Chemical evolution of the Milky Way disc with radial gas flows: a Lagrangian approach

Chemical abundance patterns result from the interplay between gas accretion, star formation, and radial mixing of gas and stars. Disentangling these processes is crucial to recover the mechanisms shaping the formation and evolution of galaxies. We model the chemical evolution of the Galactic disc in the presence of radial gas flows, to assess their impact on the [O/Fe]-[Fe/H] abundance patterns and on the radial gradients of [Fe/H] and [O/H]. We develop fast, semi-analytic solutions for the gas surface mass density and the abundances of alpha-elements and iron, accounting for radial gas flows and chemical enrichment from core-collapse and Type Ia supernovae. The model follows a Lagrangian approach, using the method of characteristics, reducing the solutions to one-dimensional integrals. We apply our model to the Milky Way disc assuming a two-infall scenario. When radial gas flows are present, the chemical abundances of the gas at a given radius result from its whole inward journey in the disc, reflecting the star formation and accretion experienced at every radius it crossed. The integrated stellar mass along the characteristic is lower than the local value by up to an order of magnitude at v = 1.5 km/s. Models with mild flows of v = 1.5 km/s reproduce simultaneously the observed [O/Fe]-[Fe/H] distribution across the disc, the present-day stellar surface-density profile, and the [Fe/H] and [O/H] gradients, improving also the agreement with the observed age-abundance relations. The stellar mass formed per Type Ia supernova sets the [O/Fe] ratio and departs from its in-situ value by up to 50 per cent, making the alpha-enhancement the quantity on which radial flows leave their strongest signature. Following the gas along its trajectory is essential to recover the correct enrichment history even for models with mild radial gas inflows. The code is made publicly available.