On the Formation of Helium Double Degenerate Stars and Pre-Cataclysmic Variables

The evolution of low mass (M < 2.5 Msun) 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 Msun) with degenerate helium cores (0.28 - 0.45 Msun) and companions (0.1 - 0.45 Msun) for initial orbital periods ranging from about 15 - 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 Msun and that the mass of the red giant progenitor is less than about 2 Msun. 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.