Thermal Tides in Fluid Extrasolar Planets
Abstract
Asynchronous rotation and orbital eccentricity lead to timedependent irradiation of the closein gas giant exoplanets—the hot Jupiters. This timedependent surface heating gives rise to fluid motions which propagate throughout the planet. We investigate the ability of this "thermal tide" to produce a quadrupole moment which can couple to the stellar gravitational tidal force. While previous investigations discussed planets with solid surfaces, here we focus on entirely fluid planets in order to understand gas giants with small cores. The Coriolis force, thermal diffusion, and selfgravity of the perturbations are ignored for simplicity. First, we examine the response to thermal forcing through analytic solutions of the fluid equations which treat the forcing frequency as a small parameter. In the "equilibrium tide" limit of zero frequency, fluid motion is present but does not induce a quadrupole moment. In the next approximation, finite frequency corrections to the equilibrium tide do lead to a nonzero quadrupole moment, the sign of which torques the planet away from synchronous spin. We then numerically solve the boundary value problem for the thermally forced, linear response of a planet with neutrally stratified interior and a stably stratified envelope. The numerical results find quadrupole moments in agreement with the analytic nonresonant result at a sufficiently long forcing period. Surprisingly, in the range of forcing periods of 130 days, the induced quadrupole moments can be far larger than the analytic result due to response of internal gravity waves which propagate in the radiative envelope. We discuss the relevance of our results for the spin, eccentricity, and thermal evolution of hot Jupiters.
 Publication:

The Astrophysical Journal
 Pub Date:
 May 2010
 DOI:
 10.1088/0004637X/714/1/1
 arXiv:
 arXiv:0912.2313
 Bibcode:
 2010ApJ...714....1A
 Keywords:

 hydrodynamics;
 planetary systems;
 planets and satellites: atmospheres;
 planet─star interactions;
 waves;
 Astrophysics  Earth and Planetary Astrophysics
 EPrint:
 12 pages, 7 figures, submitted to ApJ