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 &fdg; 7 < λ < 3 &fdg; 3 ?> or 27 &fdg; 6 < λ < 57 &fdg; 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 &farcs; 7 ?> from HAT-P-57 consisting of two point sources separated in turn from each other by 0 &farcs; 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.