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.
The Chemical Composition of Red Giants. IV. The Neutron Density at the s-Process Site
BUSSO, Maurizio Maria;
1995
Abstract
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 85Kr 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 96Zr whose synthesis is controlled by the branch in the s-process path at 95Zr. This observation shows that the observed stars are not intermediate mass stars with massive (MC ≳ 1 Msun) cores. The absence of 96Zr 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.