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 ( ${\mathrm{logCO}}_{\mathrm{MMR}}=-{1.1}_{-0.6}^{+0.4}$ ), H₂O ( ${\mathrm{logH}}_{2}{{\rm{O}}}_{\mathrm{MMR}}\,=-{4.1}_{-0.9}^{+0.7}$ ), and OH ( ${\mathrm{logOH}}_{\mathrm{MMR}}=-{2.1}_{-1.1}^{+0.5}$ ), suggesting near-complete dayside photodissociation of H₂O. 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 CO₂ snow line and post-disk migration or with accretion between the soot and H₂O snow lines. We also find tentative evidence for ¹²CO/¹³CO ~ 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.