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

Abstract

The evolution of low-mass (M 2.5 binaries through the common envelope phase has been M _ ) studied for systems in which one member is on its —rst ascent of the red giant branch. Three-dimensional hydrodynamical simulations have been carried out for a range of red giant masses (1¨2 with degen- M _ ) erate helium cores (0.28¨0.45 and companions (0.1¨0.45 for initial orbital periods ranging from M _ ) M _ ) D15 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 and M _ that the mass of the red giant progenitor is Speci—c applications are made to observed double (2 M _ . helium degenerate systems, precataclysmic 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 fol- lowed by a common envelope phase is viable, suggesting that the —rst-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: closecircumstellar matterhydrodynamics ¨ novae, cataclysmic variablesstars: evolutionstars: interiors