The chemical enrichment histories across the Milky Way disk

Context . The variations in the production of metals with time and their dilution in the interstellar medium are a function of the star formation and gas accretion rates. Thus, measuring age-chemistry relations across the Milky Way disk is our most important constraint on the gas accretion history, the past star formation history, but also provides crucial information on the place of birth of the stars. Aims . We present a new study of these relations based on a sample of almost 30 000 dwarf stars from the APOGEE survey DR17 within 2 kpc of the Sun for which we measured accurate ages. Methods . Various combinations of parameters were tested to find the best determination of stellar ages from stellar isochrones. The resulting age-chemistry relations for a selected subsample of 12 000 stars were interpreted with the help of a chemical evolution model. Results . The data show a very well-defined and tight thick disk sequence, characterized by high [α/Fe] content, subsolar metallicities, and ages greater than 8 Gyr. The thin disk, characterized by a lower α-content and by ages younger than 8 Gyr, shows a large metallicity spread at all ages, with apparent structures. When detailed in inner (R guide < 7 kpc), intermediate (7.6 kpc<R guide <9 kpc), and outer (R guide > 10 kpc) disk using guiding radius, the data show distinct chemical evolutions. We find in particular that the inner disk is typical of a monotonic, homogeneous evolution, with little dispersion, while the outer disk shows little increase in metallicity over the last 8 Gyr. The evolution at the solar radius seems to be a mix not only because some stars have migrated from the inner and outer disk, but more importantly because the chemical evolution of the interstellar medium (ISM) in the intermediate region results from mixed gas from the inner and outer disk. In particular, we demonstrate that in the solar neighborhood the evolution shows a decrease in the mean metallicity of the ISM that occurred 7-9 Gyr ago. One possible explanation assumes a radial inflow from the outer disk of lower metallicity gas at this epoch that diluted the gas left over by the formation of the thick disk, giving rise to the metallicity gradient observed in this intermediate region.