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Generation of internal gravity waves by penetrative convection
Charly PinÁon (Paris Observatory), K. Belkacem (Paris Observtaory), M.J. Goupil (Paris Observatory)
The space-borne missions CoRoT and Kepler provide seismic data of thousands of stars from the main sequence to the red giants branch. The detection of mixed-modes in subgiants and red giants enabled us to show that their core rotate much more slowly than expected. In this context, internal gravity waves (hereafter, IGW) can play a role since they are known to be able to transport angular momentum in the radiative zone of the stars. The efficiency of the transport of angular momentum by IGW depends on their driving mechanism. Two different kinds of mechanism of excitation are usually invoked. The first one, due to the turbulent pressure through the convective bulk, has already been theoretically investigated. The second one, due to the penetration of convective plumes into the stably stratified region at the edge of the base of convective zone, has already been observed in numerical simulations and studied in geophysics, but a theoretical estimate was still missing. We develop a semianalytical model in order to estimate the energy of the plumes transferred into the waves at the base of the convective zone. We also investigate the effect of the steepness of the Brunt-Všisšlš frequency at the base of the convective zone on the transmission of the waves into the propagative region. For the solar case, we show that IGW are generated more efficiently by penetrative convection than by turbulent pressure, and that a smooth thermal transition at the base of the convective zone can significatively enhances the transmission of the wave energy flux into the core. We expect this mechanism to work in evolved stars, which will be subject to future investigations.