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A.P. Showman (NASA Ames), T. Guillot (Obs. Cote-d`Azur)
Roughly 40% of currently known extrasolar giant planets have orbital distances of 0.2 AU or less and are dubbed ``hot Jupiters'' because of their high effective temperatures (1000 K or more). Doppler techniques have yielded lower bounds on their masses, and the recent discovery that one such hot Jupiter (HD209458b) transits its parent star has provided estimates of radii and actual mass. More transit detections are sure to follow, and future detection of these objects in thermal and reflected light may allow observational estimates of albedo, effective temperature, and perhaps day-night temperature differences.
Atmospheric dynamics will be required to explain these observations. The radius depends on the entropy of the (presumably convecting) interior, while the temperature at the emission-to-space level is set by thermal balance with the star. The two regions are connected by a statically stable layer whose properties depend on atmospheric dynamics, so the dynamical regime must be understood if the radius and effective temperature are to be explained simultaneously. Furthermore, day-night temperature differences are directly linked to dynamical timescales (and are probably 200 K or more). The circulation pattern also determine whether (and where) clouds exist, which influences the albedo and the depth to which stellar radiation penetrates.
Here we present detailed dynamical simulations using the Explicit Planetary Isentropic Coordinate (EPIC) model of Dowling et al. (1998) to constrain the probable circulation regimes of hot Jupiters (focusing on HD209458b, with radius 1.4 RJ, mass 0.7 MJ, and period of 3.5 days). The simulations assume a synchronously rotating gas-giant (as expected for hot Jupiters) and parameterize the intense heating and cooling using a simple relaxation to a radiative equilibrium profile. The goals are to determine the number and speed of the jets, the day-night temperature differences, and regions of upwelling and downwelling (which is relevant for cloud formation). We will compare the simulation results to simple order-of-magnitude estimates, which suggest the importance of planetary rotation, the existence of several jets, and horiztonal winds of 200 m/sec or more.