The Chemical Composition of Red Giants. IV. The Neutron Density at the s-Process Site

Rubidium abundances are determined from the Rb 1 7800 A line via synthetic spectra for a sample of M, MS, and S giants. The Rb abundance increases with increasing s-process enrichment. A ratio Rb/Sr ≃ 0.05 is derived for the s-processed material from the He-burning shell. Thanks to the branch in the s-process path at <SUP>85</SUP>Kr the Rb/Sr ratio may be used to determine the neutron density at the time of s-processing. The derived ratio is consistent with predicted neutron densities for operation of the s-process during the interpulse intervals in low-mass asymptotic giant branch (AGB) stars but clearly inconsistent with much higher neutron densities predicted for the running of the s-process in the He-shell thermal pulses of intermediate mass AGB stars and probably also of low-mass AGB stars. Zirconium isotopic abundances are determined from ZrO bandheads near 6925 Å via synthetic spectra for a sample of S stars. No evidence is found for the isotope <SUP>96</SUP>Zr whose synthesis is controlled by the branch in the s-process path at <SUP>95</SUP>Zr. This observation shows that the observed stars are not intermediate mass stars with massive (M<SUB>C</SUB> ≳ 1 M<SUB>sun</SUB>) cores. The absence of <SUP>96</SUP>Zr sets an upper limit on the neutron density at the s-process site which is higher than and, therefore, consistent with the limit set by the Rb abundances in related stars.