The GAPS programme at TNG LXXXII. The first atmospheric investigation of the gas giant WASP-13 b at high resolution with GIARPS

Low-density transiting gas giant exoplanets are key benchmarks for atmospheric studies. Despite their large-scale heights, many of them remain poorly characterised. We present the first atmospheric investigation at high-resolution of the inflated (Rp~1.4 RJ), hot Jupiter (T~1500 K) WASP-13 b aiming at constraining its atmospheric composition and refine its orbital geometry. We analysed four transits observed with GIARPS at the TNG. Updated stellar and planetary parameters were re-derived from the most precise literature values, and a new Rossiter-McLaughlin analysis was performed to measure the projected spin-orbit angle (λ) and stellar rotational velocity (vsin i⋆). Atmospheric signals were searched using high-resolution spectroscopy, both with cross-correlation and single-line analyses, targeting key atomic and molecular species (e.g. He i, Hα, Na i, H2O, CO). We find a well-aligned orbit (λ = 2.7±1.1○) and vsin i⋆ = 4.5±0.1 km s−1, improving upon previous constraints. No significant atmospheric absorption is detected in either VIS or NIR data. Injection-and-recovery tests exclude an isothermal atmosphere with metallicity [M/H]≲ 10 ×[M/H]⊙ and simultaneously a grey cloud top pressure Pcloud ≳ 10 mbar, under the assumption of chemical equilibrium. WASP-13 b appears to belong to the class of low-density hot Jupiters with weak or featureless transmission spectra, most likely shaped by the presence of high-altitude clouds. Although a highly metal-enriched atmosphere cannot be fully excluded, current evidence disfavours this scenario. Further observations with next-generation facilities will be essential to characterise the atmospheric properties of this challenging target.