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 brief orbit that appears to revolve around one another every 51 minutes. The system appears to be one of the extremely uncommon “cataclysmic variable” binary systems, in which a star comparable to our sun circles closely around the scorching, dense white dwarf of a burned-out star.
When two stars approach one another over many billions of years, the white dwarf begins to accrete or take matter from its partner star. This is known as a cataclysmic variable. The massive, fluctuating flashes of light that can be produced by this mechanism were once thought by astronomers to be the consequence of an unidentified disaster.
The team has named the recently found system ZTF J1813+4251. It is a cataclysmic variable with the shortest orbit ever discovered. The researchers captured this catastrophic variable as the stars obscured each other numerous times, unlike other similar systems detected in the past, enabling the group to carefully assess the characteristics of each star.
The researchers ran models using these findings to determine what the system is probably doing right now and how it should change over 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 sun-like star will eventually be stripped down to a mostly dense, helium-rich core. 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.
Such cataclysmic variables were expected to move to ultrashort orbits, according to predictions made decades ago by scientists at MIT and other institutions. This is the first time such a transitioning system has been observed directly.
“This is a rare case where we caught one of these systems in the act of switching from hydrogen to helium accretion,” says 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.”
Burdge and colleagues report their discovery in Nature. The study’s co-authors include collaborators from multiple institutions, including the Harvard and Smithsonian Center for Astrophysics.
Sky search
The astronomers discovered the new system within 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 pictures of wide swaths of the sky.
This is a rare case where we caught one of these systems in the act of switching from hydrogen to helium accretion. 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.
Kevin Burdge
Each of the more than 1 billion stars in the sky has been captured in more than 1,000 photos, allowing researchers to track how bright each star changes over the course of days, months, and years.
Burdge searched through the catalog in search of signs of systems with incredibly brief orbits, whose intense dynamics should produce spectacular light bursts and gravitational wave emissions.
“Gravitational waves are allowing us to study the universe in a totally new way,” says Burdge, who is searching the sky for new gravitational-wave sources.
For this new study, Burdge looked through the 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 closely orbiting objects, with one crossing the other and briefly blocking its light.
He used an algorithm to weed through over 1 billion stars, each of which was recorded in more than 1,000 images. The algorithm sifted out about 1 million stars that appeared to flash every hour or so.
Among these, Burdge then looked by eye for signals of particular interest. His search zeroed in on ZTF J1813+4251 a system that resides about 3,000 light years from Earth, in the Hercules constellation.
“This thing popped up, where I saw an eclipse happening every 51 minutes, and I said, ok, this is definitely a binary,” Burdge recalls.
A dense core
He and his colleagues further focused on the system using the W.M. Keck Observatory in Hawaii and the Gran Telescopio Canarias in Spain. 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.
By measuring 1/100th the size of the sun and having around half its mass, they discovered that the initial object was probably a white dwarf. The second object was a sun-like star near the end of its life, at a tenth the size and mass of the sun (about the size of Jupiter). The stars also appeared to orbit each other every 51 minutes.
Yet, something didn’t quite add up.
“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 says.
He soon hit upon an explanation: Nearly 30 years ago, researchers including MIT emeritus professor Saul Rappaport, had predicted that ultrashort-orbit systems should exist as cataclysmic variables.
The sun-like star should burn out, leaving a core of helium, an element that is more dense than hydrogen and heavy enough to maintain the dead star in a tight, ultrashort orbit, as the white dwarf orbits it and consumes its light hydrogen.
Burdge realized that ZTF J1813+4251 was likely a cataclysmic variable, in the act of transitioning from hydrogen to a helium-rich body. The finding represents the shortest orbit catastrophic variable ever discovered, and it also supports predictions made by Rappaport and others.
“This is a special system,” Burdge says. “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.”
This research was supported in part by the European Research Council.
