* Astronomy

Title: KELT-2Ab: A Hot Jupiter Transiting the Bright (V=8.77) Primary Star of a Binary System
Authors: Thomas G. Beatty, Joshua Pepper, Robert J. Siverd, Jason D. Eastman, Allyson Bieryla, David W. Latham, Lars A. Buchhave, Eric L. N. Jensen, Mark Manner, Keivan G. Stassun, B. Scott Gaudi, Perry Berlind, Michael L. Calkins, Karen Collins, Darren L. DePoy, Gilbert A. Esquerdo, Benjamin J. Fulton, Gábor Frész, John C. Geary, Andrew Gould, Leslie Hebb, John F. Kielkopf, Jennifer L. Marshall, Richard Pogge, K. Z. Stanek, Robert P. Stefanik, Rachel Street, Andrew H. Szentgyorgyi, Mark Trueblood, Patricia Trueblood, Amelia M. Stutz

We report the discovery of KELT-2Ab, a hot Jupiter transiting the bright (V=8.77) primary star of the HD 42176 binary system. The host is a slightly evolved late F-star likely in the very short-lived "blue-hook" stage of evolution, with Teff=6151±50 K, \log{g_*}=4.030_{-0.028}^{+0.013} and \feh=-0.018±0.069. The inferred stellar mass is M_*=1.308_{-0.025}^{+0.028}solar masses and the star has a relatively large radius of R_*=1.828_{-0.034}^{+0.070} solar radii. The planet is a typical hot Jupiter with period 4.113791±0.00001 days and a mass of M_P=1.522±0.078 Jupiter masses and radius of R_P=1.286_{-0.047}^{+0.065}Jupiter radii. This is mildly inflated as compared to models of irradiated giant planets at the ~4 Gyr age of the system. KELT-2A is the third brightest star with a transiting planet identified by ground-based transit surveys, and the ninth brightest star overall with a transiting planet. KELT-2Ab's mass and radius are unique among the subset of planets with V<9 host stars, and therefore increases the diversity of bright benchmark systems. We also measure the relative motion of KELT-2A and -2B over a baseline of 38 years, robustly demonstrating for the first time that the stars are bound. This allows us to infer that KELT-2B is an early K dwarf. We hypothesise that through the eccentric Kozai mechanism KELT-2B may have emplaced KELT-2Ab in its current orbit. This scenario is potentially testable with Rossiter-McLaughlin measurements, which should have an amplitude of ~44 m s^{-1}.

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