KELT-24b: A 5M\(_{\rm J}\) Planet on a 5.6 day Well-Aligned Orbit around the Young V=8.3 F-star HD 93148

Abstract

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 \(T_{\rm eff}\) =\(6508\pm49\) K, a mass of \(M_{*}\) = \(1.461^{+0.056}_{-0.060}\) \(M_{\odot}\), radius of \(R_{*}\) = \(1.506\pm0.022\) \(R_{\odot}\), and an age of \(0.77^{+0.61}_{-0.42}\) Gyr. Its planetary companion (KELT-24 b) has a radius of \(R_{\rm P}\) = \(1.272^{+0.021}_{-0.022}\) \(R_{\rm J}\), a mass of \(M_{\rm P}\) = \(5.18^{+0.21}_{-0.22}\) \(M_{\rm J}\), and from Doppler tomographic observations, we find that the planet's orbit is well-aligned to its host star's projected spin axis (\(\lambda\) = \(2.6^{+5.1}_{-3.6}\)). 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.