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

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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">R</mml:mi> </mml:mrow> <mml:mo>∼</mml:mo> <mml:mstyle displaystyle="false"> <mml:mtext>80,400</mml:mtext> </mml:mstyle> </mml:math> ). We found evidence of H 2 O at a signal-to-noise ratio (S/N) of ∼4.8. We also performed grid search using a Bayesian framework and constrained the orbital velocity K p and rest velocity V rest to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>158.17</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>7.90</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>8.31</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> km s −1 and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>2.57</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.57</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.54</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> km s −1 , respectively. While the retrieved K p value is consistent with theoretical prediction, the retrieved V rest value is highly redshifted (&gt;3 σ ). This might be an indication of either anomalous atmospheric dynamics at play or an orbit with nonzero eccentricity. Additionally, we searched for HCN but no successful detection has been made, possibly due to the relatively low-S/N data set. The detection of H 2 O and subsequent retrieval of its abundance, coupled with analysis of other species such as CO in 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’s formation scenario.