Spin-orbit alignment for 110 day period KOI368.01 from gravity darkening

Abstract

We fit the Kepler photometric light curve of the KOI-368 system using an oblate, gravity-darkened stellar model in order to constrain its spin-orbit alignment. We find that the system is relatively well-aligned with a sky-projected spin-orbit alignment of λ = 10°±2°, a stellar obliquity of φ = 3°±7°, and a true spin-orbit alignment of φ = 11°±3°. Although our measurement differs significantly from zero, the low value for φ is consistent with spin-orbit alignment. We also measure various transit parameters of the KOI-368 system: RKOI-368 = 2.28 ± 0.02 R⊗, Rp = 1.83 ± 0.02 Rjup, and i = 89°.221 ± 0°.013. This work shows that our gravity-darkened model can constrain long-period, well-aligned planets and M-class stars orbiting fast-rotators, allowing for measurement of a new subcategory of transiting bodies.