2025AJ....169..333F

We present atmospheric retrievals from Keck/KPIC Phase II observations of the ultrahot Jupiter (UHJ) KELT-20/MASCARA-2 b. Previous free retrievals of molecular abundances for UHJs have been impacted by significant model biases due to variations in vertical abundance profiles, which we address by including molecular dissociation into our retrieval framework as an additional free parameter. We measure the abundance of CO ($\mathrm{log}{{\rm{CO}}}_{{\rm{MMR}}}=-2.{5}_{-0.5}^{+0.6}$) and obtain a lower limit on the abundance of H₂O ($\mathrm{log}{{\rm{H}}}_{2}{{\rm{O}}}_{\mathrm{MMR}}=-1.{5}_{-1.0}^{+0.8}$, >-3.0 at 95% confidence) in the atmosphere of KELT-20 b. These abundances yield an atmospheric ${\rm{C}}/{\rm{O}}=0.{1}_{-0.1}^{+0.4}$ (C/O < 0.9 at 95% confidence) and suggest a metallicity approximately solar to 10 × solar. H₂O is dissociated at pressures below ${\rm{log}}{P}_{{{\rm{H}}}_{2}{\rm{O}}}=-1.{2}_{-0.7}^{+0.5}$ bar, roughly consistent with predictions from chemical equilibrium models, and suggesting that the retrieved composition is not a result of assumptions about the vertical mixing profiles. We also constrain the rotational velocity of KELT-20 b to $v\sin i=7.5\pm 0.7$ km s^–1, suggesting the presence of a jet comparable to the sound speed in the direction of the planet's rotation, assuming the actual rotation of the planet is tidally locked.