2018ApJ...863L..14S

Using the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA), we show that most of the hydrogen recombination energy that is released as the envelope expands during a regular common envelope evolution-namely, the initial dynamical phase or plunge-in phase-is radiated, and hence substantially increases the stellar luminosity. Only about 10% of the hydrogen recombination energy might be used to remove the envelope. We show that the key property of energy transport is that when convection becomes inefficient in the outer parts of the envelope, where the ionization degree of hydrogen falls below about 30%, photon diffusion becomes very efficient and removes the recombination energy. The expanding envelope absorbs most of the gravitational energy that is released by the spiraling-in process of the secondary star inside the common envelope, and so it is the hydrogen recombination energy that is responsible for most of the luminosity increase of the system. The recombination energy of hydrogen adds only a small fraction of the energy required to remove the common envelope, and hence does not play a significant role in the ejection of the envelope.