S-STARS WITHOUT TECHNETIUM - THE BINARY STAR CONNECTION

Results of an exploratory survey of non-Mira MS and S star radial velocities and the He I 10 830 Å triplet are presented. The stars in this study have been surveyed for the presence or absence of technetium in their spectra by previous investigators. We find a strong anticorrelation between the presence of Tc in the MS and S stars and the appearance of strong He I λ 10 830 features (emission, absorption, or both): of seven MS and S stars without Tc, all seven (100%) show easily detectable He I features, while only one out of 13 stars (8%) with Tc shows a detectable He I feature. In addition, the MS and S stars without Tc exhibit significantly larger radial-velocity variations than the Tc-containing stars (with observations so far spanning a year): we attribute these velocity variations to the presence of an unseen companion. We argue that the He I features and radial-velocity variations in the non-Tc MS and S stars are both caused by the presence of white-dwarf companions; this argument relegates these non-Tc MS and S stars to the category of cooler examples of the classical Barium stars (G and K giants). The Tc-containing MS and S stars can still be understood as the result of single-star evolution: these stars are asymptotic giant branch stars that have experienced thermal pulses and the subsequent third dredge-up.