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

Abstract

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 (R${}_{\mathrm{p}}$$\sim$1.4 R${}_{\mathrm{J}}$), hot Jupiter (T$\sim$1500$\mathrm{K}$) WASP-13 b aiming at constraining its atmospheric composition and refine its orbital geometry. We analysed four transits observed with GIARPS at the TNG. Updated stellar and planetary parameters were re-derived from the most precise literature values, and a new Rossiter-McLaughlin analysis was performed to measure the projected spin-orbit angle ($\lambda$) and stellar rotational velocity ($v\sin i_{\star }$). Atmospheric signals were searched using high-resolution spectroscopy, both with cross-correlation and single-line analyses, targeting key atomic and molecular species (e.g. \ion{He}{I}, H$\alpha$, \ion{Na}{I}, H${}_2$O, CO). We find a well-aligned orbit ($\lambda$ = 2.7$\pm$1.1${}^{\circ }$) and $v\sin i_{\star }$ = 4.5$\pm$0.1 km s${}^{-1}$, improving upon previous constraints. No significant atmospheric absorption is detected in either VIS or NIR data. Injection-and-recovery tests exclude an isothermal atmosphere with metallicity [M/H]$\lesssim$10$\times$[M/H]${}_{\odot }$ and simultaneously a grey cloud top pressure P${}_{\mathrm{cloud}}\gtrsim 10$ 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.

arXiv comment19 pages, 16 figures. Accepted for publication on MNRAS