November 2017 • 2017AJ....154..194L
Abstract • 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 (v sin I* ≃ 120 km s-1) A2V star with an effective temperature of Teff= 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 log g* = 4.292-0.020+0.017, and age of ≲ 600 Myr. The planetary companion has a radius of RP= 1.735-0.075+0.070 RJ, a semimajor axis of a=0.0542-0.0021+0.0014 au, and a linear ephemeris of BJDTDB=2457503.120049± 0.000190 +E(3.4741070± 0.0000019). We place a 3σ upper limit of ∼ 3.5 MJ 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× 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 Teff, 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.