The s process in massive AGB stars: A new tool to study abundances in globular clusters

With a few exceptions (e.g., Omega Centauri), globular clusters show no evidence of internal variations in the abundances of the elements heavier than Fe such as Y, Sr, Rb, Zr, and Ba. On the other hand they exhibit significant variations in the abundances of the light elements, such as Li, C, N, O, and Na. For example, in the GC M4 variations of O and Na by factors larger than 2 and 3, respectively, are not accompanied by variations in the elements from Y to Pb produced by slow neutron captures (the s process). We present models of the s process in intermediate-mass (IM, >4 M_☉) asymptotic giant branch (AGB) stars, which have been proposed as possible candidates to explain the observed variations. We show that AGB stellar models with stronger mass loss rates produce lower s-process yields, because of a shorter AGB life time. These models are compatible with the observations of M4. The light elements abundances are also affected by the value of the mixing-length parameter α_MLT, where a larger value results in higher temperatures at the base of the convective envelope during hot bottom burning. This solution is opposite of what is required to match direct observations of the s-process abundances in IM-AGB stars in the Galaxy and in the Magellanic Clouds. However, this cannot be used as a strong constraint while serious problems are still present in current model atmospheres of luminous AGB stars.