HAT-P-24b: An Inflated Hot Jupiter on a 3.36 Day Period Transiting a Hot, Metal-poor Star

Abstract

We report the discovery of HAT-P-24b, a transiting extrasolar planet orbiting the moderately bright V = 11.818 F8 dwarf star GSC 0774−01441, with a period P = 3.3552464 ± 0.0000071 days, transit epoch T_c = 2455216.97669 ± 0.00024 (BJD)11, and transit duration 3.653 ± 0.025 hr. The host star has a mass of 1.191 ± 0.042M_⊙, radius of 1.317 ± 0.068R_⊙, effective temperature 6373 ± 80 K, and a low metallicity of [Fe/H] = −0.16 ± 0.08. The planetary companion has a mass of 0.681 ± 0.031 M_J and radius of 1.243 ± 0.072 R_J yielding a mean density of 0.439 ± 0.069 g cm^(−3). By repeating our global fits with different parameter sets, we have performed a critical investigation of the fitting techniques used for previous Hungarian-made Automated Telescope planetary discoveries. We find that the system properties are robust against the choice of priors. The effects of fixed versus fitted limb darkening are also examined. HAT-P-24b probably maintains a small eccentricity of e = 0.052^(+0.022)_(−0.017), which is accepted over the circular orbit model with false alarm probability 5.8%. In the absence of eccentricity pumping, this result suggests that HAT-P-24b experiences less tidal dissipation than Jupiter. Due to relatively rapid stellar rotation, we estimate that HAT-P-24b should exhibit one of the largest known Rossiter–McLaughlin effect amplitudes for an exoplanet (ΔV_(RM) ≃ 95 m s^(−1)) and thus a precise measurement of the sky-projected spin–orbit alignment should be possible.