The GAPS programme at TNG : LVII. TOI-5076b: A warm sub-Neptune planet orbiting a thin-to-thick-disk transition star in a wide binary system

Aims. We report the confirmation of a new transiting exoplanet orbiting the star TOI-5076. Methods. We present our vetting procedure and follow-up observations which led to the confirmation of the exoplanet TOI-5076b. In particular, we employed high-precision TESS photometry, high-angular-resolution imaging from several telescopes, and high-precision radial velocities from HARPS-N.Results. From the HARPS-N spectroscopy, we determined the spectroscopic parameters of the host star: T-eff = (5070 +/- 143) K, log g = (4.6 +/- 0.3), [Fe/H] = (+0.20 +/- 0.08), and [alpha/Fe] = 0.05 +/- 0.06. The transiting planet is a warm sub-Neptune with a mass m(p) = (16 +/- 2) M-circle dot, a radius r(p) =(3.2 +/- 0.l) R-circle dot yielding a density rho(p) = (2.8 +/- 0.5) g cm(-3). It revolves around its star approximately every 23.445 days.Conclusions. The host star is a metal-rich, K2V dwarf, located at about 82 pc from the Sun with a radius of R-star = (0.78 +/- 0.01) R-circle dot and a mass of M-star = (0.80 +/- 0.07) M-circle dot. It forms a common proper motion pair with an M-dwarf companion star located at a projected separation of 2178 au. The chemical analysis of the host-star and the Galactic-space velocities indicate that TOI-5076 belongs to the old population of thin-to-thick-disk transition stars. The density of TOI-5076b suggests the presence of a large fraction by volume of volatiles overlying a massive core. We found that a circular orbit solution is marginally favored with respect to an eccentric orbit solution for TOI-5076b.