KELT-2Ab: A HOT JUPITER TRANSITING THE BRIGHT (V=8.77) PRIMARY STAR OF A BINARY SYSTEM

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 T-eff = 6148 +/- 48 K, log g = 4.030(-0.026)(+0.015) and [Fe/H] = 0.034 +/- 0.78. The inferred stellar mass is M-* = 1.314(-0.060)(+0.063) M-circle dot and the star has a relatively large radius of R-* = 1.836(-0.046)(+0.066) R-circle dot. The planet is a typical hot Jupiter with period 4.1137913 +/- 0.00001 days and a mass of M-P = 1.524 +/- 0.088M(J) and radius of R-P = 1.290(-0.050)(+0.064) R-J. This is mildly inflated as compared to models of irradiated giant planets at the similar to 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 hypothesize 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 similar to 44 m s(-1).