Probing the atmosphere of WASP-69 b with low- and high-resolution transmission spectroscopy

Consideration of both low- and high-resolution transmission spectroscopy is key for obtaining a comprehensive picture of exoplanet atmospheres. In studies of transmission spectra, the continuum information is well established with low-resolution spectra, while the shapes of individual lines are best constrained with high-resolution observations. In this work, we aim to merge high- with low-resolution transmission spectroscopy. We present the analysis of three primary transits of WASP-69b in the VIS channel of the CARMENES instrument and perform a combined low- and high-resolution analysis using additional data from HARPS-N, OSIRIS/GTC, and WFC3/HST already available in the literature. During the first CARMENES observing night, we detected the planet Na D\(_{2}\) and D\(_{1}\) lines at \(\sim 7\sigma\) and \(\sim 3\sigma\) significance levels, respectively. We measured a D\(_{2}\)/D\(_{1}\) intensity ratio of 2.5\(\pm\)0.7, which is in agreement with previous HARPS-N observations. Our modelling of WFC3 and OSIRIS data suggests strong Rayleigh scattering, solar to super-solar water abundance, and a highly muted Na feature in the atmosphere of this planet, in agreement with previous investigations of this target. We use the continuum information retrieved from the low-resolution spectroscopy as a prior to break the degeneracy between the Na abundance, reference pressure, and thermosphere temperature for the high-resolution spectroscopic analysis. We fit the Na D\(_{1}\) and D\(_{2}\) lines individually and find that the posterior distributions of the model parameters agree with each other within 1\(\sigma\). Our results suggest that local thermodynamic equilibrium processes can explain the observed D\(_{2}\)/D\(_{1}\) ratio because the presence of haze opacity mutes the absorption features.