KELT-20b: A Giant Planet with a Period of P ∼ 3.5 days Transiting the V ∼ 7.6 Early A Star HD 185603

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 (ν sin I* ≃ 120 km s^(-1)) A2V star with an effective temperature of T_(eff) = 8730^(+250)_(-260) K, mass of, M* = ^(+0.14)_(-0.20) M⊙ radius of, R* = 1.561^(+0.058)_(-0.064) R⊙ surface gravity of, log g* = 4.292^(+0.017)_(-0.020), and age of ≾600 Myr. The planetary companion has a radius of R_P = 1.735^(+0.070)_(-0.075) R_J, a semimajor axis of a = 0.0542^(+0.0014)_(-0.0021) 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 < ψ < 69.°8. KELT-20b receives an insolation flux of ~8 x 10^9 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 x 10^4 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.