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 g = 4.149 +/- 0.019. It has a mass of 1.535(-0.054)(+0.066) M-circle dot, a radius of 1.732(-0.045)(+0.043) R-circle dot, 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 lambda = 9 degrees.7 +/- 5 degrees.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.