Almost half of the stars in our galaxy are solitary like the Sun. The other half includes stars that surround other stars, in pairs and multiples, with orbits so narrow that some star systems could fit between Earth and the Moon.
Astronomers at MIT and elsewhere have discovered a stellar binary, or pair of stars, with an extremely short orbit that appears to orbit every 51 minutes. The system appears to be part of a rare class of binaries known as “Cataklysmic Variables,” in which a star similar to our Sun orbits tightly around a white dwarf—the hot, dense core of a burning star.
A cataclysmic variable occurs when the two stars merge over billions of years, causing the white dwarf to accret or eat material away from its partner star. This process can emit huge, variable flashes of light that astronomers centuries ago assumed were the result of an unknown cataclysm.
The newly discovered system, which the team named ZTF J1813+4251, is a cataclysmic variable with the shortest orbit detected to date. Unlike other such systems seen in the past, the astronomers captured this catastrophic variable as the stars eclipsed multiple times, allowing the team to precisely measure the properties of each star.
With these measurements, the researchers ran simulations of what the system is likely to do today and how it is expected to evolve over the next few hundred million years. They conclude that the stars are currently in transition and that the Sun-like star is spinning around and “donating” much of its hydrogen atmosphere to the ferocious white dwarf. The sun-like star will eventually be reduced to a mostly dense, helium-rich core. In 70 million years, the stars will migrate even closer to each other, with an ultra-short orbit reaching only 18 minutes before they begin to expand and recede.
Decades ago, researchers at MIT and elsewhere predicted that such cataclysmic variables would transition to ultrashort orbits. This is the first time that such a transition system has been observed directly.
“This is a rare case where we caught one of these systems going from hydrogen accretion to helium,” says Kevin Burdge, Pappalardo Fellow in MIT’s Department of Physics. “People have predicted that these objects would go on ultra-short orbits, and it has long been debated whether they could become short enough to emit detectable gravitational waves. This discovery puts an end to that. »
Burgge and colleagues report their discovery in Nature. The study’s co-authors include collaborators from several institutions, including Harvard and the Smithsonian Center for Astrophysics.
Searching for the sky
Astronomers discovered the new system in a large catalog of stars observed by the Zwicky Transient Facility (ZTF), a survey that uses a camera attached to a telescope at the Palomar Observatory in California to take high-resolution images of broad bands of the sky.
The study took more than 1,000 images of each of the more than a billion stars in the sky and recorded how each star’s brightness changes over days, months and years.
Burdge scoured the catalog for signals from systems with ultra-short orbits, whose dynamics can be so extreme that they should emit spectacular bursts of light and emit gravitational waves.
“Gravitational waves allow us to study the universe in a whole new way,” says Burdge, who is searching the sky for new sources of gravitational waves.
For this new study, Burdge looked at ZTF data for stars that appeared to blink repeatedly, with a period of less than an hour—a frequency that typically signals a system of at least two objects in close orbit, one of which crosses the other, briefly blocking its light.
He used an algorithm to eliminate more than a billion stars, each captured in more than 1,000 images. The algorithm filtered out about 1 million stars that appeared to twinkle every hour or so. Among these, Burge then visibly searched for signals of special interest. His research focused on ZTF J1813+4251 – a system located about 3,000 light-years from Earth, in the constellation Hercules.
“This thing came up where I saw an eclipse happening every 51 minutes and I said, OK, it’s definitely a binary,” Burdge recalled.
A dense core
He and his colleagues then focused on the system using the WM Keck Observatory in Hawaii and the Gran Telescopio Canarias in Spain. They found the system to be unusually “pure,” meaning they could clearly see its light changing with each eclipse. With such clarity, they were able to accurately measure each object’s mass and radius, as well as their orbital period.
They discovered that the first object was likely a white dwarf, 1/100 the size of the Sun and about half its mass. The other object was a Sun-like star near the end of its life, one-tenth the size and mass of the Sun (about the size of Jupiter). The stars also appeared to orbit every 51 minutes.
However, something was not quite as it should be.
“This star looked like the sun, but the sun can’t orbit for less than eight hours — what’s going on here? Burge said.
He quickly found an explanation: Nearly 30 years ago, researchers, including MIT professor emeritus Saul Rappaport, predicted that ultrashort orbit systems would exist as cataclysmic variables. As the white dwarf orbits the Sun-like star and eats away at its light hydrogen, the Sun-like star is expected to burn up, leaving behind a core of helium—an element denser than hydrogen and heavy enough to hold the dead star in a narrow, ultra short track.
Burdge realized that ZTF J1813+4251 was likely a cataclysmic variable, in the process of transitioning from a hydrogen-rich body to a helium-rich body. The discovery confirms both predictions made by Rappaport and others, and also stands as the shortest orbital cataclysmic variable detected to date.
“It’s a special system,” Burge said. “We were doubly fortunate to find a system that answers a great open question and is one of the most admirably known cataclysmic variables. »
This research was partially funded by the European Research Council.