Evidence of Water Vapor in the Atmosphere of a Metal-Rich Hot Saturn with High-Resolution Transmission Spectroscopy

Transmission spectroscopy presents one of the most successful approaches for investigating the atmospheres of exoplanets. We analyzed the near-infrared high-resolution transmission spectrum of a hot Saturn, HD 149026 b, taken using CARMENES spectrograph (R80, 400). We found evidence of H₂O at an S/N of 4. 8. We also performed grid search using a Bayesian framework and constrained the orbital velocity Kₚ and rest velocity Vₑ₄ₒₓ to 158. 17^+8. 31-₇. ₉₀ km\ s^-1 and 2. 57^+0. 54-₀. ₅₇ km\ s^-1, respectively. Whilst the retrieved Kₚ value is consistent with theoretical prediction, the retrieved Vₑ₄ₒₓ value is highly red-shifted (>3-). This might be an indication of either anomalous atmospheric dynamics at play or an orbit with non-zero eccentricity. Additionally, we searched for HCN but no successful detection has been made possibly due to the relatively low S/N dataset. The detection of H₂O and subsequent abundance retrieval, coupled with analysis of other species such as CO at the K-band, for example, might help us to get some information about the atmospheric C/O ratio and metallicity, which in turn could give us some insight into the planet formation scenario.