Diurnal Variations in the Thermal Spectrum of WASP-33b

We present Keck/KPIC (Keck Planet Imager and Characterizer) high-resolution (R~30000) K-band spectroscopy of the ultra-hot Jupiter WASP-33b. The use of KPIC’s single-mode fiber greatly improves both blaze and line-spread stabilities, enhancing the cross-correlation detection strength. We perform separate retrievals for the dayside and nightside thermal emission spectra with a cross-correlation-to-logL nested-sampling pipeline which fits for orbital parameters, the atmospheric pressure-temperature profile, and molecular abundances. On the nightside, we find a non-inverted pressure-temperature profile, measure H2O and CO abundances, derive the atmospheric C/O ratio, and place a tentative constraint on the 13CO/12CO ratio. On the dayside, we find an inverted pressure-temperature profile and CO spectral features, but an absence of H2O features, suggesting H2O is photodissociated at dayside longitudes. Comparing the retrieved nightside CO and H2O abundances with chemical models suggests a moderately carbon- and metal-enriched atmosphere compared with solar values. This contrasts predictions from migration theories that C/O and atmospheric metallicity should be inversely correlated.