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

We present Keck Planet Imager and Characterizer (KPIC) high-resolution (R similar to 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)), H2O (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 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 (CO)-C-12/(CO)-C-13 similar to 50, consistent with values expected in protoplanetary disks, as well as tentative evidence for a metal-enriched atmosphere (2-15 x 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.