The Formation of Low-Mass Double White Dwarfs through an Initial Phase of Stable Non-Conservative Mass Transfer

TL;DR

This paper proposes a new evolutionary pathway for low-mass double white dwarfs involving stable, non-conservative mass transfer, challenging the traditional double common-envelope model and aligning with observed system distributions.

Contribution

It introduces a model where initial stable mass transfer in RG-MS binaries can form low-mass DWDs without requiring double-CE evolution, expanding understanding of binary evolution pathways.

Findings

Stable, non-conservative mass transfer can produce low-mass DWDs.

A lower bound on mass loss fraction is identified for this process.

The model aligns with observed low-mass DWD systems.

Abstract

Although many double white dwarfs (DWDs) have been observed, the evolutionary channel by which they are formed from low-mass/long-period red-giant-main-sequence (RG-MS) binaries remains uncertain. The canonical explanations involve some variant of double common-envelope (CE) evolution, however it has been found that such a mechanism cannot produce the observed distribution. We present a model for the initial episode of mass transfer (MT) in RG-MS binaries, and demonstrate that their evolution into double white dwarfs need not arise through a double-CE process, as long as the initial primary's core mass (Md,c) does not exceed 0.46M$_{\odot}$. Instead, the first episode of dramatic mass loss may be stable, non-conservative MT. We find a lower bound on the fraction of transferred mass that must be lost from the system in order to provide for MT, and demonstrate the feasibility of this channel in producing observed low-mass (with M$_{d,c}$ < 0.46M$_{\odot}$) DWD systems.