Rubidium, zirconium, and lithium production in massive AGB stars

A recent survey of a large sample of massive Galactic asymptotic giant branch (AGB) stars shows that significant overabundances of rubidium (up to 100 times solar) are present in these stars. Zirconium, on the other hand, is not enriched compared to the solar abundance. These observations can set constraints on our theoretical notion of the slow neutron capture process (the s process) in AGB stars, as well as on the rates of the neutron capture reactions involved in the production of Rb and Zr. Li abundances are also reported for these stars. Li is made via proton captures at the base of the convective envelope (hot bottom burning; HBB) in AGB models of mass higher than ≈ 5 M _⊙. We present predictions of Rb, Zr, and Li abundances for AGB stars of masses 5, 6, and 6.5 M_⊙. We find increasing Rb overabundances with increasing stellar mass, as observed, but we are far from matching the highest observed Rb enhancements, unless the third dredge up efficiency remains as high as before the onset of the superwind phase during the final few pulses of a massive AGB star. We predict large variations in the Li abundances, which are observed. Finally, we show that the Rb production increases with a decrease in the stellar metallicity, at a given mass. This result may be useful to constrain the source of abundance anomalies observed in Globular Cluster Stars.