On the use of hydrogen recombination energy during common envelope events

TL;DR

This paper investigates the fate of hydrogen recombination energy during common envelope events, concluding it mainly contributes to envelope expansion or acceleration rather than being radiated away, with exceptions for high-entropy donors.

Contribution

It provides a model-independent analysis of hydrogen recombination energy's role in CE events, emphasizing its impact on envelope dynamics over energy loss mechanisms.

Findings

Recombination energy is primarily used for envelope expansion or acceleration.

Convection and radiation are negligible in removing recombination energy.

High-entropy donors can behave differently in energy utilization.

Abstract

In this Letter we discuss what happens to hydrogen recombination energy that is released during regular common envelope (CE) events, as opposed to self-regulated CE events. We show that the amount of recombination energy that can be transferred away by either convection or radiation from the regions where recombination takes place is negligible. Instead, recombination energy is destined to be used either to help CE expansion, as a work term, or to accelerate local fluid elements. The exceptions are donors that initially have very high entropy material, $S/(k_B N_A)>37$ mol g$^{-1}$. The analysis and conclusions are independent of specific stellar models or evolutionary codes, and rely on fundamental properties of stellar matter such as the equation of state, Saha equation and opacities, as well as on stellar structure equations and the mixing length theory of convection.