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

Abstract

We present a homogeneous catalogue of fundamental astrophysical parameters -- age, metallicity ([Fe/H]), heliocentric distance, and colour excess $E(G_{\mathrm{BP}}-G_{\mathrm{RP}})$ -- for 5,056 open star clusters drawn from the Unified Cluster Catalogue (UCC). All parameters are derived uniformly from Gaia Data Release 3 (DR3) colour-magnitude diagrams via Bayesian Nested Sampling with PARSEC stellar isochrones, with no manual intervention on individual clusters. Initial metallicity $Z_{\mathrm{ini}}$ is treated as a free parameter throughout, yielding a photometric [Fe/H] estimate for every cluster. Physically motivated priors -- parallax-based distances from Gaia DR3 astrometry, spectrophotometric metallicity constraints from Gaia XP spectra where available, and interstellar reddening from the Schlegel-Finkbeiner-Davis dust map -- reduce CMD degeneracies without anchoring the fit to any external parameter catalogue. Of the 5,056 clusters, 3,766 (74.5\%) satisfy the fit-quality criterion $η_{\mathrm{fit}} \ge 0.80$. This high-quality subset spans ages 0.003-5.5~Gyr ($\log(\mathrm{Age/yr})$ median $8.33 \pm 0.34$~dex), heliocentric distances 88-19,011 pc (median 2,150~pc), metallicities $-1.17 \le \mathrm{[Fe/H]} \le +0.42$~dex (median $+0.002$ dex), and extinctions up to $A_G = 7.37$~mag (median 1.07~mag). The catalogue is made publicly available via CDS/VizieR; the complete nested-sampling posterior chains are archived on Zenodo.