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Confirmation of sub-solar metallicity for WASP-77Ab from JWST thermal emission spectroscopy Published: 5/12/2023 11:38:15 PM Updated: 5/12/2023 11:38:15 PM

Paper abstract: We present the dayside thermal emission spectrum of WASP-77Ab from 2.8 -- 5.2 \mum as observed with the NIRSpec instrument on the James Webb Space Telescope (JWST). WASP-77Ab was previously found to have a sub-solar metallicity and a solar carbon-to-oxygen (C/O) ratio from H_2O and CO absorption lines detected using high-resolution spectroscopy. By performing atmospheric retrievals on the JWST spectrum assuming chemical equilibrium, we find a sub-solar metallicity [M/H]=-0.91^{+0.24}_{-0.16} and C/O ratio 0.36^{+0.10}_{-0.09}. We identify H_2O and CO and constrain their abundances, and we find no CO_2 in the spectrum. The JWST and high-resolution spectroscopy results agree within ~1\sigma for the metallicity and within 1.8\sigma for the C/O ratio. However, our results fit less well in the picture painted by the shorter wavelength spectrum measured by HST WFC3. Comparing the JWST thermal emission spectra of WASP-77Ab and HD 149026b shows that both hot Jupiters have nearly identical brightness temperatures in the near-infrared, but distinctly different atmospheric compositions. Our results reaffirm high-resolution spectroscopy as a powerful and reliable method to measure molecular abundances. Our results also highlight the incredible diversity of hot Jupiter atmospheric compositions.