Energy transport by convection in the common envelope evolution

Abstract

We argue that outward transport of energy by convection and photon diffusion in a common envelope evolution (CEE) of giant stars substantially reduces the fraction of the recombination energy of hydrogen and helium that is available for envelope removal. We base our estimate on the properties of an unperturbed asymptotic giant branch (AGB) spherical model, and on some simple arguments. Since during the CEE the envelope expands and energy removal by photon diffusion becomes more efficient, our arguments underestimate the escape of recombination energy. We hence strengthen earlier claims that recombination energy does not contribute much to common envelope removal. A large fraction of the energy that jets deposit to the envelope, on the other hand, might be in the form of kinetic energy of the expanding and buoyantly rising hot bubbles. These rapidly rising bubbles remove mass from the envelope. We demonstrate this process by conducting a three-dimensional hydrodynamical simulation where we deposit hot gas in the location of a secondary star that orbits inside the envelope of a giant star. Despite the fact that we do not include the large amount of gravitational energy that is released by the in-spiraling secondary star, the hot bubbles alone remove mass at a rate of about 0.1 Mo/yr, which is much above the regular mass loss rate.