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 $1.28\pm 0.18$ M_J, radius of ${1.533}_{-0.047}^{+0.046}$ R_J, and an orbital period of $2.7347749\pm 0.0000039$ 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 $logg=4.149\pm 0.019$. 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 ($73\pm 0.5$ 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.