| Abstract [eng] |
The origin of metal-poor stars on thin-disc-like orbits remains an open question in Galactic archaeology, with the potential to provide important constraints on the earliest phases of Milky Way disc formation. While low-metallicity stars are usually associated with the stellar halo or the thick disc, recent large spectroscopic surveys have revealed stars with low metallicities on cold, prograde, nearly circular orbits. These objects challenge the classical view of the thin disc as a young and metal-rich component. Therefore, they could possibly serve as tracers of the low-metallicity tail of an old thin disc, the high-angular-momentum tail of the metal-weak thick disc, or prograde halo-related populations. We aim to identify and characterise the metal-poor stars with thin-disc-like kinematics observed in the -ESO Survey. We investigate their dynamical, chemical, and evolutionary properties to assess whether they might be associated with the old, metal-poor tail of the Galactic thin disc. Gaia We used orbital parameters derived from Gaia DR3 data, combined with spectroscopic information from the -ESO Survey. Stellar ages were obtained through isochrone fitting, while detailed chemical abundances for several elements and kinematics were used to place the stars in the context of Galactic populations. Gaia Out of 1784 turn-off stars, we identified one metal-poor candidate star with thin-disc-like kinematics. It has mathrm Fe/H =-1.38 and follows a dynamically cold, prograde orbit, with low eccentricity, high azimuthal velocity (V_ϕ ≃ 220, ), high angular momentum, low vertical action, and a guiding radius close to the solar neighbourhood. Chemically, however, it is clearly distinct from the thin disc: it is α-enhanced ($ km s^ -1 Mg/Fe = +0.58$; α/Fe = +0.48), displays a low $ Y/Mg = -0.58$ ratio, and its multi-element abundance pattern is offset from that of the thin-disc reference. Its position in the $ Al / Fe -- Mg / Mn $ plane further confirms that its chemistry is distinct from the thin-disc sequence. The star also has an old age estimate, with a most probable value of ∼ 10.2 and a broad 1σ-like interval of sim4.4--13.4 . Gyr Gyr The combination of low metallicity, old age, α enhancement, low $ Y/Mg $, its position on the metal-poor sequence in the $ Al / Fe -- Mg / Mn $ plane, and dynamically cold thin-disc-like kinematics suggests that this star is not a typical member of the canonical thin disc. Instead, it is a strong candidate for a metal-poor relic of the early Galactic disc, possibly associated with the proto-thin disc or with the transition between the proto-Galactic halo and the emerging disc. Its properties indicate that stars with halo-like metallicities and chemically distinct abundance patterns can occupy orbits characteristic of the thin disc, highlighting the existence of metal-poor stars on dynamically cold orbits, while emphasizing that their origin might involve the old thin disc, the metal-weak thick disc, or prograde halo-related populations. This object provides a useful benchmark for studying the chemical enrichment and dynamical state of early metal-poor populations on disc-like orbits. |
