Atmospheric Metallicity and C/O of HD 189733 b from High-resolution Spectroscopy

Abstract We present high-resolution K -band emission spectra of the quintessential hot Jupiter HD 189733 b from the Keck Planet Imager and Characterizer. Using a Bayesian retrieval framework, we fit the dayside pressure–temperature profile, orbital kinematics, mass-mixing ratios of H 2 O, CO, CH 4 , NH 3 , HCN, and H 2 S, and the 13 CO/ 12 CO ratio. We measure mass fractions of logH 2 O = − 2.0 − 0.4 + 0.4 and logCO = − 2.2 − 0.5 + 0.5 , and place upper limits on the remaining species. Notably, we find logCH 4 < −4.5 at 99% confidence, despite its anticipated presence at the equilibrium temperature of HD 189733 b assuming local thermal equilibrium. We make a tentative (∼3 σ ) detection of 13 CO, and the retrieved posteriors suggest a 12 C/ 13 C ratio similar to or substantially less than the local interstellar value. The possible 13 C enrichment would be consistent with accretion of fractionated material in ices or in the protoplanetary disk midplane. The retrieved abundances correspond to a substantially substellar atmospheric C/O = 0.3 ± 0.1, while the carbon and oxygen abundances are stellar to slightly superstellar, consistent with core-accretion models which predict an inverse correlation between C/O and metallicity. The specific combination of low C/O and high metallicity suggests significant accretion of solid material may have occurred late in the formation process of HD 189733 b.