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White dwarf binaries
set to merge

Posted: 17 November 2010

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A team of astronomers have uncovered a dozen double-star systems of which half are spiraling towards each other, set to explode as faint, 'underluminous' supernovae.

The astronomers were studying hypervelocity stars that are being kicked out of our Milky Way Galaxy when they made the serendipitous discovery of the binary white dwarfs, the hot dense remains of a Sun-like star packed into a volume the size of the Earth.

In about 100 million years the binary star system J0923+3028 will merge. Credit: Clayton Ellis (CfA).

These particular systems are unusual in that they are circling each other at a distance less than the radius of Sun, and are more lightweight than typical white dwarf stars that are composed of carbon and oxygen, instead primarily made up of helium.

“These white dwarfs have gone through a dramatic weight loss program,” says Carlos Allende Prieto of the Instituto de Astrofisica de Canarias in Spain. “These stars are in such close orbits that tidal forces, like those swaying the oceans on Earth, led to huge mass losses.”

The astronomers say that because they orbit so close to each other they are stirring up the space-time continuum, creating ripples known as gravitational waves, which carry away the stars' orbital energy, forcing them to spiral closer together.

The binary star system J0923+3028 consists of two white dwarfs: a visible star weighing 23 percent as much as our Sun and about four times the diameter of Earth, and an unseen companion weighing 44 percent of the Sun and about one Earth-diameter in size. The stars are currently separated by about 220,000 miles and orbit each other once per hour. Image: Clayton Ellis (CfA). .

The tightest binary pair discovered in this study is expected to merge in about 100 million year, but the merging of their combined mass is too low to trigger a supernova event. Instead, a more massive white dwarf will likely be left over, marked by a brief flash of light much fainter than typical supernovae explosions.

Lead author Warren Brown speculates that these merging binaries might be one source of a rare breed of 'underluminous' supernovae, which are one hundred times fainter than a normal Type Ia supernova and eject just one-fifth as much material.

“The rate at which our white dwarfs are merging is the same as the rate of underluminous supernovae, about one every 2,000 years,” he says. “While we can’t know for sure whether our merging white dwarfs will explode as underluminous supernovae, the fact that the rates are the same is highly suggestive.”

The discovery has tripled the number of known merging white-dwarf systems and will help astronomers learn more about how this breed of star evolve.