2007PASP..119..147V

Correlations between the presence of the 6707 Å line of lithium and the resonance lines of technetium (4238 and 4262 Å) in a large sample of Galactic S stars are analyzed. Half of the sample stars are intrinsic S stars (those exhibiting technetium in their spectra), and (1)/(3) of these stars also have strong lithium lines in their spectra. Stars having both lithium and technetium in their spectra are interpreted as intermediate-mass thermally pulsating asymptotic giant branch (TP-AGB) stars in which lithium is produced by the Cameron-Fowler mechanism. The production of lithium is predicted to occur in high-luminosity (M_bol≤-6) TP-AGB stars by the hot-bottom burning (HBB) mechanism. Data on the carbon isotope ratios of stars in our sample agree with the predictions of HBB; however, oxygen isotope ratios in these stars do not agree with the predictions of HBB. Furthermore, the available luminosities for our sample stars are below the minimum value necessary for HBB to occur in available models. Cool-bottom processing (CBP) is one possible explanation for the presence of lithium in the spectra of these stars. Intrinsic S stars having technetium but no lithium in their spectra are interpreted as lower mass (1.5-3 M_☉) thermally pulsating AGB stars that have not undergone CBP. Extrinsic S stars constitute the remaining half of the sample. Carbon and oxygen isotope ratios, as well as the lack of technetium and lithium in the spectra of these stars, are consistent with these being low-mass red giant branch stars (1-2 M_☉), with mass transfer from a now extinct thermally pulsating AGB star being responsible for the enhanced abundance of s-process elements.