Heavy elements in Globular Clusters: the role of AGB stars
  Recent observations of heavy elements in Globular Clusters reveal intriguing
deviations from the standard paradigm of the early galactic nucleosynthesis. If
the r-process contamination is a common feature of halo stars, s-process
enhancements are found in a few Globular Clusters only. We show that the
combined pollution of AGB stars with mass ranging between 3 to 6 M$_\odot$ may
account for most of the features of the s-process overabundance in M4 and M22.
In these stars, the s process is a mixture of two different neutron-capture
nucleosynthesis episodes. The first is due to the 13C(a,n)16O reaction and
takes place during the interpulse periods. The second is due to the
22Ne(a,n)25Mg reaction and takes place in the convective zones generated by
thermal pulses. The production of the heaviest s elements (from Ba to Pb)
requires the first neutron burst, while the second produces large
overabundances of light s (Sr, Y, Zr). The first mainly operates in the
less-massive AGB stars, while the second dominates in the more-massive. From
the heavy-s/light-s ratio, we derive that the pollution phase should last for
$150\pm 50$ Myr, a period short enough compared to the formation timescale of
the Globular Cluster system, but long enough to explain why the s-process
pollution is observed in a few cases only. With few exceptions, our theoretical
prediction provides a reasonable reproduction of the observed s-process
abundances, from Sr to Hf. However, Ce is probably underproduced by our models,
while Rb and Pb are overproduced. Possible solutions are discussed.
