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Abstract: Tidal friction is thought to be important in determining the long-termspin-orbit evolution of short-period extrasolar planetary systems. Using asimple model of the orbit-averaged effects of tidal friction, we study theevolution of close-in planets on inclined orbits, due to tides. We analyse theeffects of the inclusion of stellar magnetic braking by performing aphase-plane analysis of a simplified system of equations, including the brakingtorque. The inclusion of magnetic braking is found to be important, and itsneglect can result in a very different system history. We then present theresults of numerical integrations of the tidal evolution equations, where wefind that it is essential to consider coupled evolution of the orbital androtational elements, including dissipation in both the star and planet, toaccurately model the evolution. The main result of our integrations is that fortypical Hot Jupiters, tidal friction aligns the stellar spin with the orbit ona similar time as it causes the orbit to decay. This means that if a planet isobserved to be aligned, then it probably formed coplanar. This reinforces theimportance of Rossiter-McLaughlin effect observations in determining the degreeof spin-orbit alignment in transiting systems. We apply these results to theXO-3 system, and constrain the tidal quality factors Q- in both the star andplanet in this system. Using a model in which inertial waves are excited bytidal forcing in the outer convective envelope and dissipated by turbulentviscosity, we calculate Q- for a range of F-star models, and find it to varyconsiderably within this class of stars. This means that assuming a single Q-applies to all stars is probably incorrect. We propose an explanation for thesurvival of WASP-12 b and OGLE-TR-56 b, in terms of weak dissipation in the star.

Author: A.J. Barker, G.I. Ogilvie

Source: https://arxiv.org/


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