IGRINS observations of WASP-127 b: H$_2$O, CO, and super-Solar atmospheric metallicity in the inflated sub-Saturn (2406.14072v1)

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$\mathrm{p}$= 1.311 R$\mathrm{Jup}$, M$\mathrm{p}$= 0.16 M$\mathrm{Jup}$) with previously reported detections of H$2$O, CO$_2$, and Na. However, the seeming absence of the primary carbon reservoir expected at WASP-127 b temperatures (T${eq}$ $\sim$ 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 H$2$O (8.67 $\sigma$) and report the detection of CO (4.34 $\sigma$). 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$\mathrm{H_2O}$ = --1.23$^{+0.29}{-0.49}$ and a lower limit on the CO abundance of log${10}$X$\mathrm{CO}$ $\ge$ --2.20 at 2$\sigma$ confidence. We also report a 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 ($\sim$ 39$\times$ 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.