HAT-P-57b: A SHORT-PERIOD GIANT PLANET TRANSITING A BRIGHT RAPIDLY ROTATING A8V STAR CONFIRMED VIA DOPPLER TOMOGRAPHY*

We present the discovery of HAT-P-57b, a P = 2.4653 day transiting planet around a V = 10.465 ± 0.029 mag, T_eff = 7500 ± 250 K main sequence A8V star with a projected rotation velocity of v sin i = 102.1 ± 1.3 km s^-1. We measure the radius of the planet to be R = 1.413 ± 0.054 R_J and, based on RV observations, place a 95% confidence upper limit on its mass of M < 1.85 M_J. Based on theoretical stellar evolution models, the host star has a mass and radius of 1.47 ± 0.12 M_⊙ and 1.500 ± 0.050 R_⊙, respectively. Spectroscopic observations made with Keck-I/HIRES during a partial transit event show the Doppler shadow of HAT-P-57b moving across the average spectral line profile of HAT-P-57, confirming the object as a planetary system. We use these observations, together with analytic formulae that we derive for the line profile distortions, to determine the projected angle between the spin axis of HAT-P-57 and the orbital axis of HAT-P-57b. The data permit two possible solutions, with -16°.7 < λ < 3°.3 or 27°.6 < λ < 57°.4 at 95% confidence, and with relative probabilities for the two modes of 26% and 74%, respectively. Adaptive optics imaging with MMT/Clio2 reveals an object located 2".7 from HAT-P-57 consisting of two point sources separated in turn from each other by 0".22. The H- and L'-band magnitudes of the companion stars are consistent with their being physically associated with HAT-P-57, in which case they are stars of mass 0.61 ± 0.10 M_⊙ and 0.53 ± 0.08 M_⊙. HAT-P-57 is the most rapidly rotating star, and only the fourth main sequence A star, known to host a transiting planet.