Spin-Orbit Misalignment and Precession in the Kepler-13Ab Planetary System

Gravity darkening induced by rapid stellar rotation provides us with a unique opportunity to characterize the spin-orbit misalignment of a planetary system through analysis of its photometric transit. We use the gravity-darkened transit modeling code simuTrans to reproduce the transit light curve of Kepler-13Ab by separately analyzing phase-folded transits for 12 short-cadence Kepler quarters. We verify the temporal change in impact parameter indicative of spin-orbit precession identified by Szab'o et al. (2012) and Masuda (2015), reporting a rate of change $db\mathrm{/}dt=(-4.1\pm 0.2)\times {10}^{-5}$ day${}^{-1}$. We further investigate the effect of light dilution on the fitted impact parameter and find that less than 1% of additional light is sufficient to explain the seasonal variation seen in the Kepler quarter data. We then extend our precession analysis to the phase curve data from which we report a rate of change $db\mathrm{/}dt=(-3.2\pm 1.3)\times {10}^{-5}$ day${}^{-1}$. This value is consistent with that of the transit data at a lower significance and provides the first evidence of spin-orbit precession based solely on the temporal variation of the secondary eclipse.