IGRINS Observations of WASP-127 b: H2O, CO, and Super-solar Atmospheric Metallicity in the Inflated Sub-Saturn

Abstract High-resolution spectroscopy of exoplanet atmospheres provides insights into their composition and dynamics from the resolved line shape and depth of thousands of spectral lines. WASP-127 b is an extremely inflated sub-Saturn ( R p = 1.311 R Jup , M p = 0.16 M Jup ) with previously reported detections of H 2 O and CO 2 . However, the seeming absence of the primary carbon reservoir expected at WASP-127 b temperatures ( T eq ∼1400 K) from chemical equilibrium, CO, posed a mystery. In this manuscript, we present the analysis of high-resolution observations of WASP-127 b with the Immersion Grating Infrared Spectrometer on Gemini South. We confirm the presence of H 2 O (8.67 σ ) and report the detection of CO (4.34 σ ). Additionally, we conduct a suite of Bayesian retrieval analyses covering a hierarchy of model complexity and self-consistency. When freely fitting for the molecular gas volume mixing ratios, we obtain super-solar metal enrichment for H 2 O abundance of log 10 X H 2 O = −1.23 − 0.49 + 0.29 and a lower limit on the CO abundance of log 10 X CO ≥–2.20 at 2 σ confidence. We also report tentative evidence of photochemistry in WASP-127 b based upon the indicative depletion of H 2 S. This is also supported by the data preferring models with photochemistry over free-chemistry and thermochemistry. The overall analysis implies a super-solar (∼39× Solar; M/H = 1.59 − 0.30 + 0.30 ) metallicity for the atmosphere of WASP-127 b and an upper limit on its atmospheric C/O ratio as < 0.68.