Abstract

Context. The chemical evolution of neutron capture elements in the Milky Way disc is still a matter of debate. There is a lack of statistically significant catalogues of such element abundances, especially those of the r-process. Aims. We aim to understand the chemical evolution of r-process elements in Milky Way disc. We focus on three pure r-process elements Eu, Gd, and Dy. We also consider a pure s-process element, Ba, in order to disentangle the different nucleosynthesis processes. Methods. We take advantage of high-resolution FEROS, HARPS, and UVES spectra from the ESO archive in order to perform a homogeneous analysis on 6500 FGK Milky Way stars. The chemical analysis is performed thanks to the automatic optimization pipeline GAUGUIN. We present abundances of Ba (5057 stars), Eu (6268 stars), Gd (5431 stars), and Dy (5479 stars). Based on the [α/Fe] ratio determined previously by the AMBRE Project, we chemically characterize the thin and the thick discs, and a metal-rich α-rich population. Results. First, we find that the [Eu/Fe] ratio follows a continuous sequence from the thin disc to the thick disc as a function of the metallicity. Second, in thick disc stars, the [Eu/Ba] ratio is found to be constant, while the [Gd/Ba] and [Dy/Ba] ratios decrease as a function of the metallicity. These observations clearly indicate a different nucleosynthesis history in the thick disc between Eu and Gd–Dy. The [r/Fe] ratio in the thin disc is roughly around +0.1 dex at solar metallicity, which is not the case for Ba. We also find that the α-rich metal-rich stars are also enriched in r-process elements (like thick disc stars), but their [Ba/Fe] is very different from thick disc stars. Finally, we find that the [r/α] ratio tends to decrease with metallicity, indicating that supernovae of different properties probably contribute differently to the synthesis of r-process elements and α-elements. Conclusions. We provide average abundance trends for [Ba/Fe] and [Eu/Fe] with rather small dispersions, and for the first time for [Gd/Fe] and [Dy/Fe]. This data may help to constrain chemical evolution models of Milky Way r- and s-process elements and the yields of massive stars. We emphasize that including yields of neutron-star or black hole mergers is now crucial if we want to quantitatively compare observations to Galactic chemical evolution models.

Highlights

  • The surface abundances of FGK dwarf stars do not show major changes along their main sequence evolution, reflecting their original pristine chemical composition

  • We find that the α-rich metal-rich stars are enriched in r-process elements, but their [Ba/Fe] is very different from thick disc stars

  • asymptotic giant branch (AGB) stars are thought to be the main contributors for enriching the interstellar medium (ISM) in nuclei with atomic mass number 90 < A < 204

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Summary

Introduction

The surface abundances of FGK dwarf stars do not show major changes along their main sequence evolution, reflecting their original pristine chemical composition. Less theoretical and observational data are available for elements heavier than Z ∼ 35, usually called neutron-capture elements because they are formed by the addition of neutrons in stellar interiors To create such nuclei, two main processes are known, first through the slow neutron-capture. We aim here to study homogeneously the evolution of Ba, Eu, Gd, and Dy for a statistically significant sample of stars, covering a large domain of metallicity To this end we automatically derived a very large set of abundances for these elements, thanks to ESO archive spectra, for a sample of ∼6000 stars. In order to put more constraints on the r-process elements origin, we focus our study on the two main components of the Milky Way: the thin and thick discs.

Observational data set from the AMBRE Project
Automatic abundance analysis of n-capture elements
Solar scale and average abundance calculation
Error budget
Repeated observations
AMBRE catalogue of Ba and r-process abundances
Comparison with literature Eu and Ba abundances
Selecting our working sample
Thin to thick disc dichotomy in the solar neighbourhood
Selecting the best abundances
Ba and r-process abundances trends with metallicity
Ratios of pure r-element to barium
Findings
Average r-process abundance trends
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