HAT-P-24b: An inflated hot Jupiter on a 3.36 day period transiting a hot, metal-poor star

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_-0.017^+0.022, 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.