[P-EM11] Exoplanet

[PEM11-04] Unveiling the Atmosphere of a Hot-Saturn: Detection of Water Vapour in HD 149026b with High-Resolution Spectroscopy

*Sayyed Ali Rafi¹, Stevanus Kristianto Nugroho^2,3, Motohide Tamura^1,2,3 (1.Department of Astronomy, University of Tokyo, 2.Astrobiology Center, 3.National Astronomical Observatory of Japan)

Keywords:Exoplanet atmosphere, Transmission spectroscopy, High resolution spectroscopy, Atmosphere composition

Transmission spectroscopy presents one of the most straightforward approaches for investigating the atmospheres of exoplanets. This method entails analysis of the planetary absorption features present in the stellar light that passed through the planetary atmosphere. As such, planets with hot and expanded atmospheres, such as hot gas-giants, are particularly suitable targets for this technique. With high-resolution spectroscopy, hundreds to thousands of planetary spectral lines can be discerned, and combining these via cross-correlation technique will boost the atmosphere signal, facilitating the robust detection of individual chemical species. In this work, we report the detection of a water vapor (H₂O) signature at >5-σ in the near-infrared transmission spectrum of the hot-Saturn HD 149026b, using archival data obtained with the CARMENES spectrograph (R~80,400). Through likelihood analysis, we were able to constrain the planet's semi-amplitude radial velocity and its systemic velocity. Interestingly, the systemic velocity was found to be highly red-shifted relative to the expected value. This might be caused by several possible factors such as errors in the transit mid-point or even an indicative of atmospheric dynamics from e.g., high-altitude winds. Searches for HCN and CH₄ were not successful, and subsequent model injection tests suggest that this may have been due to the insensitiveness of the data to these molecules (due to the relatively low S/N), at least for the assumed atmospheric composition. Finally, the detection of H₂O in this high-density planet, along with future determinations of its abundance, will offer new avenues for understanding the planet's formation history.