KELT-24b: A 5M J Planet on a 5.6 day 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}_-49⁺⁵⁰ K, a mass of M _* = {1.460}_-0.059^+0.055 M _⊙, a radius of R _* = 1.506 ± 0.022 R _⊙, 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 (λ ={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.