The GAPS programme at TNG: LXVI. A homogeneous search for Na I and its possible variability in ten gas giant exoplanets

The neutral sodium resonance doublet (Na I D) has been detected in the upper atmosphere of several close-in gas giants, through high-resolution transmission spectroscopy. We aim to investigate whether its variability is linked to the planets'properties, the data quality, or the accuracy of the system parameters used. Using the public code SLOPpy, we extracted the transmission spectrum in the Na I D region of ten gas giants for which a large number of HARPS-N observations are available. We modelled the absorption signals found, performing an MCMC analysis, and converted the measured absorption depth to the corresponding atmospheric height over which most sodium absorption occurs. While two targets (GJ 436 b and KELT-7 b) show no Na I D feature, we found variability in the transmission spectrum of the other targets. Three of them (HD 209458 b, WASP-80 b, and WASP-127 b) present absorption on only some nights, while in the other five targets (HD 189733 b, KELT-9 b, KELT-20 b, WASP-69 b, a...