Every interacting double white dwarf binary may merge

TL;DR

This paper suggests that most interacting double white dwarf binaries, previously thought to be stable, are likely to merge due to nova-like outbursts and dynamical friction, challenging existing stability assumptions.

Contribution

It introduces a new mechanism involving nova outbursts and dynamical friction that causes the merger of low mass ratio double white dwarf binaries.

Findings

Hydrogen-rich mass transfer causes nova-like outbursts.

Dynamical friction shrinks the orbit and increases mass transfer rates.

Most interacting double white dwarf binaries are likely to merge.

Abstract

Interacting double white dwarf binaries can give rise to a wide variety of astrophysical outcomes ranging from faint thermonuclear and Type Ia supernovae to the formation of neutron stars and stably accreting AM Canum Venaticorum systems. One key factor affecting the final outcome is whether mass transfer remains dynamically stable or instead diverges, leading to the tidal disruption of the donor and the merger of the binary. It is typically thought that for low ratios of the donor mass to the accretor mass, mass transfer remains stable, especially if accretion occurs via a disk. In this Letter, we examine low mass ratio double white dwarf binaries and find that the initial phase of hydrogen-rich mass transfer leads to a classical nova-like outburst on the accretor. Dynamical friction within the expanding nova shell shrinks the orbit and causes the mass transfer rate to increase dramatically above the accretor's Eddington limit, possibly resulting in a binary merger. If the binary survives the first hydrogen-rich nova outbursts, dynamical friction within the subsequent helium-powered nova shells pushes the system even more strongly towards merger. While further calculations are necessary to confirm this outcome for the entire range of binaries previously thought to be dynamically stable, it appears likely that most, if not all, interacting double white dwarf binaries will merge during the course of their evolution.