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

摘要

We present the discovery of HAT-P-57b, a P = 2.4653 day transiting planet around a mag, K main sequence A8V star with a projected rotation velocity of . We measure the radius of the planet to be and, based on RV observations, place a 95% confidence upper limit on its mass of . Based on theoretical stellar evolution models, the host star has a mass and radius of and , 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 or 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 from HAT-P-57 consisting of two point sources separated in turn from each other by The H- and -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 and . HAT-P-57 is the most rapidly rotating star, and only the fourth main sequence A star, known to host a transiting planet.