2024A&A...691A.283B

Advances in modern technology have enabled the characterization of exoplanetary atmospheres, which can be achieved by exploitation of the transmission spectroscopy technique. We performed visible (VIS) and near-infrared (NIR) high-resolution spectroscopic observations of one transit of HD 149026b, a close-in orbit sub-Saturn exoplanet by using the GIARPS configuration at the Telescopio Nazionale Galileo (TNG). We first analyzed the radial-velocity data, refining the value of the projected spin-orbit obliquity (λ). We then performed transmission spectroscopy, looking for absorption signals from the planetary atmosphere. We find no evidence for Hα, Na I D2-D1, Mg I, or Li I in the VIS and metastable helium triplet He I(2³S) in the NIR using a line-by-line approach. The non-detection of HeI is also supported by theoretical simulations. With the use of the cross-correlation technique (CCF), we do not detect Ti I, V I, Cr I, Fe I, or VO in the visible, or indeed CH₄, CO₂, H₂O, HCN, NH₃, or VO in the NIR. Our non-detection of Ti I in the planetary atmosphere is in contrast with a previous detection. We performed injection-retrieval tests, finding that our dataset is sensitive to our Ti I model. The non-detection supports the Ti I cold-trap theory, which is valid for planets with T_eq < 2200 K, such as HD 149026b. Although we do not attribute it directly to the planet, we find a possibly significant Ti I signal that is highly redshifted (≃+20 km s^–1 ) with respect to the planetary rest frame. Redshifted signals are also found in the Fe I and Cr I maps. While we can exclude an eccentric orbit as the cause of this redshifted Ti I signal, we investigated the possibility of material accretion falling onto the star - which is possibly supported by the presence of strong Li I in the stellar spectrum - but obtained inconclusive results. The analysis of multiple transits datasets could shed more light on this target.