Stellar mergers and common-envelope evolution

TL;DR

This paper reviews the physical processes, simulations, and outcomes of stellar mergers and common-envelope evolution, emphasizing their importance in various astrophysical phenomena and highlighting open research questions.

Contribution

It provides a comprehensive overview of the mechanisms, recent simulation advances, and the role of magnetic fields in stellar mergers and common-envelope evolution, identifying key open questions.

Findings

Three-dimensional simulations reveal complex interaction outcomes.

Magnetic fields significantly influence the evolution of stellar mergers.

Understanding these processes is crucial for explaining diverse astrophysical phenomena.

Abstract

Stellar mergers and common-envelope evolution are fast (dynamical-timescale) interactions in binary stars that drastically alter their evolution. They are key to understanding a plethora of astrophysical phenomena. Stellar mergers are thought to produce blue straggler stars, blue supergiants, and stars with peculiar rotation and surface chemical abundances. Common-envelope evolution is proposed as a key stage in the formation of gravitational wave sources, X-ray binaries, type Ia supernovae, cataclysmic variables, and other systems. A significant fraction (tens of percent) of binary stars undergo such a phase during their evolution. In this chapter, we first discuss processes leading to a stellar merger or common-envelope phase. We then explain these complex interactions, starting from underlying physical principles like entropy sorting in stellar mergers and the energy formalism in common envelopes. This is followed by a more complete picture revealed by three-dimensional (magneto)hydrodynamical simulations. The outcomes of these interactions are discussed comprehensively and special emphasis is given to the role of magnetic fields. Both stellar mergers and common-envelope evolution remain far from fully understood, and we conclude by highlighting open questions in their study.