“This is a special system,” and “We got doubly lucky to find a system that answers a big open question, and is one of the most beautifully behaved cataclysmic variables known.”

In our galaxy, about half of the stars are solitary, like the sun. The remaining half consists of stars that orbit other stars in pairs and groups. Some of these stellar systems have orbits that are so small that they might fit between Earth and the moon.

Researchers from MIT and other institutions have found a stellar binary, or pair of stars, with an unusually short orbit that appears to circle around one another every 51 minutes.

The system appears to be one of the elusive “cataclysmic variable” binary systems, in which a star like our sun closely orbits a white dwarf, the hot, dense core of a burned-out star.

When two stars approach one other over billions of years, a cataclysmic variable emerges, leading the white dwarf to begin accreting, or eating material away from its partner star.

In the past, astronomers believed that these massive, fluctuating bursts of light were the product of some undiscovered cataclysm.

The team has named the recently found system ZTF J1813+4251. It is a cataclysmic variable with the shortest orbit ever discovered.

Unlike other systems of this kind that astronomers have seen in the past, this one was caught as the stars eclipsed each other more than once. This made it possible for the team to accurately measure the properties of each star.

The researchers used these findings to run models of what the system is probably doing right now and how it should develop over the course of the next hundreds of millions of years.

The sun-like star has been circling and “donating” a large portion of its hydrogen atmosphere to the hungry white dwarf, they conclude, indicating that the stars are in the process of transformation.

The core of the sun-like star, which is rich in helium, will eventually be stripped away.

The stars will move even closer together in another 70 million years, with an ultrashort orbit lasting only 18 minutes, before they start to enlarge and drift away.

MIT researchers anticipated decades ago that cataclysmic variables would have ultrashort orbits. This is the initial direct observation of such a transitional system.

“This is a rare case where we caught one of these systems in the act of switching from hydrogen to helium accretion,” adds Kevin Burdge, a Pappalardo Fellow in MIT’s Department of Physics. “People predicted these objects should transition to ultrashort orbits, and it was debated for a long time whether they could get short enough to emit detectable gravitational waves. This discovery puts that to rest.”​

In Nature, Burdge and co-workers present their findings. Members of the Harvard and Smithsonian Center for Astrophysics, among others, contributed to the work as co-authors.

Sky hunt

The Zwicky Transient Facility (ZTF) survey employs a camera linked to a telescope at Palomar Observatory in California to take high-resolution photos of huge patches of sky, and it was within this massive catalog of stars that the scientists discovered the new system.

More than a billion stars have been photographed for the study, documenting the gradual dimming and brightening of each star over the course of months and years.

Burdge searched the database for signs of systems with extremely short orbits, the dynamics of which can be so intense that they should produce spectacular luminous outbursts and gravitational wave emissions.

Burdge, who is looking for new gravitational-wave sources in the sky, argues that gravity waves are enabling a completely new way to investigate the universe.

Burdge searched the ZTF data for stars that appeared to flash repeatedly with a period of less than an hour for this new study. This frequency often denotes a system of at least two closely orbiting objects, with one crossing the other and momentarily obscuring its light.

An algorithm helped him sort through the over a billion stars that were recorded in more than a thousand photographs. The system was able to identify approximately one million stars that appeared to blink roughly once every hour. After going through all of these, Burdge manually searched for signals that were of particular interest to him. His search led him to a system called ZTF J1813+4251. It is about 3,000 light years from Earth and is in the constellation Hercules.

“This thing popped up, where I saw an eclipse happening every 51 minutes, and I said, ok, this is definitely a binary,” Burdge remarks.

A dense center

Using the Gran Telescopio Canarias in Spain and the W.M. Keck Observatory in Hawaii, he and his colleagues continued to concentrate on the system.

They discovered that the system was remarkably “clean,” which allowed them to clearly observe how its light changed throughout each eclipse. They were able to precisely quantify each object’s mass, radius, and orbital period thanks to the sharpness of the images.

They discovered that the first object was most likely a white dwarf, measuring 1/100th the size of the sun and weighing around half its mass. The second object was a sun-like star nearing the end of its existence, measuring one-tenth the size and mass of the sun (about the size of Jupiter). Additionally, the stars appeared to orbit one another every 51 minutes.

But something didn’t seem right.

“This one star looked like the sun, but the sun can’t fit into an orbit shorter than eight hours — what’s up here?” Burdge adds.

He swiftly came up with an explanation: Scientists, including MIT emeritus professor Saul Rappaport, had anticipated the existence of ultrashort-orbit systems as potentially catastrophic variables some 30 years prior.

As the white dwarf rounds the sun-like star and consumes its light hydrogen, the sun-like star will eventually burn out, leaving behind a core of helium, which is denser than hydrogen and heavy enough to hold the dead star in a tight, ultrashort orbit.

Burdge came to the conclusion that ZTF J1813+4251 was probably a cataclysmic variable that was changing from a hydrogen-rich body to a helium-rich body.

In addition to confirming the predictions of Rappaport and others, this is the shortest orbit cataclysmic variable ever detected.

“This is a special system,” Burdge adds. “We got doubly lucky to find a system that answers a big open question, and is one of the most beautifully behaved cataclysmic variables known.”

Image Credit: Getty

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