Abundance Determination of the Neutron-Capture Elements Y, Zr, La, and Ce in Open Clusters.

We have derived abundances of four neutron-capture elements: yttrium, zirconium, lanthanum, and cerium, in a large sample of dwarfs and RGB stars that are members of Galactic open clusters. These stars have not produced s-elements yet, and so the abundances we measured were inherited from previous generations. We can trace the s-element evolution from the formation of the Galactic disk up to a few million years ago. Clusters younger than about 1.0 Gyr show an enrichment of their mean s-element abundances. Chemical evolutionary models are able to fit this growth only by assuming that low-mass AGB stars with masses in the range of about 1.2-1.5 M<SUB>&sun;</SUB> produce yields larger than those predicted by the standard AGB nucleosynthesis. Therefore new nucleosynthesis calculations are required for these stars, so that chemical evolutionary models can fit the observed trends of the s-elements.