On the metal-poor edge of the Milky Way "thin disc"
arxiv(2024)
摘要
The emergence of the disc in our Galaxy and the relation of the thick and
thin disc formation and evolution is still a matter of debate. The
chemo-dynamical characterization of disc stars is key to resolve this question,
in particular at parameter regimes where both disc components overlap, such as
the region around [Fe/H] ∼ -0.7 corresponding to the thin disc
metal-poor end. 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 that was made based on metallicity estimates obtained from
photometric data and their rotational velocity distribution. We explore the
chemo-dynamical properties of metal-poor stars within the recent Gaia third
data release (DR3), which includes the first catalogue of metallicity estimates
from the Radial Velocity Spectrometer (RVS) experiment. We complement them with
the two largest high-resolution (λ/dλ > 20,000) spectroscopic
surveys available, the GALAH DR3 and the APOGEE DR17. We confirm that there are
high angular-momentum stars moving in thin-disc-like orbits, i.e., with high
angular momentum 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 of stars moving on such orbits at
lower metallicities, down to [Fe/H] ∼ -2.5, although in smaller numbers.
Based on their chemical trends the fast rotators with [Fe/H] < -1 would have
formed in a medium less chemically evolved than the bulk of the thick disc.
Fast rotators with chemical abundances typical of the thin disc appear at
metallicities between -1 < [Fe/H] < -0.7.
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