2025A&A...700A..89K

The stellar disc is the dominant luminous component of the Milky Way (MW). Although our understanding of its structure is rapidly expanding due to advances in large-scale surveys of stellar populations across the Galaxy, our picture of the disc remains substantially obscured by selection functions and an incomplete spatial coverage of observational data. In this work, we present the comprehensive chrono-chemo-kinematic structure of the MW disc, recovered using a novel orbit superposition approach combined with data from APOGEE DR 17. We detected periodic azimuthal metallicity variations within 6-8 kpc with an amplitude of 0.05-0.1 dex peaking along the bar major axis. The radial metallicity profile of the MW also varies with azimuth, displaying a pattern typical among other disc galaxies, namely: a decline outside the solar radius and an almost flat profile in the inner region, attributed to the presence of old, metal-poor high-α populations, comprising ≈40% of the total stellar mass. The geometrically defined thick disc and the high-α populations have comparable masses, but with differences in their stellar population content, which we quantified using the reconstructed 3D MW structure. The well-known [α/Fe]-bimodality in the MW disc, once it has been weighted by the stellar mass, is less pronounced at a given metallicity for the whole galaxy but distinctly visible in a narrow range of galactic radii (5-9 kpc), explaining its relative lack of prominence in external galaxies and galaxy formation simulations. Analysing a more evident double age-abundance sequence, we constructed a scenario for the MW disc formation, advocating for an inner and outer disc dichotomy genetically linked to the MW's evolutionary stages. In this picture, the extended solar vicinity is a transition zone that shares the chemical properties of both the inner (old age-metallicity sequence) and outer discs (young age-metallicity sequence).