Confirmation of sub-solar metallicity for WASP-77Ab from JWST thermal emission spectroscopy

Abstract

We present the dayside thermal emission spectrum of WASP-77Ab from 2.8 -- 5.2 $\hat{I}¼$m 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${}_2$O 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]=$-0.91_{-0.16}^{+0.24}$ and C/O ratio $0.36_{-0.09}^{+0.10}$. We identify H${}_2$O and CO and constrain their abundances, and we find no CO${}_2$ in the spectrum. The JWST and high-resolution spectroscopy results agree within $\sim 1\ddot{I}ƒ$ for the metallicity and within 1.8$\ddot{I}ƒ$ 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.

Accepted for publication in ApJL