Hydrodynamic Simulations of Unevenly Irradiated Jovian Planets

Abstract

We employ a two-dimensional, grid-based hydrodynamic model to simulate upper atmospheric dynamics on extra- solar giant planets.The hydrodynamic equations of motionare integrated ona rotating, irradiated sphere using a pseudo- spectral algorithm.We use a two-frequency, two-stream approximation of radiative transfer to model the temperature forcing. This model is well suited to simulate the dynamics of the atmospheres of planets with high orbital eccentricity, which are subject to widely varying irradiation conditions.We identify six such planets, with eccentricities between e = 0.28 and e = 0.93 and semimajor axes from a = 0.0508AUto a = 0.432 AU, as particularly interesting. For each, we determine the temperature profile and resulting infrared light curves in the 8 mSpitzer band. Especially notable are the results for HD 80606b, which has the largest eccentricity (e = 0.9321) of any known planet, and HAT-P-2b, which transits its parent star, so that its physical properties are well constrained. Despite the varied orbital parameters, the atmospheric dynamics of these planets display a number of interesting common properties. In all cases, the atmo- spheric response is primarily driven by the intense irradiation at periastron. The resulting expansion of heated air produces high-velocity turbulent flow, including long-lived circumpolar vortices. In addition, a superrotating acoustic front de- velops on some planets; the strength of this disturbance depends on both the eccentricity and the temperature gradient from uneven heating. The specifics of the resulting infrared light curves depend strongly on the orbital geometry.We show, however, that the variations on HD 80606b and HAT-P-2b should be readily detectable at 4.5 and 8 m using Spitzer. These two objects present the most attractive observational targets of all known high-e exoplanets.