The s-Process in AGB Stars

Abstract

The evidence of an exponential distribution of neutron exposures for reproducing the solar-system s-isotopes between Zr and Pb, the main component, comes from a phenomenological analysis of the s-path in the σN versus A plot (see Käppeler, Beer and Wisshak, 1989). The resulting mean neutron exposure is τ0 = 0.30 ± 0.01 mb−1. The study of branchings with no or weak temperature dependence (95Zr; 147Nd, 147,148Pm; 185W, 186Re) allows one to derive an effective neutron density nn =(3.4±1.0)×108cm−3. On the other hand, an effective temperature is obtained from the branchings 134’135Cs, 151Sm, 154Eu and 176Lu: T8 = 3.4 ± 0.5 (Käppeler et al., 1990). Although the phenomenological approach is very useful because it is independent of stellar models, nonetheless it can only lead to effective physical conditions, symplifying the complexity of the astrophysical sites.