Theory, Simulations, and Observations of Stellar Mergers

Stellar mergers are responsible for a great variety of astrophysical phenomena. They form blue straggler stars, give rise to spectacular transients, and produce some of the most massive stars in the Universe. Here, we focus on mergers from binary evolution and stellar collisions but do not cover mergers involving compact objects. We review how mergers come about, explain the physics and outcomes of the merger process, discuss the evolution and ultimate fates of merged stars, and relate to observations. ▪ Mergers of main sequence stars often fully rejuvenate and have interior structures similar to genuine single stars. ▪ Contrarily, mergers involving post–main sequence stars can have interior structures that cannot be achieved by single-star evolution. Some of these merger products may become long-lived blue supergiants and even end their evolution as such stars. They could thus explode in SN 1987A-like events, lead to interacting and superluminous supernovae, or collapse into very massive black holes. Such black holes may even populate the pair-instability-supernova black-hole mass gap. ▪ Strong magnetic fields are produced in stellar mergers. Merged stars may thus be at the origin of some magnetic OBA stars and their descendants, highly magnetic white dwarfs and neutron stars. ▪ Initially, stellar merger products rotate rapidly, but there are several mechanisms that can quickly spin them down. Hence, merged stars may be rather slow rotators for most of their evolution.