Heavy elements in Globular Clusters: the role of AGB stars

Abstract

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.

Accepted by the ApJ

References in corpus (5)

• The s process in massive stars at low metallicity. Effect of primary N14 from fast rotating stars
• Heavy element abundances in giant stars of the globular clusters M4 and M5
• Measurement of the reaction O-17(α,n)Ne-20 and its impact on the s process in massive stars
• A Method to Derive the Absolute Composition of the Sun, the Solar System and the Stars
• First direct measurement of resonance strengths in 17O(α, γ)21Ne

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