Tidal Damping of Stellar Obliquities in Hot Jupiter Systems

Abstract

Hot Jupiters can be misaligned around stars with effective temperatures greater than ∼ 6100 K, and are seemingly always in alignment with stars cooler than this threshold. The high stellar obliquities of hot stars are thought to be left over from high primordial obliquities in hot Jupiter systems, due to the lack of a thick convective envelope in stars where Teff ≳ 6100 K, whereas inertial waves excited and dissipated in the convective envelopes of cooler stars may be able to tidally damp the obliquities of stars below this break. Such obliquity distributions have long been invoked as a potential channel of evidence for explaining the origins of hot Jupiters. We explore this theory through time evolutions of the relevant orbital and stellar parameters, implementing more realistic tidal evolution theory than previous explorations of this idea. We also consider the theory of core-envelope decoupling as a means of more easily tidally aligning systems by way of inertial wave damping. We find that there is no set of initial orbital, spin, and arrival parameters that can sufficiently reproduce the observed obliquity distributions of cool and hot stars simultaneously, and that core- envelope decoupling produces exceptionally strong damping of hot star obliquities and is thus starkly opposed to observations. We are unable to rectify observed obliquity distributions with theories of diverging stellar structures and inertial wave dissipation alone.