ASYMMETRIC TRANSIT CURVES AS INDICATION OF ORBITAL OBLIQUITY: CLUES FROM THE LATE-TYPE DWARF COMPANION IN KOI-13

Abstract

KOI-13.01, a planet-sized companion in an optical double star was announced\nas one of the 1235 Kepler planet candidates in February 2011. The transit\ncurves show significant distortion that was stable over the ~130 days time-span\nof the data. Here we investigate the phenomenon via detailed analyses of the\ntwo components of the double star and a re-reduction of the Kepler data with\npixel-level photometry. Our results indicate that KOI-13 is a common proper\nmotion binary, with two rapidly rotating components (v sin i ~ 65--70 km/s). We\nidentify the host star of KOI-13.01 and conclude that the transit curve\nasymmetry is consistent with a companion orbiting a rapidly rotating, possibly\nelongated star on an oblique orbit. After correcting the Kepler light curve to\nthe second light of the optical companion star, we derive a radius of 2.2 R_J\nfor the transiter, implying an irradiated late-type dwarf, probably a hot brown\ndwarf rather than a planet. KOI-13 is the first example for detecting orbital\nobliquity for a substellar companion without measuring the Rossiter-McLaughlin\neffect from spectroscopy.\n