How Hot Jupiters get out of Alignment With Their Host Stars

On the tidal origin of hot Jupiter stellar obliquity trends

Authors:

Dawson et al

Abstract:

It is debated whether the two populations of hot Jupiters --- those on orbits misaligned from their host star's spin axis and those well-aligned --- result from two migration channels or from two tidal realignment regimes. Here I demonstrate that equilibrium tides raised by a planet on its star can account for the observed spin-orbit alignment trends: the aligned orbits of hot Jupiters orbiting cool stars, the planetary mass cut-off for retrograde planets, and the stratification by planet mass of host star rotation frequency. I suggest that the first trend is caused by strong vs. weak magnetic braking (the Kraft break), rather than the realignment of the star's convective envelope vs. the entire star. The second trend can result from a small effective stellar moment of inertia participating in the tidal realignment in hot stars, enabling massive retrograde planets to flip to prograde while remaining misaligned. The third trend is attributable to higher mass planets more effectively counteracting braking to spin up their stars. For both hot and cool stars, the effective stellar moment of inertia participating in the tidal realignment must be small, e.g. an outer layer weakly coupled to the rest of the star. I demonstrate via Monte Carlo that this model can match the observed trends and distributions of sky-projected misalignments and stellar rotation frequencies. I discuss the implications of this framework for inferring hot Jupiter migration mechanisms from obliquities, emphasizing that even the hot stars do not constitute a pristine sample.