Common envelope events with low-mass giants: understanding the energy budget

TL;DR

This paper models common envelope events with low-mass giants to refine the energy formalism, emphasizing the roles of recombination energy and energy loss via ejecta, aiding binary evolution predictions.

Contribution

It introduces a revised energy formalism for common envelope events that includes recombination energy and ejecta energy loss, based on 3D simulations of low-mass giant donors.

Findings

Recombination energy significantly affects the energy budget.

A substantial fraction of orbital energy is carried away by ejecta.

Provides tools for binary population synthesis modeling.

Abstract

Common envelope events are important interactions between two binary stars that lead to the formation of close binary systems. We present here a systematic three-dimensional study in which we model common envelope events with low-mass giant donors. The results allow us to revise the energy formalism that is usually used to determine common envelope event outcomes. We show that the energy budget for this type of system should include the recombination energy, and that it also must take into account that a significant fraction of the released orbital energy is taken away by the ejecta. We provide three ways in which our results can be used by binary population synthesis studies: a relation that links the observed post-common envelope binary with the initial binary parameters, a fitting formula for the $\alpha_{\rm ce}\lambda$ parameter of the standard energy formalism, and a revised energy formalism that takes into account both the recombination energy and the energy that is taken away by the ejecta.