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^+0.015 _{- 0.026} and [Fe/H] = 0.034 ± 0.78. The inferred stellar mass is M _* = 1.314^+0.063 _{- 0.060} M _⊙ and the star has a relatively large radius of R _* = 1.836^+0.066 _{- 0.046} R _⊙. The planet is a typical hot Jupiter with period 4.1137913 ± 0.00001 days and a mass of M_P = 1.524 ± 0.088 M _J and radius of R_P = 1.290^+0.064 _{- 0.050} R _J. 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 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 ~44 m s^-1.