High-resolution transmission spectroscopy of the hot-Saturn HD 149026b

¹ DiSAT, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
² INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
³ INFN, Sezione Milano-Bicocca, P.za della Scienza 3, 20126 Milano, Italy

^★ Corresponding author; fbiassoni@studenti.uninsubria.it

Received: 1 August 2024
Accepted: 11 October 2024

Abstract

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.