2017AJ....154..194L

We report the discovery of KELT-20b, a hot Jupiter transiting a V∼7.6 early A star, HD 185603, with an orbital period of P≃3.47 days. Archival and follow-up photometry, Gaia parallax, radial velocities, Doppler tomography, and AO imaging were used to confirm the planetary nature of KELT-20b and characterize the system. From global modeling we infer that KELT-20 is a rapidly rotating (vsinI_*≃120 km s^–1) A2V star with an effective temperature of T_eff=8730_–260⁺²⁵⁰ K, mass of M_*=1.76_–0.20^+0.14  M_☉, radius of R_*=1.561_–0.064^+0.058  R_☉, surface gravity of logg_*=4.292_–0.020^+0.017, and age of 600 Myr. The planetary companion has a radius of R_P=1.735_–0.075^+0.070  R_J, a semimajor axis of a=0.0542_–0.0021^+0.0014 au, and a linear ephemeris of BJD_TDB=2457503.120049±0.000190+E(3.4741070±0.0000019). We place a 3σ upper limit of ∼3.5  M_J on the mass of the planet. Doppler tomographic measurements indicate that the planetary orbit normal is well aligned with the projected spin axis of the star (λ=3○ / . 4±2○ / . 1). The inclination of the star is constrained to 24○ / . 4 I_* 155○ / . 6, implying a three-dimensional spin-orbit alignment of 1○ / . 3 ψ69○ / . 8. KELT-20b receives an insolation flux of ∼8×10⁹ erg s^–1 cm^–2, implying an equilibrium temperature of of ∼2250 K, assuming zero albedo and complete heat redistribution. Due to the high stellar T_eff, KELT-20b also receives an ultraviolet (wavelength d 91.2 nm) insolation flux of ∼9.1×10⁴ erg s^–1 cm^–2, possibly indicating significant atmospheric ablation. Together with WASP-33, Kepler-13 A, HAT-P-57, KELT-17, and KELT-9, KELT-20 is the sixth A star host of a transiting giant planet, and the third-brightest host (in V) of a transiting planet.