2024AJ....167...43F

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 H₂O, CO, CH₄, NH₃, HCN, and H₂S, and the ¹³CO/¹²CO ratio. We measure mass fractions of ${\mathrm{logH}}_{2}{\rm{O}}=-{2.0}_{-0.4}^{+0.4}$ and $\mathrm{logCO}=-{2.2}_{-0.5}^{+0.5}$, and place upper limits on the remaining species. Notably, we find logCH₄ < -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 (∼3σ) detection of ¹³CO, and the retrieved posteriors suggest a ¹²C/¹³C ratio similar to or substantially less than the local interstellar value. The possible ¹³C 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.