On the Formation and Evolution of Common Envelope Systems

We discuss the formation of a common envelope system following dynamicallyunstable mass transfer in a close binary, and the subsequent dynamicalevolution and final fate of the envelope. We base our discussion on newthree-dimensional SPH calculations that we have performed for a close binarysystem containing a $4\,M_\odot$ red giant with a $0.7\,M_\odot$ main-sequencestar companion. The initial parameters are chosen to model the formation of asystem resembling V~471~Tau, a typical progenitor of a cataclysmic variablebinary. In our highest-resolution calculation, we find evidence for acorotating region of gas around the central binary. This is in agreement withthe theoretical model proposed by Meyer \& Meyer-Hofmeister (1979) for theevolution of common envelope systems, in which this central corotating regionis coupled to the envelope through viscous angular momentum transport only. Wealso find evidence that the envelope is convectively unstable, in which casethe viscous dissipation time could be as short as $\sim100$ dynamical times,leading to rapid ejection of the envelope. For V~471~Tau, our results, and theobserved parameters of the system, are entirely consistent with rapid envelopeejection on a timescale $\sim1\,$yr and an efficiency parameter$\alpha_{CE}\simeq1$.