2025AJ....170..359B

KELT-20 b is an ultrahot Jupiter with an equilibrium temperature of 2260 K orbiting a bright (V = 7.6), fast-rotating ($v\sin i$ = 117 km s^–1) A2 V star. The atmosphere of KELT-20 b has been studied extensively via transmission spectroscopy at optical wavelengths, showing strong hydrogen absorption and metals such as Na I, Ca II, Fe I, Fe II, Mg I, Si I, and Cr II. The atmospheric and ionization conditions of this planet may differ from Jupiter-mass exoplanets owing to the relatively weak extreme-ultraviolet radiation from its host star, as the stellar dynamo that generates chromospheric and coronal activity is thought to shut down at spectral types earlier than A4. We present the first spectroscopic observations of KELT-20 b in the far-ultraviolet (FUV) using the Hubble Space Telescope Cosmic Origins Spectrograph, searching for previously undetected low-ionization and neutral atoms in the upper atmosphere. We find that the FUV transit depth increases with decreasing wavelengths, from 1.88% ± 0.04% at 1600-1760 Å to 2.28% ± 0.04% at 1410-1570 Å, yielding planetary radii of 0.1139R_* ± 0.06R_* and 0.1222R_* ± 0.07R_*, respectively. We report tentative detections of Fe II and N I at 2.4σ each and nondetections of C I, S I, Al II, and Si II. We find no evidence for molecular absorption from CO or H₂ and no sign of hydrodynamic escape.