Common envelopes in binary star evolution

The characteristics of many close, evolved binaries can be understood most easily if there exists some agency that can abstract angular momentum or mass, or both, from the precursor system. Close binaries may be defined as systems in which at least one of the components has filled or will fill its Roche lobe and attempt to transfer matter to its companion. If the timescale for mass transfer is considerablyy shorter than the timescale on which the accretor can adjust thermally to the proferred mass, the accreted layer will heat up, expand, and fill the Roche lobe of the accretor. The mass lost by the donor thereafter flows into a "common envelope" (CE) which encomapsses both stars. The frictional interaction between this common envelope and the stellar cores produces drag forces that cause the cores to spiral in toward one another; some of the orbital energy helps drive matter from the CE into interstellar space. Examples of the systems which are experiencing or have experienced this process include some planetary nebulae, cataclysmic variables, and close binary degenerate stars. Similar situations can arise if one of the components can support, of its own accord, a dense wind that flows out of the system; the drag luminosity produced by interaction between the companion and the wind may intensify the wind and contribute tot eh mass loss from the donor. Systems undergoing this "wind-CE" process include novae and close binaries containing a Wolf-Rayet star. Planetary nebulae with close binary central stars are actually ejected CEs, and precursors of many cataclysmic variables were once the central stars of planetary nebulae formed in a CE event. In this review, we (1) describe various initial configurations which will produce a CE, (2) discuss the physics of the CE event, (3) describe attempts to model the event quantiatively, and (4) apply what we have learned to describe, in several real situations, the tarnsformations wrought by evolution through a CE phase.