Double core evolution. VIII. The spiral-in of a main-sequence star through the envelope of AN asymptotic giant branch companion

A progenitor system for a cataclysmic variable binary consisting of a main-sequence star of 0.7 M_⊙ and a 3 M_⊙ asymptotic giant branch star (with a carbon-oxygen core of mass 0.7 M_⊙) is evolved through the common envelope phase. The two-dimensional axisymmetric simulations show that the entire common envelope is ejected without the two cores merging together. It is demonstrated that the common envelope evolution terminates as a direct result of the spin-up to near corotation of gas in the vicinity of the cores. The evolution of the common envelope has been followed for sufficiently long times to show that the ejected matter remains confined to the equatorial plane at large distances (≳10¹⁴ cm) from the double core. A new feature in the ejected nebula is the appearance of multiple shock fronts which develop as a result of nonuniform energy injection into the envelope. Outward-moving pressure waves form which steepen into shocks as the density falls off in the envelope. The implications of the results for the origin of cataclysmic variables are discussed.