THE CHEMICAL-COMPOSITION OF RED GIANTS .4. THE NEUTRON DENSITY AT THE S-PROCESS SITE

Rubidium abundances are determined from the Rb I 7800 Angstrom 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 similar or equal to 0.05 is derived for the s-processed material from the He-burning shell. Thanks to the branch in the s-process path at Kr-85 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 (AGE) 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 AGE stars and probably also of low-mass AGE stars. Zirconium isotopic abundances are determined from ZrO bandheads near 6925 Angstrom via synthetic spectra for a sample of S stars. No evidence is found for the isotope Zr-96 whose synthesis is controlled by the branch in the s-process path at Zr-95. This observation shows that the observed stars are not intermediate mass stars with massive (M(c) greater than or similar to 1 M.) cores. The absence of Zr-96 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.