S-process-enriched cool stars with and without technetium - Clues to asymptotic giant branch and binary star evolution

Abstract

High-resolution spectra at λ4260 have been obtained for 40 stars of type MS or S to search for the presence of Tc I. In addition, previously published Tc surveys provide an additional 18 stars for a total sample of 58 MS or S stars. Most of these stars have also been observed in Zr I, Y II, La II, and Nd II lines. These stars show general s-process abundance enhancements. Of the total sample, 38% of these s-process-enriched stars do not show Tc. The authors suggest that the MS and S stars are composed of two major groups: (1) Tc-containing stars which are currently thermally pulsing AGB stars undergoing third dredge-up and (2) Tc-poor stars which represent the coolest members of the barium star class, where s-process enrichment is probably the result of binary mass transfer in the past.