About half of the stars that are in our galaxy shine alone, like the Sun. But the other half is made up of stars that revolve around other stars, most in pairs or trios, and with orbits so tight that some of these star systems would fit comfortably on the screen. distance between the Earth and the Moon.
Now, a team of researchers led by scientists at the Massachusetts Institute of Technology (MIT) has discovered a binary system in which its two stars are so close that they can orbit each other in just 51 minutes. The system appears to belong to a rare class of binary stars, known as ‘variable cataclísmica’, in which a star similar to our Sun orbits a very small distance from a white dwarf, the hot, dense core of an ancient star that has since disappeared. The study has just been published in ‘Nature’.
cataclysmic variables
A cataclysmic variable occurs when the two stars move closer together over billions of years, causing the white dwarf to start consuming material from its companion star and therefore getting bigger and bigger. This process can emit huge flashes of light that, for centuries, astronomers have interpreted as the result of some kind of unknown cataclysm.
The newly discovered system, labeled ZTF J1813+4251, is a cataclysmic variable with the shortest orbit detected to date. Unlike other similar systems, indeed, astronomers caught this ‘odd pair’ when the stars hid from each other multiple times, allowing the team to precisely measure the properties of each.
Thanks to these measurements, the researchers were able to make simulations of what the system is probably doing today, and also predicted how it would evolve over the next few hundred million years. Thus, they concluded that the stars are currently in transition and that the Sun-like star has long been circling and ‘donating’ a large part of its hydrogen atmosphere to the ravenous white dwarf. In the future, the Sun-like star will shrink to little more than a dense and especially helium-rich core. In another 70 million years, the stars will come even closer, with an ultrashort orbit lasting 18 minutes, before they begin to expand and move apart again.
Already several decades ago, researchers predicted that such cataclysmic variables should move into ultrashort orbits. But this is the first time such a transition system has been directly observed.
“This is a rare case where we detect one of these systems in the act of changing from accumulating hydrogen to helium,” he explains. Kevin Burdge, from the MIT Department of Physics and one of the study’s authors. It was already predicted that these objects should transition to ultrashort orbits, and whether they could be shortened enough to emit detectable gravitational waves has long been debated. This discovery confirms it.”
one in a billion
Astronomers discovered the new system among data from a vast catalog of stars, compiled thanks to the study Zwicky Transient Facility (ZTF), which uses a camera attached to a telescope at the Palomar Observatory in California to take high-resolution pictures of stars across wide swaths of the sky. ZTF has already taken more than 1,000 images of each of the more than 1,000 million stars visible in the sky, recording the changing brightness of each one of them during days, months and years.
Burdge and his colleagues combed through the catalog for signs of systems with ultrashort orbits so extremely energetic that they should regularly emit powerful bursts of light and gravitational waves that, in the scientist’s words, “allow us to study the Universe in a whole new way.” .
Specifically, Burdge combed through the ZTF data for stars that appear to flicker repeatedly, with a period of less than an hour, a frequency that typically signals a system of at least two closely orbiting objects, one crossing the other and briefly blocking. its light. Thus, among ZTF’s 1 billion stars, Burdge initially selected about a million that seemed to flicker at intervals of about an hour. Afterwards, he again selected the signals that seemed most interesting to him and in the end he settled on the ZTF J1813 + 4251 system, about 3,000 light years from Earth, in the constellation of Hercules. “I saw an eclipse happening every 51 minutes and I said, okay, this is definitely binary,” recalls the researcher.
A thorough study
Having selected the target, Burdge and his colleagues observed it with the powerful telescopes at the Keck Observatory in Hawaii, and with the Great Canary Telescope, and found that the system was exceptionally ‘clean’, meaning that they could clearly see its light changes with each eclipse. Thanks to that clarity, they were able to accurately measure the mass and radius of each object, as well as its orbital period.
In this way they discovered that the first object was probably a white dwarf, a hundred times smaller than the Sun and with about half its mass. The second object was a star similar to the Sun, although ten times smaller (about the size of Jupiter) and much closer to the end of its life. Both stars orbited each other every 51 minutes.
But still there was something that didn’t quite add up. “The biggest star looked like the Sun,” Burdge recalls, “but the Sun can’t fit into an orbit of less than eight hours. What’s going on here?”
The answer was not long in coming. nearly 30 years ago, Saul Rappaport, also from MIT, had predicted with his colleagues that ultrashort-orbit systems should exist as catastrophic variables. Therefore, as the white dwarf feeds on the star similar to the Sun, eating its hydrogen, it should reduce in size until it is reduced to a nucleus of helium, an element denser than hydrogen and heavy enough to keep the dead star in a tight and extremely short orbit.
Burdge realized that ZTF J1813+4251 was probably a cataclysmic variable, in the act of transitioning from a hydrogen-rich body to a helium-rich one. The discovery confirms predictions made by Rappaport and his team, and also stands as the shortest-orbiting cataclysmic variable detected so far.
“It’s a very special system,” says Burdge. We were doubly lucky to find a system that answers a big open question, and is at the same time one of the most beautifully behaved cataclysmic variables known.”