MS and three of type S. Four of the MS stars show no measurable enhancements of the s-process elements. The seven stars which exhibit s-process enhancements also show increasing ¹²C abundances and the ¹²C abundances correlate with both absolute s-process enhancement and integrated neutron exposure. The results for the s-process and ¹²C-rich stars are compared to theoretical models of low-mass (1 - 2 M_sun;), thermally pulsing AGB stars.