IGRINS observations of WASP-127 b: H${}_2$O, 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${}_{\mathrm{p}}$= 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 $\ddot{I}ƒ$) and report the detection of CO (4.34 $\ddot{I}ƒ$). 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${}_{{\mathrm{H}}_2\mathrm{O}}$ = --1.23${}_{-0.49}^{+0.29}$ and a lower limit on the CO abundance of log${}_{10}$X${}_{\mathrm{CO}}$ $\ge$ --2.20 at 2$\ddot{I}ƒ$ 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.

18 pages, 15 figures, submitted to AJ, poster at Exo5 conference area-A