Metallicity effects in the chemical evolution from AGB stars to PNe

Abstract

We present the main results derived from a chemical abundance analysis carried out on a large sample of massive galactic O-rich AGB stars (M $>$ 3$-$4 M$_\odot$). Combining these results with previous studies made on a similar sample of luminous AGB stars belonging to the Magellanic Clouds we provide strong observational evidences that metallicity effects are playing a more important role than generally assumed in chemical evolution models. This concerns not only the onset of the so-called “hot bottom burning”, the efficiency of the third dredge-up and the s-process nucleosynthesis as derived from our optical observations, but also the dust production efficiency and the chemical properties of the dust grains in the shell, as inferred from the available infrared data. We find Li overabundances in the galactic stars studied, indicating that they are actually “hot bottom burning” AGB stars. Similar Li overabundances are also observed in the most luminous Magellanic Cloud AGB stars. However, the AGB stars in our galactic sample are not enriched in Zr, in contrast to what is observed in the Magellanic Clouds. In addition, many stars in the galactic sample appear heavily obscured in the optical, suggesting a much more efficient dust production and/or stronger mass loss rates which eventually can be translated into shorter AGB lifetimes.