17082

Ultrahot Jupiters (UHJs) provide us with an exciting opportunity to probe the refractory content of exoplanetary atmospheres. These planets are hot enough to retain species such as iron and silicon in gaseous phase comma which contribute significant opacity in the optical and near-IR but especially in near-UV. By observing a planet.s transmission spectrum across this wide wavelength range with HST comma we can measure the refractory abundance in its atmosphere. Additionally comma we can characterize the refractory-to-volatile ratio of the planet by simultaneously measuring the water abundance in the near-IR. The joint characterization of refractory and volatile abundances can place significantly tighter constraints on the origin of gas giant planets than volatiles alone can. This technique has been used to characterize two UHJs (WASP-121b and WASP-178b) so far but both of them have polar orbits around their host stars. Here comma we propose to observe the UHJ KELT-20b (Teq = 2260 K) that orbits a very bright and hot star (V mag = 7.6 comma Teff = 8720 K) and yet has a projected obliquity of 0.6 +-- 4 degrees. This extremely rare dynamical configuration for KELT-20b points to a formation and migration history that is potentially different from its compatriots. We will obtain a comprehensive transmission spectrum of KELT-20b and characterize both its refractory and water content by observing it with the WFC3 instrument in the UVIS comma G102 comma and G141 bandpasses. With our proposed observations comma we can strongly distinguish between different formation scenarios comma evaluate the role of rainout comma and determine if KELT-20b.s distinct dynamical configuration is reflected in its atmospheric composition.