Astrophysical Parameters of 5056 Open Star Clusters from Bayesian Nested Sampling with PARSEC Isochrones

Abstract We present a homogeneous catalogue of ages, metallicities, distances, and reddenings for 5 056 open clusters in the Unified Cluster Catalogue (UCC), derived uniformly from Gaia Data Release 3 (DR3) colour–magnitude diagrams via Bayesian nested sampling with PARSEC isochrones. Unlike previous large-scale catalogues, metallicity is fitted as a free parameter for every cluster rather than fixed or adopted from the literature: where available, Gaia XP spectrophotometric metallicities inform the fit (1 201 clusters), while the remaining clusters are constrained by broad priors from Gaia parallaxes and the Schlegel–Finkbeiner–Davis dust map. No cluster is anchored to an external open-cluster parameter catalogue, and no per-cluster manual intervention is applied. A fit-quality metric, ηfit, identifies 3 807 clusters (75 per cent) with reliable colour–magnitude solutions, spanning ages of 0.004–5.9 Gyr and distances out to 17 kpc. We validate the resulting parameters against six literature catalogues and, more critically, against independent high-resolution spectroscopic metallicities from OCCAM DR19: for 132 clusters in common we find a median absolute deviation of only 0.054 dex (−0.021 ± 0.132 dex mean offset), comparable to the intrinsic systematic uncertainty of isochrone-based metallicities. As a direct consequence of this fidelity, the radial metallicity gradient recovered from our XP-informed clusters (−0.064 dex kpc−1) steepens towards the spectroscopically calibrated value (−0.079 dex kpc−1) for the highest-quality subset – demonstrating that a purely photometric pipeline, built without reference to spectroscopy, recovers a physically meaningful abundance scale. The full catalogue and posterior chains are publicly released.