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

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 m 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 +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 1 but favored a slightly more sub-stellar metallicity and solar C/O ratio (+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 (Vₒₘₒ) and Keplerian (K₏) velocity offsets which were broadly consistent with expectations from 3D general circulation models of WASP-76b, with the exception of a redshifted Vₒₘₒ 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.