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

Abstract

Recent spectroscopic explorations of large Galactic stellar samples stars have revealed the existence of red giants with [$\alpha$/Fe] ratios that are anomalously high, given their relatively young ages. We revisit the GALAH DR3 survey to look for both dwarfs and giants with extreme [$\alpha$/Fe] ratios, that is, the upper 1% in the [$\alpha$/Fe]-[Fe/H] plane over the range in [Fe/H] between -1.1 and +0.4 dex. We refer to these outliers as "ex$\alpha$fe" stars. We used the GALAH DR3 data and their value-added catalog to trace the properties (abundances, masses, ages, and kinematics) of the ex$\alpha$fe stars. We investigated the effects of secular evolution and the magnitude limitations of GALAH to understand the mass and metallicity distributions of the sample stars. We also discuss the corresponding biases in previous studies of stars with high [$\alpha$/Fe] in other surveys. We find both dwarf and giant ex$\alpha$fe stars younger than 3 Gyr, which we refer to as "y-ex$\alpha$fe" stars. 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)~3.5 dex (i.e., a factor of 2 above the protosolar value), while cooler/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 dwarfs and giants are indeed young, their mass distribution shows no peculiarity, and they differ from young [$\alpha$/Fe]-normal stars by their extreme [$\alpha$/Fe] content only. However, their origin remains unclear.