HAT-P-57b: A Short-period Giant Planet Transiting a Bright Rapidly Rotating A8V Star Confirmed Via Doppler Tomography

Abstract

We present the discovery of HAT-P-57b, a P = 2.4653 day transiting planet around a V =  10.465 0.029 mag, Teff =  7500 250 K main sequence A8V star with a projected rotation velocity of v i sin 102.1 1. =  3 km s-1 . We measure the radius of the planet to be R =  1.413 0.054 RJ and, based on RV observations, place a 95% confidence upper limit on its mass of M < 1.85 MJ. Based on theoretical stellar evolution models, the host star has a mass and radius of 1.47 0.1  2 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 3 .  < <l 3 or 27 . 6 57 . 4  < <l  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.