Radiating the hydrogen recombination energy during common envelope evolution

TL;DR

This study uses MESA simulations to show that during common envelope evolution, most hydrogen recombination energy is radiated away, contributing minimally to envelope ejection but significantly increasing stellar luminosity.

Contribution

It demonstrates through detailed modeling that hydrogen recombination energy is mostly radiated and does not significantly aid in envelope removal during CEE.

Findings

Most recombination energy is radiated, not used for ejection.

Luminosity increases substantially due to recombination energy.

Recombination energy contributes only about 10% to envelope removal.

Abstract

By using the stellar evolution code MESA we show that most of the hydrogen recombination energy that is released as the envelope expands during a regular common envelope evolution (CEE), namely, the initial dynamical phase or plunge-in phase, is radiated, and hence increases substantially the stellar luminosity. Only about ten per cent 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 per cent, 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 behind 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.