A Tight Pair of White Dwarfs

White dwarfs are the end result of evolution for stars less than 8 times the Sun’s mass. A study by Debes et al. (2015) report on the discovery that a relatively nearby object known as WD 1242-105 is a binary system consisting of two white dwarfs of similar mass and temperature. Both white dwarfs circle around one another ever 2.85 hours. As they do, gravitational waves are emitted. These gravitational waves carry away energy, causing the separation between the two white dwarfs to gradually shrink. In fact, both white dwarfs are predicted to merge 737 million years from now.

WD 1242-105 is currently the strongest known source of gravitational waves in the milli-hertz regime. The system’s total mass is 0.95 times the Sun’s mass. This is below the critical mass necessary to produce a Type Ia supernova when the two white dwarfs eventually merge. Instead, the merger will lead to either an under-luminous supernova or the formation of an extreme helium star. WD 1242-105 is estimated to lie at a distance of 127 light-years.

Both components of WD 1242-105 are low-mass white dwarfs, possibly helium-core white dwarfs. WD 1242-105 follows the trend whereby binaries containing low-mass helium-core white dwarfs tend to have shorter orbital periods than binaries containing more massive carbon/oxygen-core white dwarfs. This is because shorter period binaries interact more strongly and experience greater mass loss, resulting in low-mass helium-core white dwarfs. 

Reference:
Debes et al. (2015), “A New Merging Double Degenerate Binary in the Solar Neighborhood”, arXiv:1503.00349 [astro-ph.SR]