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 α /Fe = 0.05±0.06. The transiting planet is a warm sub-Neptune with a mass m p = (16±2) M ⊙ , a radius r p =(3.2±0.l) R ⊙ yielding a density ρ 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 ⋆ = (0.78±0.01) R ⊙ and a mass of M ⋆ = (0.80±0.07) M ⊙ . 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.