We announce the discovery of a highly inflated transiting hot Jupiter by the KELT-North survey. A global analysis including constraints from isochrones indicates that the V = 10.8 host star (HD 343246) is a mildly evolved, G dwarf with Teff = 5754-55+54 K, log g = 4.078-0.054+0.049, [Fe/H] = 0.272 ± 0.038, an inferred mass M∗ = 1.211 -0.066+0.078 M⊙, and radius R∗ = 1.67-0.12+0.14 R⊙. The planetary companion has a mass MP = 0.867-0.061+0.065 MJ, radius RP = 1.86-0.16+0.18 RJ, surface gravity log gP = 2.793-0.075+0.072, and density ρP = 0.167-0.038+0.047 g cm-3. The planet is on a roughly circular orbit with semimajor axis a = 0.04571-0.00084+0.00096 AU and eccentricity e = 0.035-0.025+0.050. The best-fit linear ephemeris is T0 = 2456883.4803 ± 0.0007 BJDTDB and P = 3.24406 ± 0.00016 days. This planet is one of the most inflated of all known transiting exoplanets, making it one of the few members of a class of extremely low density, highly irradiated gas giants. The low stellar log g and large implied radius are supported by stellar density constraints from follow-up light curves, as well as an evolutionary and space motion analysis. We also develop a new technique to extract high-precision radial velocities from noisy spectra that reduces the observing time needed to confirm transiting planet candidates. This planet boasts deep transits of a bright star, a large inferred atmospheric scale height, and a high equilibrium temperature of Teq = 1675-55+61 K, assuming zero albedo and perfect heat redistribution, making it one of the best targets for future atmospheric characterization studies.
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Fulton, B., Collins, K., Gaudi, B., Stassun, K., Pepper, J., Beatty, T., Siverd, R., Penev, K., Howard, A., Baranec, C., Corfini, G., Eastman, J., Gregorio, J., Law, N., Lund, M., Oberst, T., Penny, M., Riddle, R., Rodriguez, J., Stevens, D., Zambelli, R., Ziegler, C., Bieryla, A., D'Ago, G., DePoy, D., Jensen, E., Kielkopf, J., Latham, D., Manner, M., Marshall, J., McLeod, K., & Reed, P. (2015). KELT-8b: A HIGHLY INFLATED TRANSITING HOT JUPITER AND A NEW TECHNIQUE FOR EXTRACTING HIGH-PRECISION RADIAL VELOCITIES FROM NOISY SPECTRA. Astrophysical Journal, 810 (1) https://doi.org/10.1088/0004-637X/810/1/30