On the Surface Heating of Synchronously Spinning Short-Period Jovian Planets

We consider the atmospheric flow on short-period extrasolar planets through two-dimensional numerical simulations of hydrodynamics with radiation transfer. The observed low eccentricity of these planets indicates that tidal dissipation within them has been effective in circularizing their orbits and synchronizing their spins. Consequently, one side of these planets (the day side) is always exposed to the irradiation from the host star, whereas the other (the night side) is always in shadow. The temperature of the day side is determined by the equilibrium that the planetary atmosphere establishes with the stellar radiation. For planets around solar-type stars with periods less than 7 days, the flux of stellar irradiation exceeds that released from their Kelvin-Helmholtz contraction by several orders of magnitude. A fraction of the thermal energy deposited on the day side is advected to the night side by a current. We show that the radiation transfer and the nightside temperature distribution in a planet's atmosphere are sensitive functions of its opacity. If the atmosphere contains grains with an abundance and size distribution comparable to that of the interstellar medium, only shallow heating occurs on the day side, whereas the heat flux carried by the circulation does not effectively heat the night side, which cools well below the day side. The temperature difference affects the spectroscopic signature of these planets. However, the temperature difference decreases as the abundance of grains in the atmosphere is reduced. This effect occurs because if the grains are depleted, the stellar radiative flux penetrates more deeply into the atmosphere on the day side, and the higher density atmospheric circulation carries a larger flux of heat over to the night side. A simple analytic model of the dissipation of the circulation flow and associated kinetic heating is also considered. This heating effect occurs mostly near the photosphere, not deep enough to significantly affect the size of planets. The depth of the energy deposition increases as the abundance of grains is reduced. Finally, we show that the surface irradiation suppresses convection near the photospheric region on the day side. However, in some cases, depending on the opacity, convection zones are present near the surface on the night side. This structural modification may influence the response and dissipation of tidal disturbances and alter the circularization and synchronization timescales.