IGRINS observations of WASP-127 b: H₂O, CO, and super-Solar atmospheric metallicity in the inflated sub-Saturn

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 (Rₚ= 1.311 RJup, Mₚ= 0.16 MJup) with previously reported detections of H₂O, CO₂, and Na. However, the seeming absence of the primary carbon reservoir expected at WASP-127 b temperatures (Teq ~ 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 (IGRINS) on Gemini South. We confirm the presence of H₂O (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 H₂O abundance of log₁₀XH₂O = --1.23+0.29-0.49 and a lower limit on the CO abundance of log₁₀XCO ≥ --2.20 at 2σ confidence. We also report a tentative evidence of photochemistry in WASP-127 b based upon the indicative depletion of H₂S. 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.