Full computation of massive AGB evolution

II. The role of mass loss and cross-sections

Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy e-mail: [ventura;dantona]@mporzio.astro.it

Received: 19 November 2004
Accepted: 26 April 2005

Abstract

In the course of a systematic exploration of the uncertainties associated with the input micro- and macro-physics in the modeling of the evolution of intermediate mass stars during their Asymptotic Giant Branch (AGB) phase, we focus on the role of the nuclear reactions rates and mass loss. We consider masses   for a metallicity typical for globular clusters, , and compare the results obtained by computing the full nucleosynthesis with hot bottom burning (HBB), for a network of 30 elements, using either the NACRE or the Cameron & Fowler (CF88) cross-sections. The results differ in particular with respect to the ²³Na nucleosynthesis (which is more efficient in the NACRE case) and the magnesium isotopes ratios. For both choices, however, the CNO nucleosynthesis shows that the C+N+O is constant within a factor of two, in our models employing a very efficient convection treatment. Different mass loss rates alter the physical conditions for HBB and the length of the AGB phase, indirectly changing the chemical yields. These computations show that the predictive power of our AGB models is undermined by these uncertainties. In particular, it appears at the moment very difficult to strongly accept or dismiss that these sources play a key-role in the pollution of Globular Clusters (GCs), and that they have been the main stellar site responsible for the chemical anomalies that are observed at the surface of giant and turn-off stars of GCs, in the self-enrichment scenarios.