The Astronomical Journal, 153:215 (15pp), 2017 May https://doi.org/10.3847/1538-3881/aa6572 © 2017. The American Astronomical Society. All rights reserved. KELT-11b: A Highly Inflated Sub-Saturn Exoplanet Transiting the V = 8 Subgiant HD 93396 Joshua Pepper1, Joseph E. Rodriguez2, Karen A. Collins2,3, John Asher Johnson4, Benjamin J. Fulton5, Andrew W. Howard5, Thomas G. Beatty6,7, Keivan G. Stassun2,3, Howard Isaacson8, Knicole D. Colón1,9,10,11, Michael B. Lund2, Rudolf B. Kuhn12, Robert J. Siverd13, B. Scott Gaudi14, T. G. Tan15, Ivan Curtis16, Christopher Stockdale17,18, Dimitri Mawet19,20, Michael Bottom19,20, David James21, George Zhou4, Daniel Bayliss22, Phillip Cargile4, Allyson Bieryla4, Kaloyan Penev23, David W. Latham4, Jonathan Labadie-Bartz1, John Kielkopf24, Jason D. Eastman4, Thomas E. Oberst25, Eric L. N. Jensen26, Peter Nelson27, David H. Sliski28, Robert A. Wittenmyer29,30,31, Nate McCrady32, Jason T. Wright6,7, Howard M. Relles4, Daniel J. Stevens14, Michael D. Joner33, and Eric Hintz33 1 Department of Physics, Lehigh University, 16 Memorial Drive East, Bethlehem, PA 18015, USA 2 Department of Physics and Astronomy, Vanderbilt University, 6301 Stevenson Center, Nashville, TN 37235, USA 3 Department of Physics, Fisk University, 1000 17th Avenue North, Nashville, TN 37208, USA 4 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 5 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822-1839, USA 6 Department of Astronomy & Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA 7 Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA 8 Department of Astronomy, University of California at Berkeley, Berkeley, CA 94720, USA 9 NASA Ames Research Center, M/S 244-30, Moffett Field, CA 94035, USA 10 Bay Area Environmental Research Institute, 625 2nd Street,Suite209, Petaluma, CA 94952, USA 11 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 12 South African Astronomical Observatory, Cape Town, South Africa 13 Las Cumbres Observatory Global Telescope Network, 6740 Cortona Drive, Suite 102, Santa Barbara, CA 93117, USA 14 Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA 15 Perth Exoplanet Survey Telescope, Perth, Australia 16 ICO, Adelaide, Australia 17 American Association of Variable Star Observers, 49 Bay State Road, Cambridge, MA 02138, USA 18 Hazelwood Observatory, Australia 19 Department of Astronomy, California Institute of Technology, 1200 E. California Boulevard, MC 249-17, Pasadena, CA 91125, USA 20 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA 21 Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USA 22 Observatoire Astronomique de l’Université de Genève, 51 ch. des Maillettes, 1290 Versoix, Switzerland 23 Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA 24 Department of Physics and Astronomy, University of Louisville, Louisville, KY 40292, USA 25 Department of Physics, Westminster College, New Wilmington, PA 16172, USA 26 Department of Physics and Astronomy, Swarthmore College, Swarthmore, PA 19081, USA 27 Ellinbank Observatory, Victoria, Australia 28 Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA 29 School of Physics, University of New South Wales, Sydney, NSW 2052, Australia 30 Australian Center for Astrobiology, University of New South Wales, Sydney, NSW 2052, Australia 31 Computational Engineering and Science Research Centre, University of Southern Queensland, Toowoomba, Queensland 4350, Australia 32 Department of Physics and Astronomy, University of Montana, Missoula, MT 59812, USA 33 Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, USA Received 2016 July 6; revised 2017 March 3; accepted 2017 March 6; published 2017 April 18 Abstract We report the discovery of a transiting exoplanet, KELT-11b, orbiting the bright (V = 8.0) subgiant HD 93396. A global analysis of the system shows that the host star is an evolved subgiant star with Teff =5370 51 K, M = 1.438+0.061 M , R = 2.72+0.21 R , logg = 3.727+0.040, and [Fe H] = 0.180 0.075. The planet is a low- * -0.052 * -0.17 * -0.046 = ± = ± +0.15 mass gas giant in a P 4.736529 0.00006 day orbit, with MP 0.195 0.018 MJ, RP = 1.37-0.12 RJ, +0.028 −3 +0.080 +51 rP = 0.093-0.024 gcm , surface gravity loggP = 2.407-0.086, and equilibrium temperature Teq = 1712-46 K. KELT-11 is the brightest known transiting exoplanet host in the southern hemisphere by more than a magnitudeand is the sixth brightest transit host to date. The planet is one of the most inflated planets known, with an exceptionally large atmospheric scale height (2763 km), and an associated size of the expected atmospheric transmission signal of 5.6%. These attributes make the KELT-11 system a valuable target for follow-up and atmospheric characterization, and it promises to become one of the benchmark systems for the study of inflated exoplanets. Key words: planetary systems – planets and satellites: detection – stars: individual (KELT-11) – techniques: photometric – techniques: radial velocities Supporting material: data behind figure, machine-readable tables 1 The Astronomical Journal, 153:215 (15pp), 2017 May Pepper et al. 1. Introduction 2. Discovery and Follow-up Observations The discovery of transiting exoplanets has been marked by In this section, we describe the KELT data and follow-up two eras. The first era began with observations showing that the photometric and RV observations that led to the discovery and planet HD 209458b, first discovered by the radial velocity (RV) confirmation of KELT-11b. A detailed description of the KELT method, transited its host star (Henry et al. 2000; Charbonneau pipeline and candidate selection process can be found in Siverd et al. 2000), and with the discovery of the first planet with the et al. (2012) and Kuhn et al. (2016). transit method, OGLE-TR-56b (Udalski et al. 2002; Konacki et al. 2003). Thisbegan a period of rapid discovery of new transiting exoplanets using small, automated, and dedicated 2.1. KELT-South telescopes, most notably by the HATNet (Bakos et al. 2004), KELT-11b is located in the KELT-South field 23, which is SuperWASP (Pollacco et al. 2006), TrES (Alonso et al. 2004), centered at J2000 α =10h43m48s, δ = −20°00′00″. This field and XO (McCullough et al. 2006) projects. The second era was was monitored from UT 2010 March 12 to UT 2014 July 9, marked by the 2007 launch of the CoRoT mission (Rouan et al. resulting in 3910 images after post-processing and removal of 1998), and then in 2009 with the launch of the Kepler mission bad images. Following the same strategy as described in Kuhn (Borucki et al. 2010). Space-based detection of transiting et al. (2016), we reduced the raw images, extracted the light planets was a huge leap forward, especially with the ability to curves, and searched for transit candidates. One candidate from detect smaller and longer-period transiting planets. this processing, KS23C00790 (HD 93396, TYC 5499-1085-1, Although the Kepler mission has been tremendously fruitful 2MASS J10464974-0923563), located at α = 10h46m 49 7, with the number and variety of detected planets, especially for δ = −9°23′56 5 J2000, was rated as a top candidate from the the determination of the underlying population and demo- field. The host star properties can be seen in Table 1. We use graphics of exoplanets, the small ground-based telescopes have the Box-fitting Least-squares algorithm (Kovács et al. 2002) as continued to make many important discoveries. Notably, the implemented in the VARTOOLS software package (Hartman population of transiting planets discovered by the ground-based & Bakos 2016) to identify transit candidates, making use of surveys tend to be large planets with short orbital periods statistics that come from the VARTOOLS package, as well as orbiting bright stars, due to selection and observation bias from Pont et al. (2006) and Burke et al. (2006). The associated (Pepper et al. 2003; Gaudi 2005; Pepper & Gaudi 2005; Pont selection criteria and the corresponding values for KELT-11b et al. 2006; Fressin et al. 2007). These planets, unlike the vast are shown in Table 2. For the choices of the particular selection majority of the Kepler planets, offer great potential for detailed criteria, refer to Siverd et al. (2012). The discovery light curve characterization of the atmospheres of exoplanets. The bulk of itself is shown in Figure 1, and the data are provided in Table 3. our understanding of exoplanetary atmospheres comes from observations of planets with host stars with V<13 (Seager & Deming 2010; Sing et al. 2016). 2.2. Photometric Follow-up For that reason, the ongoing discoveries from ground-based We obtained follow-up time-series photometry of KELT-11b transit surveys will continue to provide great value, at least to check for false positives such as eclipsing binary stars, and to until the launch of the Transiting Exoplanet Survey Satellite better determine the transit shape, depth, and duration. We used (TESS) mission (Ricker et al. 2015). Among these projects is the TAPIR software package (Jensen 2013) to predict transit the Kilodegree Extremely Little Telescope (KELT) survey. events, and we obtained ninefull or partial transits in multiple KELT fills a niche in planet discovery space by observing stars bands between 2015 January and 2016 February. All data were generally brighter than those observed by the other ground- calibrated and processed using the AstroImageJ package based surveys, with a target magnitude regime of (AIJ;34 Collins & Kielkopf 2013; Collins et al. 2017) unless 7.5<V<10.5. The KELT-North telescope (Pepper et al. otherwise stated. The follow-up light curves are displayed in 2007) has been operating since 2006, and has discovered 10 Figure 2, and Table 4 lists the details of the observations.