Observational constraints on the origin of the elements – VI. Origin and evolution of neutron-capture elements as probed by the Gaia-ESO survey

Abstract

Most heavy elements beyond the iron peak are synthesized via neutron capture processes. The nature of the astrophysical sites of neutron capture processes is still very unclear. In this work, we explore the observational constraints of the chemical abundances of s-process and r-process elements on the sites of neutron-capture processes by applying Galactic chemical evolution (GCE) models to the data from Gaia-ESO large spectroscopic stellar survey. For the r-process, the [Eu/Fe]–[Fe/H] distribution suggests a short delay time of the site that produces Eu. Other independent observations (e.g. NS–NS binaries), however, suggest a significant fraction of long delayed (>1 Gyr) neutron star mergers (NSM). When assuming NSM as the only r-process sites, these two observational constraints are inconsistent at above 1σ level. Including short delayed r-process sites like magnetorotational supernova can resolve this inconsistency. For the s-process, we find a weak metallicity dependence of the [Ba/Y] ratio, which traces the s-process efficiency. Our GCE model with up-to-date yields of AGB stars qualitatively reproduces this metallicity dependence, but the model predicts a much higher [Ba/Y] ratio compared to the data. This mismatch suggests that the s-process efficiency of low-mass AGB stars in the current AGB nucleosynthesis models could be overestimated.