Lithium, masses, and kinematics of young Galactic dwarf and giant stars with extreme [α/Fe] ratios

Résumé : Recent spectroscopic explorations of large Galactic stellar samples stars have revealed the existence of red giants with [{\alpha}/Fe] ratios anomalously high given their relatively young ages. We revisit the GALAH DR3 survey to look for both dwarf and giant stars with extreme [{\alpha}/Fe] ratios, i.e., upper 1% in the [{\alpha}/Fe]-[Fe/H] plane over the range in [Fe/H] between -1.1 and +0.4 dex. We call these outliers ex{\alpha}fe stars. We use the GALAH DR3 data along with their Value-Added Catalogue to trace the properties (chemical abundances, masses, ages, and kinematics) of the ex{\alpha}fe. We investigate the effects of secular stellar evolution and of the magnitude limitations of the GALAH survey to understand the mass and metallicity distributions of the sample stars and discuss the corresponding biases in previous studies of stars with high - though not extreme - [{\alpha}/Fe] in other spectroscopic surveys. We find both dwarf and giant ex{\alpha}fe stars younger than 3 Gyr, that we call y-ex{\alpha}fe. Dwarf y-ex{\alpha}fe stars exhibit lithium abundances similar to those of young [{\alpha}/Fe]-normal dwarfs at the same age and [Fe/H]. In particular, the youngest and most massive stars of both populations exhibit the highest Li abundances, A(Li){\sim}3.5 dex, while cooler and/or older stars exhibit the same Li depletion patterns increasing with both decreasing mass and increasing age. In addition, the [Fe/H] and mass distributions of both the dwarf and giant y-ex{\alpha}fe stars do not differ from those of their [{\alpha}/Fe]-normal counterparts found in the thin disk, and they share the same kinematic properties. We conclude that y-ex{\alpha}fe dwarf and giant stars are indeed young, that their mass distribution shows no peculiarity, and that they differ from young [{\alpha}/Fe]-normal stars by their extreme [{\alpha}/Fe] content only. However, their origin remains unclear.