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

Abstract

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 ∼ 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 M ⊙ and that the mass of the red giant progenitor is ∼ < 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. Subject headings: binaries: close — circumstellar matter — hydrodynamics — stars: interiors 1.