Discovery of a Detached, Eclipsing 40 min Period Double White Dwarf Binary and a Friend: Implications for He+CO White Dwarf Mergers

TL;DR

This paper reports the discovery of two ultra-compact double white dwarf binaries with very short orbital periods, providing insights into their merger rates, gravitational wave signals, and implications for He+CO WD merger outcomes.

Contribution

It presents the discovery of two new detached double white dwarf binaries with very short periods and updates the Milky Way He+CO WD merger rate, highlighting the dominance of unstable mergers.

Findings

One system is an eclipsing binary with a 40 min period.

Both systems have merger times under 50 million years.

He+CO WD binaries merge at a rate 40 times greater than stable AM CVn formation.

Abstract

We report the discovery of two detached double white dwarf (WD) binaries, SDSS J082239.546+304857.19 and SDSS J104336.275+055149.90, with orbital periods of 40 and 46 min, respectively. The 40 min system is eclipsing; it is composed of a 0.30 Msun and a 0.52 Msun WD. The 46 min system is a likely LISA verification binary. The short 20 Myr and ~34 Myr gravitational wave merger times of the two binaries imply that many more such systems have formed and merged over the age of the Milky Way. We update the estimated Milky Way He+CO WD binary merger rate and affirm our previously published result: He+CO WD binaries merge at a rate at least 40 times greater than the formation rate of stable mass-transfer AM~CVn binaries, and so the majority must have unstable mass-transfer. The implication is that spin-orbit coupling in He+CO WD mergers is weak, or perhaps nova-like outbursts drive He+CO WDs into merger as proposed by Shen.