The Merger Rate of Extremely Low Mass White Dwarf Binaries: Links to the Formation of AM CVn Stars and Underluminous Supernovae

TL;DR

This study quantifies the merger rate of extremely low mass white dwarf binaries in the Milky Way and explores their potential roles in forming AM CVn stars and causing underluminous supernovae, linking stellar evolution to transient phenomena.

Contribution

It provides the first measurement of the Milky Way's ELM white dwarf merger rate and connects these mergers to the origins of specific stellar and supernova phenomena.

Findings

Milky Way ELM WD merger rate is approximately 4 x 10^(-5)/yr.

Stable mass transfer ELM WD systems can account for over 3% of AM CVn stars.

Merger rate of potential underluminous SNe progenitors is comparable to observed rates.

Abstract

We study a complete, colour-selected sample of double-degenerate binary systems containing extremely low mass (ELM) <0.25 Msol white dwarfs (WDs). We show, for the first time, that Milky Way disk ELM WDs have a merger rate of approximately 4 x 10^(-5)/yr due to gravitational wave radiation. The merger end-product depends on the mass ratio of the binary. The ELM WD systems that undergo stable mass transfer can account for >3% of AM CVn stars. More importantly, the ELM WD systems that may detonate merge at a rate comparable to the estimated rate of underluminous SNe, rare explosions estimated to produce only ~0.2 Msol worth of ejecta. At least 25% of our ELM WD sample belong to the old thick disk and halo components of the Milky Way. Thus, if merging ELM WD systems are the progenitors of underluminous SNe, transient surveys must find them in both elliptical and spiral galaxies.