Chromospheric activity of nearby Sun-like stars

R^′_HK index signature of a recent burst of star formation^★

European Space Agency, ESTEC - Postbus 299, 2200 AG Noordwijk, The Netherlands
e-mail: Philippe.Gondoin@esa.int

Received: 29 April 2020
Accepted: 16 June 2020

Abstract

Context. The chromospheric emission in the cores of the Ca II H & K lines of late-type dwarfs is a well known indicator of magnetic activity that decreases with increasing stellar age.

Aims. I use this indicator to investigate the formation history of nearby G- and early K-type stars with origins at galactocentric distances similar to that of the region where the Sun was born.

Methods. A parent sample of single main-sequence stars with near-solar metallicity and known magnetic activity levels is built from catalogues of stellar atmospheric parameters and chromospheric activity indices. A kinematical approach uses Gaia astrometric data to differentiate thin disc stars from thick disc stars. Measured distributions of R^′_HK chromospheric activity indices are compared with Monte Carlo simulations based on an empirical model of chromospheric activity evolution.

Results. The thin disc includes a significant fraction of Sun-like stars with intermediate activity levels (2 × 10⁻⁵ ≤ R^′_HK ≤ 6 × 10⁻⁵), while most early K- and G-type stars from the thick disc are inactive (R^′_HK < 2 × 10⁻⁵). The chromospheric activity distribution among nearby Sun-like dwarfs from the thin disc can be explained by a combination of an old (>6–7 Gyr) star formation event (or events) and a more recent (<3 Gyr) burst of star formation. Such an event is not required to account for the R^′_HK index distributions of nearby thick disc stars.

Conclusions. The distribution of magnetic activity among local G- and early K-type stars with a near-solar metallicity bears the imprint of an important star formation event that occurred ~1.9–2.6 Gyr ago in the thin disc of the Milky Way.