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DRAFT VERSION JULY 7, 2017 Preprint typeset using LATEX style emulateapj v. 12/16/11 KELT-20b: A GIANT PLANET WITH A PERIOD OF P ∼ 3:5 DAYS TRANSITING THE V ∼ 7:6 EARLY A STAR HD 185603 MICHAEL B. LUND1 ,JOSEPH E. RODRIGUEZ2 ,GEORGE ZHOU2 , B. SCOTT GAUDI3 ,KEIVAN G. STASSUN1,4 ,MARSHALL C. JOHNSON3 ,ALLYSON BIERYLA2 ,RYAN J. OELKERS1 ,DANIEL J. STEVENS3 ,KAREN A. COLLINS1 ,KALOYAN PENEV5 ,SAMUEL N. QUINN2 ,DAVID W. LATHAM2 ,STEVEN VILLANUEVA JR.3 ,JASON D. EASTMAN2 ,JOHN F. KIELKOPF6 ,THOMAS E. OBERST7 ,ERIC L. N. JENSEN8 ,DAVID H. COHEN8 ,MICHAEL D. JONER9 ,DENISE C. STEPHENS9 ,HOWARD RELLES2 ,GIORGIO CORFINI10,* ,JOAO GREGORIO11 ,ROBERTO ZAMBELLI10 ,GILBERT A. ESQUERDO2 ,MICHAEL L. CALKINS2 ,PERRY BERLIND2 ,DAVID R. CIARDI12 , COURTNEY DRESSING13,14 ,RAHUL PATEL15 ,PATRICK GAGNON16,12 ,ERICA GONZALES17 ,THOMAS G. BEATTY18,19 ,ROBERT J. SIVERD20 ,JONATHAN LABADIE-BARTZ21 ,RUDOLF B. KUHN22,23 ,KNICOLE D. COLON´ 24 ,DAVID JAMES25 ,JOSHUA PEPPER21 , BENJAMIN J. FULTON26, 27 ,KIM K. MCLEOD28 ,CHRISTOPHER STOCKDALE29 ,SEBASTIANO CALCHI NOVATI15,30 , D. L. DEPOY31, 32 , ANDREW GOULD3 ,JENNIFER L. MARSHALL31, 32 ,MARK TRUEBLOOD33 ,PATRICIA TRUEBLOOD33 Draft version July 7, 2017 ABSTRACT 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 modeling we infer that −1 the host star HD 185603 is a rapidly-rotating (v sin I∗ ' 120 km s ) A2V star with an effective temperature +250 +0:14 +0:058 of Teff = 8730−260 K, mass of M∗ = 1:76−0:20 M , radius of R∗ = 1:561−0:064 R , surface gravity of +0:017 +0:070 log g∗ = 4:292−0:020, and age of . 600 Myr. The planetary companion has a radius of RP = 1:735−0:075 RJ, a +0:0014 semimajor axis of a = 0:0542−0:0021AU, 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. The Doppler tomographic measurement indicates that the planetary orbit normal is well aligned with the projected spin-axis of the star (λ = 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 < < 69:8 degrees. The planet 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 , the planet also receives an ultraviolet (wavelengths d ≤ 91:2 nm) insolation flux of ∼ 9:1 × 104 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. Subject headings: planets and satellites: detection – planets and satellites: gaseous planets – stars: individual (HD 185603) – techniques: photometric – techniques: radial velocities – methods: obser- vational 1 Department of Physics and Astronomy, Vanderbilt University, Pasadena, CA 91125, USA Nashville, TN 37235, USA; [email protected] 16 2 College of the Canyons, 26455 Rockwell Canyon Rd., Santa Clarita, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA CA 91355, USA 02138, USA 17 3 University of California, Santa Cruz, CA, USA Department of Astronomy, The Ohio State University, Columbus, OH 18 43210, USA Department of Astronomy & Astrophysics, The Pennsylvania State 4 Department of Physics, Fisk University, 1000 17th Avenue North, University, 525 Davey Lab, University Park, PA 16802, USA 19 Nashville, TN 37208, USA Center for Exoplanets and Habitable Worlds, The Pennsylvania State arXiv:1707.01518v1 [astro-ph.EP] 5 Jul 2017 5 Department of Astrophysical Sciences, Princeton University, Prince- University, 525 Davey Lab, University Park, PA 16802, USA ton, NJ 08544, USA 20 Las Cumbres Observatory, 6740 Cortona Dr., Suite 102, Goleta, CA 6 Department of Physics and Astronomy, University of Louisville, 93117, USA Louisville, KY 40292, USA 21 Department of Physics, Lehigh University, 16 Memorial Drive East, 7 Department of Physics, Westminster College, New Wilmington, PA Bethlehem, PA, 18015, USA 16172, USA 22 South African Astronomical Observatory, PO Box 9, Observatory, 8 Department of Physics and Astronomy, Swarthmore College, Swarth- 7935 Cape Town, South Africa more, PA 19081, USA 23 Southern African Large Telescope, PO Box 9, Observatory, 7935 9 Department of Physics and Astronomy, Brigham Young University, Cape Town, South Africa Provo, UT 84602, USA 24 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 10 Societ Astronomica Lunae, Castelnuovo Magra 19030, Italy 25 Astronomy Department, University of Washington, Box 351580, 11 Atalaia Group & CROW Observatory, Portalegre, Portugal Seattle, WA 98195, USA 12 NASA Exoplanet Science Institute/Caltech, Pasadena, CA, USA 26 Institute for Astronomy, University of Hawaii, 2680 Woodlawn 13 NASA Sagan Fellow, Division of Geological & Planetary Sciences, Drive, Honolulu, HI 96822-1839, USA California Institute of Technology, Pasadena, CA 91125, USA 27 California Institute of Technology, Pasadena, CA 91125, USA 14 Department of Astronomy, University of California, Berkeley, CA 28 Department of Astronomy, Wellesley College, Wellesley, MA 02481, 94720-3411, USA USA 15 IPAC, Mail Code 100-22, Caltech, 1200 E. California Blvd., 29 Hazelwood Observatory, Churchill, Victoria, Australia 2 1. INTRODUCTION the field of exoplanets has developed, there have been two The first surveys for exoplanets, which primarily used the broad goals: determining the overall demographics of exo- planets and how these demographics depend on the proper- radial velocity method35, focused on sunlike (late F, G and ties of the planets and their host stars, and finding individ- early K) dwarf stars. This was due to the fact that old stars ual exoplanets that can be characterized in detail, in particular with T below the Kraft break (Kraft 1967) at T ' 6250 K eff eff their atmospheres. The primary techniques for characterizing tend to be slowly rotating and have plentiful absorption lines, exoplanet atmospheres are transits and direct imaging. The therefore enabling the sub-tens of meters per second precision combination of transit photometry and radial velocity mea- that was expected to be needed to detect analogs of the planets surements can provide a planet’s radius and mass and, by ex- in our solar system. Stars cooler than early K also have plen- tension, its density and bulk composition. Beyond this, phase tiful lines, but are generally faint in the optical, where these curves and spectroscopy of transits and eclipses can shed light initial surveys were carried out. Given the high-resolution on the atmospheric properties of the system. Although planet (R 50; 000) spectra needed to resolve the stellar spectral & densities can be determined even for quite faint host stars, de- lines, high photon counts were difficult to acquire for cooler tailed spectra and phase curves benefit greatly from having stars with the modest-aperture telescopes that were then avail- host stars that are bright (Seager & Deming 2010). Indeed, able at the time. finding such bright transit hosts is one of the primary moti- Of course, it came as a surprise when the first exoplan- vations of the Transiting Exoplanet Survey Satellite (Ricker ets discovered around main-sequence stars (Campbell et al. et al. 2015). 1988; Latham et al. 1989; Mayor & Queloz 1995; Marcy & The Kilodegree Extremely Little Telescope Survey (KELT; Butler 1996) did not resemble the planets in our solar system, Pepper et al. 2003, 2007, 2012) was originally designed to and typically induced much higher radial velocity (RV) ampli- find transiting hot Jupiters orbiting bright (8 V 10) tudes than even our own giant planets. Indeed, the Jupiter-like . stars, precisely the targets best suited for follow-up and at- planetary companion to 51 Pegasi (Mayor & Queloz 1995), mospheric characterization. Nevertheless, the KELT survey which jump-started the field of exoplanets (despite not being did not start actively vetting targets until around 2011, by the first exoplanet discovered), has such a short period that it which point many ground-based transit surveys had discov- creates a reflex RV amplitude on its host star of hundreds of ered a number of transiting planets orbiting moderately bright meters per second. It is the prototypical “hot Jupiter”, a class stars (Alonso et al. 2004; McCullough et al. 2005; Bakos et al. of planets that are now known to orbit ∼ 0:5 − 1% of stars 2007; Collier Cameron et al. 2007). (Gould et al. 2006; Howard et al. 2012; Wright et al. 2012), Concurrently, while the overall picture of the demograph- but whose origins and characteristics remain important topics ics of planetary systems orbiting late F to M stars was start- of study. ing to become clear, the properties of planetary systems orbit- Subsequent surveys for exoplanets, including those using ing more massive and hotter stars remained relatively murky. the transit (Winn et al. 2010) and microlensing (Gaudi 2012) This was largely because the workhorse planet detection tech- methods, began to more fully explore the planet populations nique, RVs, begins to have difficulties achieving precisions of of lower-mass stars, and in particular around M dwarfs. The better than a few hundred meters per second for stars above reasons for this are clear: RV, transit, and microlensing sur- T ' 6250K, both because these stars have thin convec- veys are all more sensitive to planets orbiting low-mass stars eff tive envelopes and so do not spin down with age due to mag- (albeit for different reasons, see Wright & Gaudi 2013).

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