Astronomers at MIT and elsewhere have discovered a stellar binary, or pair of stars, with an extremely short orbit, appearing to orbit every 51 minutes. The system appears to be part of a rare class of binaries known as the “Cataclysmic Variable”, in which a star similar to our sun orbits closely around a white dwarf – a hot, dense core of a scorched star.
A cataclysmic variable occurs when the two stars come together, over billions of years, causing the white dwarf to accrete or eat material away from its partner star. This process can emit huge, variable flashes of light that centuries ago astronomers assumed were the result of an unknown cataclysm.
The newly discovered system, which the team labeled 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 cataclysmic 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 voracious 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 ultrashort orbit reaching just 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 transitioning from hydrogen accretion to helium,” says Kevin Burdge, Pappalardo Fellow in MIT’s Department of Physics. “People have predicted that these objects should transition to ultrashort orbits, and there has long been debate about their ability to become short enough to emit detectable gravitational waves. This discovery puts an end to that.
Burge and his colleagues report their discovery in Nature. The study’s co-authors include collaborators from several institutions, including the Harvard and Smithsonian Center for Astrophysics.
Astronomers discovered the new system in a vast 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 photos of wide bands of the sky.
The survey took more than 1,000 images of each of the more than a billion stars in the sky, recording how each star’s brightness changes over days, months and years.
Burdge scoured the catalog for signals from systems with ultrashort 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 searches the sky for new sources of gravitational waves.
For this new study, Burdge looked at ZTF data for stars that appeared to flash repeatedly, with a period of less than an hour — a frequency that typically signals a system of at least two objects in close orbit, the one crossing the other and briefly blocking its light.
He used an algorithm to eliminate more than a billion stars, each of which was recorded in more than 1,000 images. The algorithm filtered out about 1 million stars that seemed to flash every hour or so. Among these, Burge then searched visibly for signals of particular interest. His research focused on ZTF J1813+4251 – a system that resides about 3,000 light-years from Earth, in the constellation of Hercules.
“This thing appeared, where I saw an eclipse happening every 51 minutes, and I said, okay, it’s definitely a binary,” Burdge recalled.
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 exceptionally “clean”, 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 probably a white dwarf, 1/100th the size of the sun and about half its mass. The second 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 seemed to orbit every 51 minutes.
“This star looked like the sun, but the sun can’t go into an orbit of less than eight hours – what’s going on here?” said Burge.
He quickly found an explanation: Nearly 30 years ago, researchers including MIT Professor Emeritus Saul Rappaport predicted that ultrashort-orbit systems should exist as cataclysmic variables. As the white dwarf eats around 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 space. dead star in a tight, ultrashort orbit.
Burdge realized that ZTF J1813+4251 was likely a cataclysmic variable, in the act 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,” says Burge. “We had a double chance of finding a system that answers a big open question and is one of the most admirably known cataclysmic variables.”
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