ON THE FORMATION OF DOUBLE WHITE DWARFS THROUGH STABLE MASS TRANSFER AND A COMMON ENVELOPE

Although several dozen double white dwarfs (DWDs) have been observed, for many the exact nature of the evolutionary channel(s) by which they form remains uncertain. The canonical explanation calls for the progenitor binary system to undergo two subsequent mass-transfer events, both of which are unstable and lead to a common envelope (CE). However, it has been shown that if both CE events obey the standard αCE-prescription (parameterizing energy loss), it is not possible to reproduce all of the observed systems. The γ-prescription was proposed as an alternative to this description, instead parameterizing the fraction of angular momentum carried away in dynamical-timescale mass loss. However, this too has proven problematic, and does not provide a clear physical mechanism. In this paper, we consider in detail the first episode of mass transfer in binary systems with initially low companion masses, with a primary mass in the range 1.0-1.3 M☉ and an initial mass ratio between the secondary and primary stars of 0.83-0.92. In these systems, the first episode of dramatic mass loss may be stable, non-conservative mass transfer. This strips the donor's envelope and dramatically raises the mass ratio; the considered progenitor binary systems can then evolve into DWDs after passing through a single CE during the second episode of mass loss. We find that such a mechanism reproduces the properties of the observed DWD systems which have an older component with M ∼< 0.46 M☉ and mass ratios between the younger and older WDs of q ≥ 1.