KELT-7b: A Hot Jupiter Transiting a Bright V = 8.54 Rapidly Rotating F-star

We report the discovery of KELT-7b, a transiting hot Jupiter with a mass of 1.28+/- 0.18 M_J, radius of {1.533}_-0.047^+0.046 R_J, and an orbital period of 2.7347749+/- 0.0000039 days. The bright host star (HD 33643; KELT-7) is an F-star with V = 8.54, T_eff = 6789{}_-49⁺⁵⁰ K, [Fe/H] = {0.139}_-0.081^+0.075, and {log}\{\text{}}g\=4.149+/- 0.019. It has a mass of {1.535}_-0.054^+0.066 M_⊙, a radius of {1.732}_-0.045^+0.043 R_⊙, and is the fifth most massive, fifth hottest, and the ninth brightest star known to host a transiting planet. It is also the brightest star around which Kilodegree Extremely Little Telescope (KELT) has discovered a transiting planet. Thus, KELT-7b is an ideal target for detailed characterization given its relatively low surface gravity, high equilibrium temperature, and bright host star. The rapid rotation of the star (73+/- 0.5 km s^-1) results in a Rossiter-McLaughlin effect with an unusually large amplitude of several hundred m s^-1. We find that the orbit normal of the planet is likely to be well-aligned with the stellar spin axis, with a projected spin-orbit alignment of λ = 9.°7 ± 5.°2. This is currently the second most rapidly rotating star to have a reflex signal (and thus mass determination) due to a planetary companion measured.