Atmospheric characterization of six ultra-hot Jupiters from K-band high-resolution spectroscopy

We present new Keck/KPIC high-resolution spectroscopic detections of three ultra-hot Jupiters (UHJs) in the K band: WASP-189b (SNR = 7.2), MASCARA-1b (SNR = 8.6), and TOI-1518b (SNR = 7.1), as well as a tentative detection of KELT-9b (SNR = 5.0). We perform a uniform set of atmospheric retrieval analysis on these objects, as well as previously reported KPIC observations of WASP-33b (SNR = 11.2) and KELT-20b (SNR = 10.5), We perform atmospheric retrievals for the pressure-temperature (P-T) profile, orbital velocity parameters, v i, and abundances of CO, H 2 O, OH, and Fe, with parameterized mixing profiles to account for the expected vertical abundance variations of H 2 O and OH. We also perform a set of retrievals assuming chemical equilibrium, which are generally in good agreement with the free retrievals. Except for, the retrieved spectra are dominated by CO emission features, with additional weak H 2 O or OH features consistent with thermal dissociation of H 2 O., which is significantly hotter, appears to have very weak molecular features. Dissociation limits our ability to reliably constrain H 2 O or OH abundances from K band data alone, resulting in poor constraints on the C/O ratio. For all objects, the atmospheric abundances from detected carbon and oxygen species are 1-10 times solar. These results highlight the importance of wide spectral coverage for high-resolution retrievals. Additional observations to expand phase and wavelength coverage are needed to better constrain oxygen species and possible spatial inhomogeneities from dissociation.