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 similar to 7.6 early A star, HD 185603, with an orbital period of P similar or equal to 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 (v sin I* similar or equal to 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-circle dot, radius of R* = 1.561(-0.064)(+0.058) R-circle dot, surface gravity of log g* = 4.292(-0.020)(+0.017), and age of less than or similar to 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 3 sigma upper limit of similar to 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 (lambda = 3 degrees.4 +/- 2 degrees.1). The inclination of the star is constrained to 24 degrees.4 < I* < 155 degrees.6, implying a three-dimensional spin-orbit alignment of 1 degrees.3 < psi < 69 degrees.8. KELT-20b receives an insolation flux of similar to 8 x 10(9) erg s(-1) cm(-2), implying an equilibrium temperature of of similar to 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 similar to 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.