On the Formation and Evolution of Common Envelope Systems

We discuss the formation of a common envelope system following dynamically unstable mass transfer in a close binary and the subsequent dynamical evolution and nal fate of the envelope. We base our discussion on new three-dimensional hydrodynamic calculations that we have performed for a close binary system containing a red giant with a main-sequence star companion. The initial 4 M _ 0.7 M _ parameters are chosen to model the formation of a system resembling V471 Tau, a typical progenitor of a cataclysmic variable binary. The calculations are performed using the smoothed particle hydrodynamics (SPH) method with up to 5 ] 104 particles. As initial condition we use an exact hydrostatic equilibrium conguration at the onset of dynamically unstable mass transfer. The nonlinear development of the instability is followed using SPH until a quasi-static common envelope conguration is formed. In our highest resolution calculation, we nd evidence for a corotating region of gas around the central binary. This is in agreement with the theoretical model proposed by Meyer & Meyer-Hofmeister for the evolution of common envelope systems, in which this central corotating region is coupled to the envelope through viscous angular momentum transport only. We also nd evidence that the envelope is convectively unstable, in which case the viscous dissipation time could be as short as D100 dynamical times, leading to rapid ejection of the envelope. For V471 Tau our results, and the observed parameters of the system, are entirely consistent with rapid envelope ejection on a timescale D1 yr and an efficiency parameter a CE ^1.