Measurement of the Spin-Orbit Misalignment of KOI-13.01 from its Gravity-Darkened Kepler Transit Lightcurve

Abstract: We model the asymmetry of the KOI-13.01 transit lightcurve assuming a gravity-darkened rapidly-rotating host star in order to constrain the system's spin-orbit alignment and transit parameters. We find that our model can reproduce the Kepler lightcurve for KOI-13.01 with a sky-projected alignment of {\lambda} = $23 \pm 4$ degrees and with the star's north pole tilted away from the observer by $48 \pm 4$ degrees (assuming $M_* = 2.05 M_\odot$). With both these determinations, we calculate that the net misalignment between this planet's orbit normal and its star's rotational pole is $56 \pm 4$ degrees. Degeneracies in our geometric interpretation also allow a retrograde spin-orbit angle of $124 \pm 4$ degrees. This is the first spin-orbit measurement to come from gravity darkening, and is one of only a few measurements of the full (not just the sky-projected) spin-orbit misalignment of an extrasolar planet. We also measure accurate transit parameters incorporating stellar oblateness and gravity darkening: $R_* = 1.756 \pm 0.014 R_\odot$, $R_p = 1.445 \pm 0.016 R_{Jup}$, and $i = 85.9 \pm 0.4$ degrees. The new lower planetary radius falls within the planetary mass regime for plausible interior models for the transiting body. A simple initial calculation shows that KOI-13.01's circular orbit is apparently inconsistent with the Kozai mechanism having driven its spin-orbit misalignment; planet-planet scattering and stellar spin migration remain viable mechanisms. Future Kepler data will improve the precision of the KOI-13.01 transit lightcurve, allowing more precise determination of transit parameters and the opportunity to use the Photometric Rossiter-McLaughlin effect to resolve the prograde/retrograde orbit determination degeneracy.