KELT-24b: A 5M_J Planet on a 5.6day Well-aligned Orbit around the Young V = 8.3 F-star HD 93148

We present the discovery of KELT-24 b, a massive hot Jupiter orbiting a bright (V = 8.3 mag, K = 7.2 mag) young F-star with a period of 5.6 days. The host star, KELT-24 (HD 93148), has a T_eff = 6509_-⁺₄₉⁵⁰ K, a mass of M_* = 1.460_-⁺_0.059^0.055 M_e, a radius of R_* = 1.506 ± 0.022 R_e, and an age of 0.78_-⁺_0.42^0.61 Gyr. Its planetary companion (KELT-24 b) has a radius of R_P = 1.272 ± 0.021 R_J and a mass of M_P = 5.18_-⁺_0.22^0.21 M_J, and from Doppler tomographic observations, we find that the planet’s orbit is well-aligned to its host star’s projected spin axis (l = 2.6_-⁺_3.6^5.1). The young age estimated for KELT-24 suggests that it only recently started to evolve from the zero-age main sequence. KELT-24 is the brightest star known to host a transiting giant planet with a period between 5 and 10 days. Although the circularization timescale is much longer than the age of the system, we do not detect a large eccentricity or significant misalignment that is expected from dynamical migration. The brightness of its host star and its moderate surface gravity make KELT-24b an intriguing target for detailed atmospheric characterization through spectroscopic emission measurements since it would bridge the current literature results that have primarily focused on lower mass hot Jupiters and a few brown dwarfs.