Metallicities of old open clusters: A new Galactic map

Old open clusters (OCs) can constrain the chemical evolution of the Galactic disc through their metallicity gradients and age-metallicity relation but they are affected by low statistics. This work aims to determine precise and homogeneous metallicities for a number of old clusters (≥ 500 Myr) from all-sky catalogues of stellar parameters leveraging Gaia spectrophotometry. Our purpose was to revisit the metallicity distribution of the oldest OCs as a function of their Galactic position and age with improved statistics. Several catalogues of stellar parameters have been cross-matched to the most recent census of OCs and their members. The median metallicities per cluster and per catalogue were evaluated by comparison to high-resolution spectroscopy. The best performance is achieved when only bright giants are considered. Metallicity maps are presented and analysed, as well as trends of the distribution. Our sample includes ∼600 old OCs with a typical precision of 0.05 dex in metallicity. We identified metal-poor or metal-rich clusters never studied before, as well as moving groups as the remnants of dissolving clusters. Galactic maps show a smooth decrease in metallicity from inside to outside the disc. Metal-rich and metal-poor clusters exist at all ages, but dominate respectively in the inner and the outer disc, with different scale heights. The radial metallicity gradient was found to have a knee shape with a steep value of -0.084 ± 0.004 dex kpc^-1 in the inner side and -0.018 ± 0.004 dex kpc^-1 outside the knee. The inner radial gradient flattens with age. Vertically, the metallicity gradient is -0.415 ± 0.030 dex kpc^-1. The large scatter in the distribution of metallicity versus age is nicely explained by the superposition of OC populations standing at different galactocentric distances, each with its own mean metallicity and small dispersion, less than 0.08 dex in radius bins of 1 kpc. Our results are consistent with a negative radial metallicity gradient of interstellar matter that was present in the disc when the clusters formed. The low metallicity dispersion in each radius bin reflects weak radial mixing. Our OC sample also indicates that most of the chemical enrichment of the Galactic disc occurred before they were formed.