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

TL;DR

This study used high-resolution transmission spectroscopy to detect water vapor in the atmosphere of the hot Saturn HD 149026 b, providing insights into its atmospheric composition and dynamics.

Contribution

First detection of water vapor in the atmosphere of a metal-rich hot Saturn using high-resolution spectroscopy and Bayesian analysis of orbital velocities.

Findings

Detected H$_2$O at S/N of 4.8

Constrained orbital velocity $K_p$ and rest velocity $V_{rest}$

No detection of HCN possibly due to low S/N

Abstract

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 ($\mathcal{R}\sim80,400$). We found evidence of H$_2$O at an S/N of $\sim$4.8. We also performed grid search using a Bayesian framework and constrained the orbital velocity $K_\mathrm{p}$ and rest velocity $V_{\mathrm{rest}}$ to $158.17^{+8.31}_{-7.90}$ $\mathrm{km\ s}^{-1}$ and $2.57^{+0.54}_{-0.57}$ $\mathrm{km\ s}^{-1}$, respectively. Whilst the retrieved $K_\mathrm{p}$ value is consistent with theoretical prediction, the retrieved $V_{\mathrm{rest}}$ value is highly red-shifted ($>$3-$\sigma$). This might be an indication of either anomalous atmospheric dynamics at play or an orbit with non-zero eccentricity. Additionally, we searched for HCN but no successful detection has been made possibly due to the relatively low S/N dataset. The detection of H$_2$O 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.