Strong H<sub>2</sub>O and CO Emission Features in the Spectrum of KELT-20b Driven by Stellar UV Irradiation
Guangwei Fu, David K. Sing, Joshua D. Lothringer, Drake Deming, Jegug Ih, Eliza M.-R. Kempton, Matej Malik, Thaddeus D. Komacek, Megan Mansfield, Jacob L. Bean
Abstract
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 ∼ 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 μ m with data from TESS, HST WFC3/G141, and Spitzer 4.5 μ m channel. KELT-20b has a 1.4 μ m water feature strength metric of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">S</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> </mml:msub> </mml:math> = −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 μ m channel (T b = 2930 ± 59 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 ± 250 K) is expected to be the key that drives its strong thermal inversion and prominent emission features based on previous PHOENIX model calculations.