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(circle dot), radius of 1.317 +/- 0.068R(circle dot), 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 (Delta V-RM similar or equal to 95 m s(-1)) and thus a precise measurement of the sky-projected spin-orbit alignment should be possible.