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

Details Why are massive O-rich AGB stars in our Galaxy not S-stars? Why are massive O-rich AGB stars in our Galaxy not S-stars?

2005-09-02

arxiv.org/abs/astro-ph/0509051v1

AIP Conf.Proc.804:137-140,2006

Astronomy and Astrophysics

Astrophysics

4 pages, 2 figures, to appear in the proceedings of the conference "Planetary Nebulae as astronomical tools" held in Gdansk, P

Scientific paper

10.1063/1.2146250

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 third 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.

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