KELT-24b: A 5M${}_{\mathrm{J}}$ 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 $T_{\mathrm{eff}}$ =$6509_{-49}^{+50}$ K, a mass of $M_{\ast }$ = $1.460_{-0.059}^{+0.055}$ $M_{\odot }$, radius of $R_{\ast }$ = $1.506\pm 0.022$ $R_{\odot }$, and an age of $0.78_{-0.42}^{+0.61}$ Gyr. Its planetary companion (KELT-24 b) has a radius of $R_{\mathrm{P}}$ = $1.272\pm 0.021$ $R_{\mathrm{J}}$, a mass of $M_{\mathrm{P}}$ = $5.18_{-0.22}^{+0.21}$ $M_{\mathrm{J}}$, and from Doppler tomographic observations, we find that the planet's orbit is well-aligned to its host star's projected spin axis ($\hat{I}»$ = $2.6_{-3.6}^{+5.1}$). 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.

18 pages, 10 Figures, 6 Tables, Accepted to the Astronomical Journal