We report the discovery of KELT-20b , a hot Jupiter transiting a V \sim 7.6 early A star with an orbital period of P \simeq 3.47 days .
We identified the initial transit signal in KELT-North survey data .
Archival and follow-up photometry , the Gaia parallax , radial velocities , Doppler tomography , and adaptive optics imaging were used to confirm the planetary nature of the companion and characterize the system .
From global modeling we infer that the host star HD 185603 is a rapidly-rotating ( v \sin { I _ { * } } \simeq 120 ~ { } km s ^ { -1 } ) A2V star with an effective temperature of T _ { eff } = 8730 ^ { +250 } _ { -260 } K , mass of M _ { * } = 1.76 ^ { +0.14 } _ { -0.20 } M _ { \sun } , radius of R _ { * } = 1.561 ^ { +0.058 } _ { -0.064 } R _ { \sun } , surface gravity of \log { g _ { * } } = 4.292 ^ { +0.017 } _ { -0.020 } , and age of \lesssim 600 Myr .
The planetary companion has a radius of R \textsubscript { 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 \pm 0.000190 + E ( 3.4741070 \pm 0.0000019 ) .
We place a 3 \sigma upper limit of \sim 3.5 ~ { } M _ { J } on the mass of the planet .
The Doppler tomographic measurement indicates that the planetary orbit normal is well aligned with the projected spin-axis of the star ( \lambda = 3.4 \pm { 2.1 } degrees ) .
The inclination of the star is constrained to be 24.4 < I _ { * } < 155.6 degrees , implying a true ( three-dimensional ) spin-orbit alignment of 1.3 < \psi < 69.8 degrees .
The planet receives an insolation flux of \sim 8 \times 10 ^ { 9 } ~ { } { erg~ { } s ^ { -1 } ~ { } cm ^ { -2 } } , implying an equilibrium temperature of of \sim 2250 K , assuming zero albedo and complete heat redistribution .
Due to the high stellar T _ { eff } , the planet also receives an ultraviolet ( wavelengths d \leq 91.2 nm ) insolation flux of \sim 9.1 \times 10 ^ { 4 } ~ { } { erg~ { } s ^ { -1 } ~ { } cm ^ { -2 } } , which may lead to significant ablation of the planetary atmosphere .
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 .
The system is a slightly longer-period analog of the KELT-9 system .