Galactic chemical evolution of primary elements in the solar neighbourhood -- II. Elements affected by the s-process

Abstract

We apply the analytical model developed previously to the Galactic chemical evolution of elements that in the Solar system come primarily from the s-process, but at low metallicities are believed to come primarily from the r-process. We find that a ‘primary’ production model is indicated for the s-process, but with two separate time delays that are treated in the ‘delayed production’ approximation and turn out to be of the order of 37 Myr and 2.7 Gyr, corresponding to progenitor masses of about 8 and 1.5 M⊙ respectively. While the latter component fits in well with current models of s-process synthesis of material in the Solar system, the early component raises some difficulties because asymptotic giant branch (AGB) stars of high mass and low metallicity are observed to generate a high ratio of heavy to light s-process elements, whereas the field stars presumed to represent the effects of Galactic chemical evolution still have Solar system-like heavy to light ratios.