commonenvelope

Common envelope evolution is a brief but pivotal phase in the evolution of many close binary star systems. It starts when the more evolved star, the donor, expands and fills its Roche lobe, triggering dynamically unstable mass transfer to the companion. The companion becomes embedded in the donor’s expanding envelope, forming a common envelope around the two stellar cores.

During this phase, friction and drag cause the cores to spiral toward each other, converting orbital energy into heat and kinetic energy. If enough energy is deposited to overcome the envelope’s gravitational binding, the envelope is ejected from the system, leaving a close binary composed of the exposed core of the donor and the companion. If the envelope is not ejected, the cores may merge.

The outcome depends on the balance of energies and angular momentum. In the standard energy formalism, the efficiency of envelope ejection is described by alpha_CE, which relates the change in orbital energy to the binding energy of the envelope. Internal energy and recombination energy can also contribute, leading to variant formalisms. An alternative gamma-formalism uses angular momentum balance rather than energy conservation.

Observationally, post-common-envelope binaries exist in several forms, including close white-dwarf binaries, cataclysmic variables, and certain neutron-star or black-hole binaries. Remnants are seen in planetary nebulae with binary cores or in luminous red novae thought to represent CE events.

CE evolution remains a major theoretical challenge, with ongoing hydrodynamic simulations and parameter studies. CE channels are important for forming some Type Ia supernova progenitors and other compact-object mergers, and they influence the population of short-period binaries across the cosmos.