# Common Envelope Evolution Redux

## TL;DR

This paper revisits the physics of common envelope evolution in binary stars, highlighting the importance of recombination energy in explaining observed binary systems and addressing previous modeling challenges.

## Contribution

It introduces a reformulated energy budget for common envelope evolution that explicitly includes recombination energy, improving the understanding of binary star outcomes.

## Key findings

- Recombination energy plays a crucial role in common envelope ejection.
- Standard models underestimate the energy needed for envelope ejection.
- Inclusion of recombination energy resolves discrepancies in binary evolution models.

## Abstract

Common envelopes form in dynamical time scale mass exchange, when the envelope of a donor star engulfs a much denser companion, and the core of the donor plus the dense companion star spiral inward through this dissipative envelope. As conceived by Paczynski and Ostriker, this process must be responsible for the creation of short-period binaries with degenerate components, and, indeed, it has proven capable of accounting for short-period binaries containing one white dwarf component. However, attempts to reconstruct the evolutionary histories of close double white dwarfs have proven more problematic, and point to the need for enhanced systemic mass loss, either during the close of the first, slow episode of mass transfer that produced the first white dwarf, or during the detached phase preceding the final, common envelope episode. The survival of long-period interacting binaries with massive white dwarfs, such as the recurrent novae T CrB and RS Oph, also presents interpretative difficulties for simple energetic treatments of common envelope evolution. Their existence implies that major terms are missing from usual formulations of the energy budget for common envelope evolution. The most plausible missing energy term is the energy released by recombination in the common envelope, and, indeed, a simple reformulation the energy budget explicitly including recombination resolves this issue.