The chemical composition of red giants. I. Dredge-up in the M and MS stars.

Carbon, nitrogen, oxygen, Fe-peak, and heavy, s-process elemental abundances have been determined for nine red giants of type M and MS. The source material are high-resolution, high signal-to-noise digital spectra in the infrared and near-infrared. The abundances of the Fe-peak and heavier elements are based on atomic lines and are referenced to a comparison red giant, alpha Tau, to minimize non-LTE effects. The CNO abundances are based on molecular lines from CO, OH, and CN. The CNO abundances clearly show the effects of CN-processed material being dredged to the surface, while one MS giant shows an excess of C-12, probably the result of He-burning products being dredged up during thermal pulses on the asymptotic giant branch (AGB). The three MS giants studied all show enhancements of the s-process elements Sr, Y, Zr, Ba, and Nd. A range of integrated neutron exposures is evident with one star showing a large enhancement of Ba and Nd relative to Sr, Y, and Zr, while the other two show much lower enhancements of Ba and Nd. It is suggested that the MS stars are AGB stars and that their compositions reflect the addition of C-12 and s-process material from He-burning thermal pulses.