* Astronomy

Title: KELT-20b: A giant planet with a period of P~ 3.5 days transiting the V~ 7.6 early A star HD 185603
Author: Michael B. Lund, Joseph E. Rodriguez, George Zhou, B. Scott Gaudi, Keivan G. Stassun, Marshall C. Johnson, Allyson Bieryla, Ryan J. Oelkers, Daniel J. Stevens, Karen A. Collins, Kaloyan Penev, Samuel N. Quinn, David W. Latham, Steven Villanueva Jr., Jason D. Eastman, John F. Kielkopf, Thomas E. Oberst, Eric L. N. Jensen, David H. Cohen, Michael D. Joner, Denise C. Stephens, Howard Relles, Giorgio Corfini, Joao Gregorio, Roberto Zambelli, Gilbert A. Esquerdo, Michael L. Calkins, Perry Berlind, David R. Ciardi, Courtney Dressing, Rahul Patel, Patrick Gagnon, Erica Gonzales, Thomas G. Beatty, Robert J. Siverd, Jonathan Labadie-Bartz, Rudolf B. Kuhn, Knicole D. Colon, David James, Joshua Pepper, Benjamin J. Fulton, Kim K. McLeod, Christopher Stockdale, Sebastiano Calchi Novati, D. L. DePoy, Andrew Gould, et al. (3 additional authors not shown)

We report the discovery of KELT-20b, a hot Jupiter transiting a V~7.6 early A star with an orbital period of P~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 modelling we infer that the host star HD 185603 is a rapidly-rotating (VsinI~120 km/s) A2V star with an effective temperature of T_eff=8730K, mass of M_star=1.76M_sun, radius of R_star=1.561R_sun, surface gravity of logg=4.292, and age of <600 Myr. The planetary companion has a radius of 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 3 sigma upper limit of ~3.5 M_J on the mass of the planet. The Doppler tomographic measurement indicates that the planetary orbit is well aligned with the projected spin-axis of the star (lambda=3.4±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 ~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, the planet also receives an ultraviolet (wavelengths d \le 91.2~nm) insolation flux of ~9.1x 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.

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