2015ApJ...805...28M

Analysis of the transit light curve deformed by the stellar gravity darkening allows us to photometrically measure both components of the spin-orbit angle ψ, its sky projection λ and inclination of the stellar spin axis. In this paper, we apply the method to two transiting hot Jupiter systems monitored with the Kepler spacecraft, Kepler-13A and HAT-P-7. For Kepler-13A, we find and adopting the spectroscopic constraint λ = 58.°6±2.°0 by Johnson et al. In our solution, the discrepancy between the above λ and that previously reported by Barnes et al. is solved by fitting both of the parameters in the quadratic limb-darkening law. We also report the temporal variation in the orbital inclination of Kepler-13Ab,, providing further evidence for the spin-orbit precession in this system. By fitting the precession model to the time series of, λ, and obtained with the gravity-darkened model, we constrain the stellar quadrupole moment for our new solution, which is several times smaller than obtained for the previous one. We show that the difference can be observable in the future evolution of λ, thus providing a possibility to test our solution with follow-up observations. The second target, HAT-P-7, is the first F-dwarf star analyzed with the gravity-darkening method. Our analysis points to a nearly pole-on configuration with or and the gravity-darkening exponent β consistent with 0.25. Such an observational constraint on β can be useful for testing the theory of gravity darkening.