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

Rubidium abundances are determined from the Rb I 7800 Angstrom line vi a synthetic spectra for a sample of M, MS, and S giants. The Rb abunda nce increases with increasing s-process enrichment. A ratio Rb/Sr simi lar 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 ti me of s-processing. The derived ratio is consistent with predicted neu tron densities for operation of the s-process during the interpulse in tervals in low-mass asymptotic giant branch (AGE) stars but clearly in consistent with much higher neutron densities predicted for the runnin g 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 s ynthetic 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-pr ocess path at Zr-95. This observation shows that the observed stars ar e not intermediate mass stars with massive (M(c) greater than or simil ar to 1 M.) cores. The absence of Zr-96 sets an upper limit on the neu tron density at the s-process site which is higher than and, therefore , consistent with the limit set by the Rb abundances in related stars.