Strong H2O and CO Emission Features in the Spectrum of KELT-20b Driven by Stellar UV Irradiation

Know thy star, know thy planetary atmosphere. Every exoplanet with atmospheric measurements orbits around a star, and the stellar environment directly affects the planetary atmosphere. Here we present the emission spectrum of ultra-hot Jupiter KELT-20b which provides an observational link between host-star properties and planet atmospheric thermal structure. It is currently the only planet with thermal emission measurements in the T-eq similar to 2200 K range that orbits around an early A-type star. By comparing it with other similar ultra-hot Jupiters around FGK stars, we can better understand how different host-star types influence planetary atmospheres. The emission spectrum covers 0.6-4.5 mu m with data from TESS, HST WFC3/G141, and Spitzer 4.5 mu m channel. KELT-20b has a 1.4 mu m water feature strength metric of S-H2O = -0.097 +/- 0.02 and a blackbody brightness temperature difference of 528 K between WFC3/G141 (T-b = 2402 +/- 14 K) and Spitzer 4.5 mu m channel (T-b = 2930 +/- 59 K). These very large H2O and CO emission features combined with the A-type host star make KELT-20b a unique planet among other similar hot Jupiters. The abundant FUV, NUV, and optical radiation from its host star (T-eff = 8720 +/- 250 K) is expected to be the key that drives its strong thermal inversion and prominent emission features based on previous PHOENIX model calculations.

Automated summary

This study analyzes the thermal emission spectrum of the ultra-hot Jupiter KELT-20b to reveal the direct relationship between planetary atmosphere and its surrounding star. By comparing it with other similar ultra-hot Jupiters, we can gain a better understanding of how different types of stars influence planetary atmospheres.