Deficiencies in the classical model of s-process nucleosynthesis

Abstract— The classical model of s-process nucleosynthesis, based on the concept of a steady neutron flux under astrophysical conditions pertaining to the He-burning phase of red giant stars, has successfully described observed isotopic abundances and provided information on the physical conditions of the s-process 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-section data have become available, the classical model has been tested under increasingly stringent conditions. Accurate determinations of the neutron capture cross sections at appropriate astrophysical conditions for the Ba isotopes have shown that the abundance of the s-only isotope ¹³⁶Ba is under-produced by ～20% according to the classical model. This paper describes the accurate assessment of the meteoritic abundance of Ba by the stable isotope dilution mass spectrometric technique, based on the Cl carbonaceous chondrites Orgueil and Ivuna. Repeated analyses of these two Cl chondrites give an abundance that is identical to the presently accepted solar system value for Ba within experimental errors, which indicates a deficiency in the classical model. When combined with similar data for the s-only nuclides ¹¹⁶Sn and ¹⁴²Nd, it is apparent that the classical model, having served 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 asymptotic giant branch (AGB) stars.