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

Abstract

Context. Recent spectroscopic explorations of large Galactic stellar samples stars have revealed the existence of red giants with [α/Fe] ratios that are anomalously high, given their relatively young ages. Aims. We revisit the GALAH DR3 survey to look for both dwarf and giant stars with extreme [α/Fe] ratios, that is, the upper 1% in the [α/Fe]–[Fe/H] plane over the range in [Fe/H] between −1.1 and +0.4 dex. We refer to these outliers as “exαfe” stars. Methods. We used the GALAH DR3 data along with their value-added catalog to trace the properties (chemical abundances, masses, ages, and kinematics) of the exαfe stars. We applied strict criteria to the quality of the determination of the stellar parameters, abundances, and age determinations to select our sample of single stars. We investigated the effects of secular stellar evolution and the magnitude limitations of the GALAH survey to understand the mass and metallicity distributions of the sample stars. Here, we also discuss the corresponding biases in previous studies of stars with high – albeit not extreme – [α/Fe] in other spectroscopic surveys. Results. We find both dwarf and giant exαFe stars younger than 3 Gyr, which we refer to as “y-exαfe” stars. Dwarf y-exαFe stars exhibit lithium abundances similar to those of young [α/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 and 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-exαFe stars do not differ from those of their [α/Fe]-normal counterparts found in the thin disk and they share the same kinematic properties, with lower eccentricities and velocities with respect to the local standard of rest than old stars of the thick disk. Conclusions. We conclude that y-exαFe dwarf and giant stars are indeed young, their mass distribution shows no peculiarity, and they differ from young [α/Fe]-normal stars by their extreme [α/Fe] content only. However, their origins still remain unclear.