New pair of ‘cataclysmic’ stars with the shortest known orbit

Oct. 5 () –

Astronomers at MIT and other institutions have discovered a stellar binary, or pair of stars, with an extremely short orbit, which seems to rotate around each other every 51 minutes.

The system appears to belong to a rare class of binaries known as “cataclysmic variables”, in which a star similar to our sun orbits closely around a white dwarfa hot, dense core of a burned-out star.

A cataclysmic variable occurs when the two stars move closer together, over billions of years, causing the white dwarf to begin accreting or consuming material from its companion star. This process can emit huge and variable flashes of light that, centuries ago, astronomers assumed that they were the result of some unknown cataclysm.

The newly discovered system, which the team has labeled ZTF J1813+4251, is a cataclysmic variable with the shortest orbit detected to date. Unlike other similar systems observed in the past, astronomers captured this catastrophic variable when the stars eclipsed each other multiple times, which allowed the team to accurately measure the properties of each star.

Using these measurements, the researchers ran simulations of what the system is likely doing today and how it should evolve in the next few hundred million years. They conclude that the stars are currently in transition and that the Sun-like star has been spinning around and “donating” much of its hydrogen atmosphere to the ravenous white dwarf. The Sun-like star will eventually shrink to a mostly dense, helium-rich core. In another 70 million years, the stars will migrate even closer together, with an ultrashort orbit reaching 18 minutes, before they begin to expand and separate.

Decades ago, researchers at MIT and elsewhere predicted that such cataclysmic variables should move into ultrashort orbits. This is the first time such a transition system has been directly observed.

“This is a rare case where we caught one of these systems in the act of switching from accumulating hydrogen to helium,” he says. it’s a statement Kevin Burdge, a Pappalardo fellow in the MIT Department of Physics. “People predicted that these objects should transition to ultrashort orbits, and it was long debated whether they could get short enough to emit detectable gravitational waves. This discovery puts an end to it.”

Burdge and colleagues report their discovery in Nature. Study co-authors include collaborators from multiple institutions, including the Harvard Center for Astrophysics and the Smithsonian.

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 broad swaths of the Earth. star. darling.

The survey has taken more than 1,000 images of each of the more than 1 billion stars in the sky, recording the changing brightness of each star over days, months and years.

Burdge combed through the catalog for signs of systems with ultrashort orbits, whose dynamics may be so extreme that they should emit dramatic bursts of light and 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 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 their light.

It used an algorithm to remove more than a billion stars, each of which was recorded in more than 1,000 images. The algorithm sifted through around 1 million stars that seemed to twinkle every hour or so. Among these, Burdge then looked for signs of particular interest. The search for him focused on ZTF J1813+4251, a system that resides about 3,000 light-years from Earth, in the constellation of Hercules.

“This came up, where I saw an eclipse happen every 51 minutes, and I was like, okay, this is definitely binary,” Burge remembers.

He and his colleagues further focused on the system using the WM Keck Observatory in Hawaii and the Gran Telescopio Canarias in Spain. They found that the system was exceptionally “clean,” meaning that they could clearly see its light change with each eclipse. so clearly, they were able to accurately measure the mass and radius of each object, as well as its orbital period.

They found 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 appeared.

The stars also seemed to orbit each other every 51 minutes. However, something was not quite right. “This star looked like the sun, but the sun can’t fit into an orbit of less than eight hours, what’s happening here?Burgge says.

He soon found an explanation: Nearly 30 years ago, researchers, including MIT professor emeritus Saul Rappaport, had predicted that ultrashort orbit systems should exist as catastrophic variables. As the white dwarf comes around the Sun-like star and eats its light hydrogen, the Sun-like star should burn up, leaving behind a core of helium, an element that is denser than hydrogen and heavy enough to hold the dead star in a narrow and ultrashort 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 others, and it also stands as the shortest-orbiting cataclysmic variable detected to date.