The GAPS programme at TNG: LXXV. Validating and confirming Gaia sub-stellar astrometric candidates with HARPS-N

Context. The astrometric measurements provided by the Gaia space mission represent a key advancement in the search for and characterisation of exoplanets, helping in particular to solve the mass degeneracy intrinsic to the radial velocity (RV) method. The fact that a fraction of astrophysical false positives contaminates the current catalogue of astrometric candidate solutions requires an RV follow-up to validate and confirm such candidates. Aims. Within the Global Architecture of Planetary Systems (GAPS) programme, we observed a selected sample of 14 stars with Gaia astrometric solutions compatible with the presence of a sub-stellar companion. The immediate aim of this survey is to identify astrophysical false positives and provide the first RV validation and confirmation of the remaining candidates. Methods. We analysed data collected with the HARPS-N spectrograph to identify stellar binary systems from the spectral cross correlation function profiles. The remaining astrometric candidates were characterised via Markov chain Monte Carlo analysis searching for the best-fit RV solution. Results. Among the stars in our sample with astrometric candidate solutions, we identify six as originating from close binary companions mimicking the astrometric motion of distant sub-stellar companions, from which we can estimate an updated value of 43-11+13% for the binary contamination fraction in the Gaia DR3 catalogue of astrometric candidates. We validate and confirm the remaining eight solutions, corresponding to giant and brown dwarf companions with minimum masses between 8 and 62 MJup and semi-major axes between 0.76 and 1.42 au, providing the first RV characterisation for six of these candidates and updated orbital solutions for two previously confirmed ones.