Most Double Degenerate Low Mass White Dwarf Binaries Merge

TL;DR

This study estimates the merger rate of extremely low mass white dwarf binaries in the Milky Way, suggesting they are likely progenitors of various stellar phenomena such as AM CVn systems and underluminous supernovae.

Contribution

It provides the first detailed estimate of the Galactic merger rate of ELM white dwarf binaries and links these mergers to multiple observed stellar phenomena.

Findings

Merger rate of 3e-3 per year in the Milky Way disk.

Most ELM white dwarf binaries originate from systems with <1 hour orbital periods.

ELM white dwarf mergers exceed the formation rate of stable AM CVn binaries.

Abstract

We estimate the merger rate of double degenerate binaries containing extremely low mass (ELM) <0.3 Msun white dwarfs in the Galaxy. Such white dwarfs are detectable for timescales of 0.1 Gyr -- 1 Gyr in the ELM Survey; the binaries they reside in have gravitational wave merger times of 0.001 Gyr -- 100 Gyr. To explain the observed distribution requires that most ELM white dwarf binary progenitors detach from the common envelope phase with <1 hr orbital periods. We calculate the local space density of ELM white dwarf binaries and estimate a merger rate of 3e-3/yr over the entire disk of the Milky Way; the merger rate in the halo is 10 times smaller. The ELM white dwarf binary merger rate exceeds by a factor of 40 the formation rate of stable mass transfer AM CVn binaries, marginally exceeds the rate of underluminous supernovae, and is identical to the formation rate of R CrB stars. On this basis, we conclude that ELM white dwarf binaries can be the progenitors of all observed AM CVn and possibly underluminous supernovae, however the majority of He+CO white dwarf binaries go through unstable mass transfer and merge, e.g. into single massive ~1 Msun white dwarfs.