Deficiencies in the classical model of s-process nucleosynthesis

The classical model of s-process nucleosynthesis, based on the concept of a steady neutron flux under astrophysical conditions pertaining to the He-bu rning phase of red giant stars, has successfully described observed isotopi c abundances and provided information on the physical conditions of the s-p rocess environment. Because most of the isotopes on the s-process path are stable, their relevant nuclear parameters can be measured in the laboratory so that as more accurate elemental abundance and neutron capture cross-sec tion data have become available, the classical model has been tested under increasingly stringent conditions. Accurate determinations of the neutron c apture cross sections at appropriate astrophysical conditions for the Ba is otopes have shown that the abundance of the s-only isotope B-136, is under- produced by similar to 20% according to the classical model. This paper des cribes the accurate assessment of the meteoritic abundance of Ba by the sta ble isotope dilution mass spectrometric technique, based on the CI carbonac eous chondrites Orgueil and Ivuna. Repeated analyses of these two C1 chondr ites give an abundance that is identical to the presently accepted solar sy stem value for Ba within experimental errors, which indicates a deficiency in the classical model. When combined with similar data for the s-only nucl ides Sn-116 and Nd-142, it is apparent that the classical model, having ser ved a valuable function for many years, must be replaced by stellar models that more accurately reflect the dynamic nature of the He-burning phase in red giant stars, in particular, during the thermal pulses of low-mass asymp totic giant branch (AGB) stars.