In our galaxy, about half of the stars are standalone, like the sun. The remaining half consists of pairs and multiples of stars that orbit other stars in such proximity that specific stellar systems may fit between Earth and the moon.
Such stellar binary has recently been discovered by astronomers at MIT and elsewhere. Located within a vast catalog of stars, this stellar binary or a pair of stars have an extremely short orbit. They orbit each other every 51 minutes.
The newly discovered system, called ZTF J1813+4251, seems to be a rare class of binaries known as a “cataclysmic variable,” in which a star similar to our sun orbits tightly around a white dwarf—a hot, dense core of a burned-out star. Unlike other similar systems seen in the past, astronomers managed to capture this cataclysmic variable as the stars frequently eclipsed one another, enabling the group to quantify the characteristics of each star carefully.
Using these measurements, astronomers created and ran simulations of what the system is likely doing today and how it should evolve over the next hundreds of years. They conclude that the stars are in a transition state and that the sun-like star has been circling and “donating” a significant portion of its hydrogen atmosphere to the voracious white dwarf. Eventually, the sun-like lead will be converted to a large dense, helium-rich core. Before expanding and drifting apart, the stars will move even closer together in another 70 million years, with an ultrashort orbit reaching just 18 minutes.
Kevin Burdge, a Pappalardo Fellow in MIT’s Department of Physics, said, “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.”
This system has been observed by the Zwicky Transient Facility (ZTF), a survey that uses a camera attached to a telescope at the Palomar Observatory in California. The survey took more than 1,000 images of each of the more than 1 billion stars in the sky, recording each star’s changing brightness over days, months, and years.
Scientists combed through the catalog, looking for signals of systems with ultrashort orbits, the dynamics of which can be so extreme that they should give off dramatic bursts of light and emit gravitational waves.
For this study, scientists 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. Later they used an algorithm to sort among more than a billion stars, each of which was captured in over a thousand pictures.
The program removed about 1 million stars that appeared to flash every hour or so. Scientists then personally scanned them for any signs of particular interest. ZTF J1813+4251, a system in the Hercules constellation located around 3,000 light years from Earth, was the focus of his search.
Burdge said, “This thing popped up, where I saw an eclipse happening every 51 minutes, and I said, ok, this is a binary.”
The team later used the W.M. Keck Observatory in Hawaii and the Gran Telescopio Canarias in Spain to further focus on the system. They found the system exceptionally “clean,” meaning they could see its light change with each eclipse. With such clarity, they could precisely measure each object’s mass, radius, and orbital period.
With a mass of around half that of the sun and one-hundredth size, they discovered that the first object was probably a white dwarf. A solar-like star near the end of its life and a tenth of the sun’s size and mass was the second object (about the size of Jupiter). The stars also seemed to revolve around one another every 51 minutes.
Burdge said, “This one star looked like the sun, but the sun can’t fit into an orbit shorter than eight hours—what’s up here?”
“This is a special system. We were lucky to find a system that answers a big open question and is one of the most beautifully behaved cataclysmic variables known.”