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 mome ntum or mass, or both, from the precursor system. Close binaries may b e defined as systems in which at least one of the components has fille d or will fill its Roche lobe and attempt to transfer matter to its co mpanion. If the time scale for mass transfer is considerably shorter t han the time scale on which the accretor can adjust thermally to the p roffered mass, the accreted layer will heat up, expand, and fill the R oche lobe of the accretor. The mass lost by the donor thereafter flows into a ''common envelope'' (CE) which encompasses both stars. The fri ctional interaction between this common envelope and the stellar cores produces drag forces that cause the cores to spiral in toward one ano ther; some of the orbital energy helps drive matter from the CE into i nterstellar space. Examples of systems which are experiencing or have experienced this process include some planetary nebulae, cataclysmic v ariables, and close binary degenerate stars. Similar situations can ar ise if one of the components can support, of its own accord, a dense w ind that flows out of the system; the drag luminosity produced by inte raction between the companion and the wind may intensify the wind and contribute to mass loss from the donor. Systems undergoing this ''wind -CE'' process include novae and close binaries containing an OB star. Planetary nebulae with close binary central stars are actually ejected CEs, and precursors of many cataclysmic variables were once the centr al 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 mode l the event quantitatively, and (4) apply what we have learned to desc ribe, in several real situations, the transformations wrought by evolu tion through a CE phase.