On The Formation of Double White Dwarfs: Reevaluating How We Parametrize the Common Envelope Phase

Abstract

One class of compact binaries of special interest is that of double white dwarfs (DWDs). For many of these systems, the exact nature of the evolutionary channels 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) phase. However, it has been shown that if both CE events obey the standard alpha prescription, it is not possible to reproduce all of the observed systems. As an alternative prescription, the gamma-formalism was proposed, which parametrizes the fraction of angular momentum carried away with mass loss, in contrast to the alpha prescription, which parametrizes energy loss. We demonstrate that the gamma-prescription is also inadequate in describing the evolution of an arbitrary DWD binary; clearly we require a deeper understanding of the physical mechanisms underlying their formation. We then present a detailed model for the evolution of Red Giant -- Main Sequence binaries during the first episode of mass transfer, and demonstrate that their evolution into DWDs need not arise through two phases of dynamical mass loss. Instead, the first episode of dramatic mass loss may be stable, non-conservative mass transfer. The second phase is then well described by the alpha prescription. We find that the considered progenitors can reproduce the properties of the observed helium DWDs in which the younger component is the more massive.