The metal-poor edge of the Milky Way’s “thin disc”

Context. The emergence of the disc in our Galaxy and the relation between thick and thin disc formation and evolution is still a matter of debate. The chemo-dynamical characterization of disc stars is key to resolving this question, particularly at parameter regimes, where both disc components overlap, such as the region around Fe/H ∼ −0.7 corresponding to the thin disc’s metal-poor end. Aims. In this paper, we re-assess the recent detection of a metal-poor extension of stars moving with thin-disc-like rotational velocities between −2 20 000) spectroscopic surveys available: the GALAH DR3 and the APOGEE DR17. Results. We confirm that there are high-angular-momentum stars moving in thin-disc-like orbits, that is, with a high angular momentum of L z / J tot > 0.95, and close to the Galactic plane, | Z max |< 750 pc, reaching metallicity values down to Fe/H ∼ −1.5. We also find tentative evidence for stars moving on such orbits at lower metallicities, down to Fe/H ∼ −2.5, albeit in smaller numbers. Based on their chemical trends, thin-disc-like stars with Fe/H < −1 would have formed in a medium that is less chemically evolved than the bulk of the thick disc. Stars with chemical abundances typical of the thin disc appear at metallicities between −1 < Fe/H < −0.7.