The metal-poor edge of the Milky Way's "thin disc"

Fernández-Alvar, Emma; Kordopatis, Georges; Hill, Vanessa; Battaglia, Giuseppina; Gallart, Carme; González Rivera de la Vernhe, Isaure; Thomas, Guillaume; Sestito, Federico; Ardern-Arentsen, Anke; Martin, Nicolas; Viswanathan, Akshara; Starkenburg, Else
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Astronomy and Astrophysics

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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 < [Fe/H] < −0.7, carried out on the basis of metallicity estimates obtained from photometric data and their rotational velocity distributions.
Methods: We explored the chemo-dynamical properties of metal-poor stars from the recent Gaia third data release (DR3), which includes the first catalog of metallicity estimates from the Radial Velocity Spectrometer (RVS) experiment. We complemented them with the two largest high-resolution (λ/dλ > 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 Lz/Jtot > 0.95, and close to the Galactic plane, |Zmax|< 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.