The oldest low-$\alpha$ thin disc stars

Abstract

The formation pathway of the $\alpha$-poor Milky Way thin disc remains under debate. The latest observational data indicates the presence of comparable numbers of old stars in both the low- and high-$\alpha$ disc components, suggesting either coeval or overlapping formation epochs. Using GALAH DR4, we provide evidence that the $\alpha$-poor stars coexist with $\alpha$-rich stars up to $\sim$12.5 Gyr. Stellar ages of 0.5 Gyr precision were re-derived for nearly $30000$ subgiant stars using accurate effective temperatures ($T_{\mathrm{eff}}$) 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 [$\alpha$/Fe] and solar-like metallicities ($-0.5\le [Fe/H]\le +0.5$ dex). These stars exhibit kinematics consistent with those of near solar-age $\alpha$-poor stars in the thin disc. These stars can be evolved Sun-like stars, i.e. old solar analogues, whose $T_{\mathrm{eff}}$ and gravity modified with the time. Comparison with a well-tested Galactic disc chemical evolution model supports the presence of a significant fraction of old, $\alpha$-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.

Accepted in A&A