The age, kinematics, and metallicity of nearby Sun-like stars and the history of the Milky Way disc

Abstract

Contact. Investigating how the Milky Way formed and has evolved is an important topic in astrophysics that requires the determination of precise ages for large samples of stars over long periods. Aims. The present study addresses the formation history of nearby Sun-like stars using the emission reversal in the cores of their Ca II H&K Fraunhofer lines as an age indicator. Methods. I used an empirical age–activity relationship derived from stellar rotation period measurements in intermediate-age open clusters to infer the age distribution of a representative sample of nearby 0.85−1.0 M⊙ stars with −0.2 ≤ [Fe/H] ≤ +0.2. The evolution of the dispersion of their velocity components and of the mean iron abundance as a function of age is estimated. Results. The inferred age distribution shows a steep rise in star formation in the solar neighbourhood between 7 and 6 Gyr ago, with a maximum formation rate ∼5 Gyr ago. This rate then decays until ∼2 Gyr and rises again in the recent past. The dispersion of the radial and vertical velocity components of the sample stars is the largest at the time of maximum star formation. Their mean iron abundance first decays from a super-solar value ([Fe/H] ∼ +0.05) ∼ 6 Gyr ago to a sub-solar value ([Fe/H] ≤ −0.05) ∼ 4 Gyr ago and rises again in the recent past. Conclusions. This timeline is consistent with a scenario where the steep rise in the age distribution of nearby Sun-like stars around 7−6 Gyr is related to an external perturbation induced by a first close pericentric passage of the Sgr galaxy ∼6.5 Gyr ago. The Sgr galaxy would have been significantly stripped from its gas in this first encounter, thus explaining the weaker star formation during a more recent encounter ∼2 Gyr ago. The gas infall from the satellite galaxy onto the MilkyWay disc would have diluted its metallicity over an extended period of time after the first encounter. The turbulence induced in this initial encounter may be partly responsible for the increased dispersion of velocity components of the stars born around the age of maximum star formation. A continuous metal enrichment of the disc would have progressively compensated the decaying infall of low-metallicity gas leading to an increase in the mean stellar metallicity in the last 4 Gyr.