Chemical abundances in nearby Sun-like stars and the history of the Milky Way disc

ESA exoplanet team, ESTEC – Postbus 299, 2200 AG Noordwijk, The Netherlands

^★ Corresponding author; ph.gondoin.astro@gmail.com

Received: 29 February 2024
Accepted: 2 September 2024

Abstract

Context. The properties of nearby stars bear the imprint of the formation and evolution of the Milky Way (MW). Reconstructing its history requires the determination of precise ages for large samples of stars over long periods.

Aims. The present study aims to address the evolution of the MW disc in the region where the Sun and nearby Sun-like stars formed.

Methods. The evolution of the disc composition in that region during the last 6 Gyr was inferred from the mean abundances of various chemical elements in nearby Sun-like stars. Their age was estimated from their mean chromospheric activity index using an empirical age–activity relationship derived from stellar rotation period measurements in intermediate-age open clusters. The mean abundances versus age of the sample stars were compared with chemical evolution models of metal-rich gaseous discs experiencing an infall of pristine gas after a quenching period of star formation.

Results. The chemical composition of the sample stars reveals two distinct evolutionary trends. Light α elements and iron-peak elements show increasing abundances relative to iron with age. In contrast, the abundance ratios of s-process elements decay with age. Models that best fit the mean abundances of the sample stars as a function of age concur to a gas infall and a concomitant burst of star formation that occurred between 6.2 and 5.5 Gyr ago.

Conclusions. This timeline is consistent with a scenario where the first close pericentric passage of the Sagittarius (Sgr) dwarf galaxy ~5.7 Gyr ago induced an infall of metal-poor gas onto the MW disc and a major burst of star formation. The most massive stars that formed in this event rapidly released α elements via type II supernovae explosions, while intermediate-mass stars returned s-process elements on much longer timescales. The first encounter of the Sgr galaxy with the MW played an important role in determining the long-term evolution of the interstellar medium (ISM) composition in the region of the disc where the Sun and Sun-like stars formed, thus explaining the observed correlations between their chemical abundances and their age.