2025AJ....170..223F

We present a joint analysis of high-resolution K- and L-band observations of the benchmark hot Jupiter HD 209458 b from the Keck Planet Imager and Characterizer. One half-night of observations was obtained in each bandpass, covering similar preeclipse phases. The two epochs were then jointly analyzed using our atmospheric retrieval pipeline based on petitRADTRANS to constrain the atmospheric pressure-temperature profile and chemical composition. Consistent with recent results from JWST observations at lower spectral resolution, we obtain an oxygen-rich composition for HD 209458 b (C/O < 10^–3 at 95% confidence) and a lower limit on the volatile metallicity similar to the solar value ([(C + O)/H] > -0.2 at 95% confidence). Leveraging the large spectral grasp of the multiband observations, we constrain the atmospheric H₂O mixing ratio to ${\mathrm{log}}\,{{{\rm{H}}}}_{2}{{{\rm{O}}}}_{{\mathrm{V\, MR}}}\gt -3.1$ at 95% confidence, and obtain 95% upper limits on the atmospheric mixing ratios of CO (<10^–4.8), CH₄ (<10^–4.5), NH₃ (<10^–5.8), H₂S (<10^–3.3), and HCN (<10^–5.6). The limits on CH₄, NH₃, and HCN are consistent with recent results from JWST transmission spectroscopy, demonstrating the value of multiband, ground-based high-resolution spectroscopy for precisely constraining trace-species abundances in exoplanet atmospheres. The retrieved low-C/O, moderate-metallicity composition for HD 209458 b is consistent with formation scenarios involving late accretion of substantial quantities of oxygen-rich refractory solids and/or ices.