J-PLUS Reconstructing the Milky Way Disc's star formation history with twelve-filter photometry

Abstract: Wide-field, multi-filter photometric surveys enable the reconstruction of the Milky Way's star formation history (SFH) on Galactic scales and offer new insights into disc assembly. The twelve-filter system of the Javalambre Photometric Local Universe Survey (J-PLUS) is particularly suitable, as its colours trace stellar chemical abundances and help alleviate the age-metallicity degeneracy in colour-magnitude diagram fitting. We aim to recover the SFH of the Galactic disc and separate its chemically distinct components by combining J-PLUS DR3 photometry with Gaia astrometry. We also evaluate the potential of isochrone fitting to estimate stellar ages and metallicities as proxies for evolutionary trends. We fit magnitudes and parallaxes of $1.38\times10^{6}$ stars using a Bayesian multiple isochrone method. The bright region of the colour-absolute-magnitude diagram ($M_{r}\leq4.2$ mag) constrains ages, while the faint region provides an empirical metallicity prior mitigating the age-metallicity degeneracy. Both PARSEC and BaSTI isochrones, in solar-scaled and $α$-enhanced forms, are adopted. The recovered SFH reveals two sequences: an $α$-enhanced population forming rapidly between $12.5$ and $8$ Gyr ago, enriching from [M/H]$\sim-0.6$ to $0.1$ dex; and a solar-scaled sequence emerging $\sim8$ Gyr ago, dominating after $\sim7$ Gyr with slower enrichment reaching solar metallicity by $3$ Gyr. Metal-rich ([M/H]$\gtrsim0$) stars are confined to $\vert z_{GC}\vert\lesssim1$ kpc, whereas metal-poor ([M/H]$\lesssim-0.5$) stars reach $\vert z_{GC}\vert\sim2$ kpc. Simultaneous fitting of both isochrone families reveals distinct formation epochs for the thin and thick discs. J-PLUS multi-filter photometry, combined with Gaia parallaxes, mitigates age-metallicity degeneracies and enables detailed mapping of the Milky Way's temporal and chemical evolution.