On the formation of helium double degenerate stars and pre-cataclysmic variables

The evolution of low-mass (M < 2.5 M.) binaries through the common envelope phase has been studied for systems in which one member is on its first ascent of the red giant branch. Three-dimensional hydrodynamical simulations have been carried out for a range of red giant masses (1-2 M.) with degenerate helium cores (0.28-0.45 M.) and companions (0.1-0.45 M.) for initial orbital periods ranging from similar to 15 to 1000 days. The results suggest that these low-mass binary systems can survive the common envelope phase provided that the helium degenerate core is more massive than about 0.2-0.25 M. and that the mass of the red giant progenitor is less than or similar to 2 M.. Specific applications are made to observed double helium degenerate systems, pre-cataclysmic variables, and subdwarf B stars in order to place constraints on progenitor systems evolving through the common envelope phase. For the observed short-period double degenerate systems, it is found that evolutionary scenarios involving two phases of common envelope evolution are not likely and that a scenario involving an Algol-like phase of mass transfer followed by a common envelope phase is viable, suggesting that the first-formed white dwarf is often reheated by nuclear burning on its surface. A formation mechanism for two subdwarf B stars observed in eclipsing short-period binaries with low-mass main-sequence stars is also described.