Spin-orbit alignment from transit photometry: the case for multi-colour observations

The asymmetric transit light curves of hot Jupiters orbiting fast-rotating, early type stars encode information about the true spin-orbit angle Ψ: a parameter that eludes measurement using most other methods. However, an accurate measurement of Ψ using from a transit is challenging, with numerous values for individual planets in literature exhibiting significant disagreement. Doppler tomography provides an alternative spectroscopy-based method which is less prone to bias, but only unlocks the sky-projected spin-orbit angle λ. I will present analyses of transit light curves for a sample of hot Jupiters with fast-rotating hosts acquired with CHEOPS, TESS, Kepler and Spitzer. The effect of fitting light curves acquired across the optical and IR with a single set of transit parameters greatly diminishes the degeneracies, and returns measurements of λ consistent with those from Doppler tomography. The placement of priors from tomography is better still, generally achieving improved precision on Ψ. With multi-colour light curves becoming available for ever-larger sections of the sky, this study provides a template for how to unveil the orbital architecture of these extreme systems and the underlying physical processes that shape them.