Double helium-core white dwarf detected

US astronomers have announced the detection of J2322+0509, a detached binary white dwarf composed of two helium-core stars with a short orbital period. 

It is the first gravitational wave source of its kind ever detected, according to a team led by the Centre for Astrophysics Harvard & Smithsonian.

“Theories predict that there are many double helium-core white dwarf binaries out there,” says Warren Brown, lead author of a study to be published in Astrophysical Journal Letters and currently available on the preprint server arXiv.

“This detection provides an anchor for those models, and for doing future experiments so that we can find more of these stars and determine their true numbers.”

In a statement, the researchers say the star will be used for verification on the LISA (Laser Interferometer Space Antenna) gravitational wave observatory planned for launch in 2034.

“Verification binaries are important because we know that LISA will see them within a few weeks of turning on the telescopes,” says co-author Mukremin Kilic, from the University of Oklahoma. 

“There’s only a handful of LISA sources that we know of today. The discovery of the first prototype of a new class of verification binary puts us well ahead of where anyone could have anticipated.”

Brown says J2322+0509 initially was a challenge to study because it had no light curve. “We couldn’t detect a photometric signal because there isn’t one.” 

That all changed thanks to data collected by the MMT telescope at the Fred Lawrence Whipple Observatory in Arizona, the Magellan Baade Telescope at the Las Campanas Observatory in Chile, and the Gemini North telescope on Maunakea, Hawaii.

Spectroscopic studies using this data revealed the binary’s orbital motion.

“We’re finding that the binaries that might be the hardest to detect may actually be the strongest sources of gravitational waves,” says Brown. 

“This binary was difficult to detect because it is oriented face-on to us, like a bull’s eye, rather than edge-on. Remarkably, the binary’s gravitational waves are 2.5 times stronger at this orientation than if it were orientated edge-on like an eclipsing binary.”

And that wasn’t the only surprise. With an orbital period of 20 minutes (1201 seconds to be precise) the pair is confirmed as having the third shortest period of all known detached binaries. 

“This pair is at the extreme end of stars with short orbital periods,” says Brown. “And the orbit of this pair of objects is decaying. 

“The gravitational waves that are being emitted are causing the pair to lose energy; in six or seven million years they will merge into a single, more-massive white dwarf.”

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