The oldest low-α thin disc stars

Abstract

The formation pathway of the α-poor Milky Way (MW) thin disc remains under debate. The latest observational data indicates the presence of comparable numbers of old stars in both the low- and high-α disc components, suggesting either coeval or overlapping formation epochs. Using GALAH DR4, we provide evidence that α-poor stars have coexisted with α-rich stars since ∼12.5 Gyr ago. Stellar ages of 0.5 Gyr precision were re-derived for nearly 30,000 subgiant stars using accurate effective temperatures ( and solar-like metallicities (-0.5 łeq łeq +0.5 dex). These stars exhibit kinematics consistent with those of near solar-age α-poor stars in the thin disc. These stars can be evolved Sun-like stars, i.e. old solar analogues, whose from the infrared flux method (IRFM) together with distances based on Gaia parallaxes. About 22% of the sample constitutes a population peaking at 9.2 Gyr, with evidence for an extended old-age tail reaching 12.5 Gyr. Among stars older than 10 and 11 Gyr, between 20 and 30% have relative low α/Fe Fe/H and gravity modified with time. Comparison with a well-tested Galactic disc chemical evolution model supports the presence of a significant fraction of old, α-poor disc stars, confirming the plausibility of our selection criteria. We provide a catalogue with the selected sample to facilitate spectroscopic follow-ups at high resolution.