The recently improved information on the stellar (n, γ) cross sections of neutron-magic nuclei at N = 82, and in particular of 142 Nd, turned out to represent a sensitive test for models of s-process nucleosynthesis. While these data were found to be incompatible with the classical approach based on an exponential distribution of neutron exposures, they provide significantly better agreement between the solar abundance distribution of s nuclei and the predictions of models for low mass AGB stars. The origin of this phenomenon is identified as being due to the high neutron exposures at low neutron density obtained between thermal pulses when the 13 C burns radiatively in a narrow layer of a few 10 −4 M ⊙ . This effect is studied in some detail, and the influence of the presently available nuclear physics data is discussed with respect to specific further requests. In this context, particular attention is paid to a consistent description of s-process branchings in the region of the rare earth elements.It is shown that -in certain cases -the nuclear data are sufficiently accurate that the resulting abundance uncertainties can be completely attributed to stellar modelling. Thus, the s process becomes important for testing the role of different stellar masses and metallicities as well as for constraining the assumptions for describing the low neutron density provided by the 13 C source.

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Among the various processes responsible for the formation of the heavy elements in stars, the slow neutron capture process (s-process) is distinguished by the fact that it involves mostly stable isotopes. Therefore, the relevant nuclear physics data can be determined by experiments. With this rather reliable data basis, s-process nucleosynthesis offers an important testground of models for the late stages of stellar evolution, which are supposed to be the s-process site. The empirical counterpart for such models is the so-called classical s-process, a purely phenomenological picture, that is successfully used to derive the resulting abundances as well as information on the physical conditions during the s-process. The status of this classical approach is reviewed with emphasis on the implications for various stellar models of the s-process and in the light of results obtained by stellar spectroscopy. A brief account of the potential s-process chronometers is also presented.

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