Keck/KPIC Emission Spectroscopy of WASP-33b

Abstract

We present Keck/KPIC high-resolution ($R\sim 35,000$) $K$-band thermal emission spectroscopy of the ultra-hot 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 molecular abundances.We strongly detect the thermally-inverted dayside and measure mass-mixing ratios for CO ($\log \mathrm{C}{\mathrm{O}}_{\mathrm{MMR}}=-1.1_{-0.6}^{+0.4}$), H${}_2$O ($\log {\mathrm{H}}_2{\mathrm{O}}_{\mathrm{MMR}}=-4.1_{-0.9}^{+0.7}$) and OH ($\log \mathrm{O}{\mathrm{H}}_{\mathrm{MMR}}=-2.1_{-1.1}^{+0.5}$), 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 ${}^{12}\mathrm{CO}{/}^{13}\mathrm{CO}\sim 50$, consistent with values expected in protoplanetary disks, as well as tentative evidence for a metal-enriched atmosphere (2--15$\times$ 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.

Accepted in AJ, 26 pages, 12 figures