On the Formation of Double White Dwarfs through Stable Mass Transfer and a Common Envelope

TL;DR

This paper investigates the formation of double white dwarfs through stable mass transfer and common envelope phases, challenging traditional prescriptions and proposing a scenario where the first mass transfer can be stable, leading to observed DWD properties.

Contribution

It demonstrates that a stable, non-conservative mass transfer can occur in certain binary systems, enabling the formation of double white dwarfs consistent with observations, and questions the adequacy of existing prescriptions.

Findings

Gamma-prescription cannot universally describe CE events.

First mass transfer phase can be stable in low-mass companion systems.

Proposed mechanism reproduces observed DWD properties.

Abstract

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 alpha-prescription (parametrizing energy loss), it is not possible to reproduce all of the observed systems. The gamma-prescription was proposed as an alternative to this description, instead parametrizing the fraction of angular momentum carried away in dynamical-timescale mass loss. In this paper, we analyze simultaneous energy and angular-momentum conservation, and show that the gamma-prescription cannot adequately describe a CE event for an arbitrary binary, nor can the first phase of mass loss always be understood in general as a dynamical-timescale event. 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 solar masses 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 less than approximately 0.46 solar masses and mass ratios between the younger and older WDs greater than 1.