Strong H$_2$O and CO emission features in the spectrum of KELT-20b driven by stellar UV irradiation

TL;DR

This study presents the emission spectrum of the ultra-hot Jupiter KELT-20b, revealing strong H$_2$O and CO emission features driven by its early A-type star's intense UV radiation, providing insights into star-planet atmospheric interactions.

Contribution

First detailed emission spectrum of KELT-20b, linking stellar UV irradiation to atmospheric thermal structure in ultra-hot Jupiters around early A-type stars.

Findings

Large H$_2$O and CO emission features observed

Strong thermal inversion likely driven by stellar UV radiation

Unique spectral characteristics among similar hot Jupiters

Abstract

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}\sim$2200K 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 to 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$_{H_2O}$ = -0.097$\pm$0.02 and a blackbody brightness temperature difference of 528K between WFC3/G141 (T$_b$=2402$\pm$14K) and Spitzer 4.5 $\mu m$ channel (T$_b$=2930$\pm59$K). These very large H$_2$O 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$_{eff}=8720\pm250$K) is expected to be the key that drives its strong thermal inversion and prominent emission features based on previous PHOENIX models calculations.