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 \pm 48 { K } , \log { g } = 4.030 _ { -0.026 } ^ { +0.015 } and { \left [ { Fe } / { H } \right ] } = 0.034 \pm 0.78 .
The inferred stellar mass is M _ { * } = 1.314 _ { -0.060 } ^ { +0.063 } { M } _ { \sun } and the star has a relatively large radius of R _ { * } = 1.836 _ { -0.046 } ^ { +0.066 } { R } _ { \sun } .
The planet is a typical hot Jupiter with period 4.11379 \pm 0.00001 days and a mass of M _ { P } = 1.524 \pm 0.088 { { M } _ { 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 \sim 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 \sim 44 m s ^ { -1 } .