Astronomers find pair of 'cataclysmic' stars orbiting each other once every 51 minutes

It's the first direct evidence to prove a decades-old theory on transitioning binary star systems.
Chris Young
An artist's impression of a binary star system.
An artist's impression of a binary star system.

NASA/CXC/M.Weiss 

Roughly half the stars in the Milky Way are solitary like our Sun. The other half, on the other hand, circles other stars, sometimes in incredibly tight orbits.

A group of astronomers, including experts from MIT, just discovered a stellar binary — a pair of stars — with an orbit so tight that they appear to orbit each other once every 51 minutes, as per a press statement.

The system is likely part of a rare class of binaries called a "cataclysmic variable," where a star similar to our Sun orbit a white dwarf — the hot, dense core of a dead star.

Decades ago, astronomers predicted these cataclysmic variables should transition and move closer and closer together until they form incredibly tight orbits. The new observation provides the first direct evidence of a cataclysmic variable in this transition phase.

Stars with incredibly short orbits

Cataclysmic variables get their name from the fact they give off variable flashes of light that astronomers centuries ago thought occurred due to unknown cataclysmic events out in the cosmos. In reality, the flashes occur due to the fact the white dwarf is slowly accreting or eating material from its partner star.

The newly discovered system, dubbed ZTF J1813+4251, has the shortest orbit in a cataclysmic variable to date. In a first, astronomers discovered the system due to the fact each of the stars momentarily eclipsed the other. This allowed them to take precise measurements of each of the stars and run simulations to see how the system would likely evolve over millions of years.

The simulations showed that the stars are in a transition period and that the sun-like star has been "donating" much of its hydrogen atmosphere to the white dwarf. In approximately 70 million years, the stars will have moved even closer together. At their closest, they will likely have an 18-minute orbit, after which they will eventually expand and drift apart.

Surveying billions of stars

Burge and colleagues detailed their findings in a paper in the journal Nature. They specifically searched for signals of systems with ultrashort orbits by looking for dramatic light flashes in the survey as well as bursts of gravitational waves.

"This is a rare case where we caught one of these systems in the act of switching from hydrogen to helium accretion," explained Kevin Burdge, a Pappalardo Fellow in MIT's Department of Physics.

"People predicted these objects should transition to ultrashort orbits," Burdge continued, "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."​

The group of astronomers discovered the new system in a massive star catalog compiled by the Zwicky Transient Facility (ZTF). ZFT uses a telescope at the Palomar Observatory in California to survey the sky in high resolution.

The survey has captured an eye-watering amount of information — it has taken more than 1,000 images of more than 1 billion stars, showing changes in brightness over days, months, and years.

Abstract:

Of more than a thousand known cataclysmic variables (CVs), where a white dwarf is accreting from a hydrogen-rich star, only a dozen have orbital periods below 75 minutes1,2,3,4,5,6,7,8,9. One way to achieve these short periods requires the donor star to have undergone substantial nuclear evolution before interacting with the white dwarf10,11,12,13,14, and it is expected that these objects will transition to helium accretion. These transitional CVs have been proposed as progenitors of helium CVs13,14,15,16,17,18. However, no known transitional CV is expected to reach an orbital period short enough to account for most of the helium CV population, leaving the role of this evolutionary pathway unclear. Here we report observations of ZTF J1813+4251, a 51-minute-orbital-period, fully eclipsing binary system consisting of a star with a temperature comparable to that of the Sun but a density 100 times greater owing to its helium-rich composition, accreting onto a white dwarf. Phase-resolved spectra, multi-band light curves and the broadband spectral energy distribution allow us to obtain precise and robust constraints on the masses, radii and temperatures of both components. Evolutionary modelling shows that ZTF J1813+4251 is destined to become a helium CV binary, reaching an orbital period under 20 minutes, rendering ZTF J1813+4251 a previously missing link between helium CV binaries and hydrogen-rich CVs.

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