HST-COS Transit Spectroscopy of KELT-20b: First Detection of Excess Far-ultraviolet Absorption From an Ultra-hot Jupiter

TL;DR

The paper addresses the challenge of characterizing the upper atmospheres of ultra-hot Jupiters around hot A-type stars, where XUV radiation is weak but FUV irradiation is strong. It uses first FUV transit spectroscopy with HST-COS to search for neutral and low-ionization species in KELT-20 b's atmosphere and to assess molecular absorption and escape signatures. The study finds that the FUV transit depth increases toward shorter wavelengths (from $1.88\pm0.04\%$ to $2.28\pm0.04\%$ across the specified bands), with tentative detections of Fe II and N I at $2.4\sigma$ and non-detections of C I, S I, Al II, and Si II, and no evidence for CO, H2, or hydrodynamic escape. These results place constraints on the upper-atmosphere composition and ionization under strong FUV irradiation and provide a baseline for atmospheric processes around A-type host stars in the UHJ regime.

Abstract

KELT-20 b is an ultra-hot 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 as well as metals including 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 due 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 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 $1.88\pm0.04$\% at 1600--1760 Å to $2.28\pm0.04$\% at 1410--1570 Å, yielding planetary radii of $0.1139\pm0.06$ $R_*$ and $0.1222\pm0.07$ $R_*$, respectively. We report tentative detections of Fe II and N I at $2.4σ$ each, and non-detections of C I, S I, Al II, and Si II. We find no evidence for molecular absorption from CO or H$_2$ and no sign of hydrodynamic escape.