The metallicity and carbon-to-oxygen ratio of the ultra-hot Jupiter WASP-76b from Gemini-S/IGRINS

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Measurements of the carbon-to-oxygen (C/O) ratios of exoplanet atmospheres can reveal details about their formation and evolution. Recently, high-resolution cross-correlation analysis has emerged as a method of precisely constraining the C/O ratios of hot Jupiter atmospheres. We present two transits of the ultra-hot Jupiter WASP-76b observed between 1.4-2.4 μm with Gemini-S/IGRINS. We detected the presence of H₂O, CO, and OH at signal-to-noise rations of 6.93, 6.47, and 3.90, respectively. We performed two retrievals on this data set. A free retrieval for abundances of these three species retrieved a volatile metallicity of [C+O/H]=-0.70+1.27-0.93, consistent with the stellar value, and a super-solar carbon-to-oxygen ratio of C/O=0.80+0.07-0.11. We also ran a chemically self-consistent grid retrieval, which agreed with the free retrieval within 1σ but favored a slightly more sub-stellar metallicity and solar C/O ratio ([C+O/H]=-0.74+0.23-0.17 and C/O=0.59+0.13-0.14). A variety of formation pathways may explain the composition of WASP-76b. Additionally, we found systemic (Vsys) and Keplerian (Kₚ) velocity offsets which were broadly consistent with expectations from 3D general circulation models of WASP-76b, with the exception of a redshifted Vsys for H₂O. Future observations to measure the phase-dependent velocity offsets and limb differences at high resolution on WASP-76b will be necessary to understand the H₂O velocity shift. Finally, we find that the population of exoplanets with precisely constrained C/O ratios generally trends toward super-solar C/O ratios. More results from high-resolution observations or JWST will serve to further elucidate any population-level trends.