The evolution of low-mass close binary systems. I. The evolutionary fate of contact binaries.

A simple angular-momentum argument shows that if the absence of thermal-equilibrium solutions for contact binary components prevents the growth of deep common envelopes, the mass ratio (secondary/primary) decreases on an evolutionary time scale until the secondary is completely dissipated, during the primary's initial ascent of the giant branch. If simultaneous thermal equilibrium for both components exists at some stage, the binary may overflow its outer critical surface, dissipating its orbital angular momentum in an 'excretion' disk. Considerations of thermal stability suggest that a contact system cannot survive as a binary beyond the main-sequence stage. Large-scale angular-momentum loss as the primary attempts to cross the Hertzsprung gap renders the remnant indistinguishable from an ordinary single star. Angular-momentum losses in a stellar wind during giant-branch and asymptotic-giant-branch evolution are sufficient to prevent rotational instability of the degenerate core or white dwarf remnant.