The Discovery of Binary White Dwarfs that will Merge within 500 Myr

TL;DR

This paper reports the discovery of four extremely low-mass white dwarf binaries, three of which will merge within 500 million years, providing insights into white dwarf mergers and their potential outcomes.

Contribution

It presents the first observations of such short-period low-mass white dwarf binaries and analyzes their future evolution and possible astrophysical phenomena.

Findings

Three systems will merge within 500 Myr due to gravitational radiation.

All systems are likely white dwarf companions, not main-sequence or pulsar stars.

Potential outcomes include R Coronae Borealis stars, helium-enriched subdwarfs, or AM CVn systems.

Abstract

We present radial velocity observations of four extremely low-mass (0.2 Msol) white dwarfs. All four stars show peak-to-peak radial velocity variations of 540 - 710 km/s with 1.0 - 5.9 hr periods. The optical photometry rules out main-sequence companions. In addition, no milli-second pulsar companions are detected in radio observations. Thus the invisible companions are most likely white dwarfs. Two of the systems are the shortest period binary white dwarfs yet discovered. Due to the loss of angular momentum through gravitational radiation, three of the systems will merge within 500 Myr. The remaining system will merge within a Hubble time. The mass functions for three of the systems imply companions more massive than 0.46 Msol; thus those are carbon/oxygen core white dwarfs. The unknown inclination angles prohibit a definitive conclusion about the future of these systems. However, the chance of a supernova Ia event is only 1% to 5%. These systems are likely to form single R Coronae Borealis stars, providing evidence for a white dwarf + white dwarf merger mechanism for these unusual objects. One of the systems, SDSS J105353.89+520031.0 has a 70% chance of having a low-mass white dwarf companion. This system will probably form a single helium-enriched subdwarf O star. All four white dwarf systems have unusal mass ratios of < 0.2-0.8 that may also lead to the formation of AM CVn systems.