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-circle dot and 1.500 +/- 0.050 R-circle dot, 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 degrees.7 < lambda < 3 degrees.3 or 27 degrees.6 < lambda < 57 degrees.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-circle dot and 0.53 +/- 0.08 M-circle dot. HAT-P-57 is the most rapidly rotating star, and only the fourth main sequence A star, known to host a transiting planet.