2024AJ....168...14W

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 ultrahot Jupiter WASP-76b observed between 1.4 and 2.4 μm with the high-resolution Immersion GRating INfrared Spectrometer on the Gemini-S telescope. We detected the presence of H₂O, CO, and OH at signal-to-noise ratios 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 $\left[\tfrac{{\rm{C}}+{\rm{O}}}{{\rm{H}}}\right]=-{0.70}_{-0.93}^{+1.27}$, consistent with the stellar value, and a supersolar carbon-to-oxygen ratio of C/O$\,=\,{0.80}_{-0.11}^{+0.07}$. We also ran a chemically self-consistent grid retrieval, which agreed with the free retrieval within 1σ but favored a slightly more substellar metallicity and solar C/O ratio ($\left[\tfrac{{\rm{C}}+{\rm{O}}}{{\rm{H}}}\right]=-{0.74}_{-0.17}^{+0.23}$ and C/O$\,=\,{0.59}_{-0.14}^{+0.13}$). A variety of formation pathways may explain the composition of WASP-76b. Additionally, we found systemic (V_sys) and Keplerian (K_p) velocity offsets which were broadly consistent with expectations from 3D general circulation models of WASP-76b, with the exception of a redshifted V_sys 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.