Keck Planet Imager and Characterizer Emission Spectroscopy of WASP-33b

We present Keck Planet Imager and Characterizer (KPIC) high-resolution ( R ∼35,000) K -band thermal emission spectroscopy of the ultrahot Jupiter WASP-33b. The use of KPIC’s single-mode fibers greatly improves both blaze and line-spread stabilities relative to slit spectrographs, enhancing the cross-correlation detection strength. We retrieve the dayside emission spectrum with a nested-sampling pipeline, which fits for orbital parameters, the atmospheric pressure–temperature profile, and the molecular abundances. We strongly detect the thermally inverted dayside and measure mass-mixing ratios for CO ( logCO MMR = − 1.1 − 0.6 + 0.4 ), H 2 O ( logH 2 O MMR = − 4.1 − 0.9 + 0.7 ), and OH ( logOH MMR = − 2.1 − 1.1 + 0.5 ), suggesting near-complete dayside photodissociation of H 2 O. The retrieved abundances suggest a carbon- and possibly metal-enriched atmosphere, with a gas-phase C/O ratio of 0.8 − 0.2 + 0.1 , consistent with the accretion of high-metallicity gas near the CO 2 snow line and post-disk migration or with accretion between the soot and H 2 O snow lines. We also find tentative evidence for 12 CO/ 13 CO ∼ 50, consistent with values expected in protoplanetary disks, as well as tentative evidence for a metal-enriched atmosphere (2–15 × solar). These observations demonstrate KPIC’s ability to characterize close-in planets and the utility of KPIC’s improved instrumental stability for cross-correlation techniques.