Why are Massive O-rich AGB Stars in our Galaxy not S-stars?

Abstract

We present the main results derived from a chemical analysis carried out on a large sample of galactic O‐rich AGB stars using high resolution optical spectroscopy (R∼40,000–50,000) with the intention of studying their lithium abundances and/or possible s‐process element enrichment. Our chemical analysis shows that some stars are lithium overabundant while others are not. The observed lithium overabundances are interpreted as a clear signature of the activation of the so‐called “hot bottom burning” (HBB) process in massive galactic O‐rich AGB stars, as predicted by the models. However, these stars do not show the zirconium enhancement (taken as a representative for the s‐process element enrichment) associated to the 3rd dredge‐up phase following thermal pulses. Our results suggest that the more massive O‐rich AGB stars in our Galaxy behave differently from those in the Magellanic Clouds, which are both Li‐ and s‐process‐rich (S‐type stars). Reasons for this unexpected result are discussed. We conclude that metallicity is probably the main responsible for the differences observed and suggest that it may play a more important role than generally assumed in the chemical evolution of AGB stars.