Star slowly devours brown dwarf in billion-year feast

Astronomers have long known that planets can be swallowed whole when they venture too close to a star. Now an international team led by MIT has found something altogether different: a star that is dining on its companion one bite at a time, in a meal that could last billions of years.

The system, catalogued as ZTF J0440+2325, lies about 300 light-years away within the Milky Way. It is the first observed case of a low-mass star steadily pulling material from another low-mass object, according to a study published Oct. 5 in Nature Astronomy.

"When we think of stars interacting with planets or brown dwarfs, the picture is always that the star eventually swallows the other thing," said Kevin Burdge, assistant professor of physics at MIT and lead author of the study. "This is what will happen to the Earth when the sun becomes a red giant. But here, we've found an alternative: Instead of swallowing the thing up, the star can gradually eat it, for billions of years."

Brown dwarfs are objects more massive than planets but not massive enough to sustain the hydrogen fusion that powers ordinary stars.

A triangular signal that sat for years

The discovery began with a confusing pattern in data from the Zwicky Transient Facility (ZTF), a survey that repeatedly photographs the sky from Palomar Observatory in California. Burdge noticed a light curve that traced a repeated triangular shape — unlike the smooth, bell-shaped curves seen in supernovae.

"I remember first looking at this and thinking, stars don't make triangular waveforms like this," Burdge said.

He initially suspected the signal might come from a "black widow binary," a system where a dense neutron star strips matter from a smaller companion. But the motion of the objects didn't match: the wobble was far too small.

"We weren't seeing that whipping back and forth here," Burdge said. "It didn't make any sense. We couldn't explain what this was."

The signal sat unexplained for years until Burdge and MIT graduate student Aaron Householder revisited it.

Two objects in an 87-minute orbit

Using ZTF observations and follow-up with multiple telescopes, the team identified two unusually compact objects orbiting at an extremely close distance. The central star has about 85 times the mass of Jupiter. Its companion, a brown dwarf, weighs roughly 25 Jupiter masses. The brown dwarf completes an orbit every 87 minutes, and its entire path would fit within the diameter of the Sun.

Measuring the wobble was key. "That was the real clincher for this system," Householder said. "When we measured that wobble, we found we were not seeing a black widow. This was a low-mass star that's orbited by a brown dwarf. The wobble was too small in amplitude to be anything else."

Computer simulations confirmed what the researchers suspected: material from the brown dwarf was falling directly onto the star's surface.

"When we track those test particles, we see they indeed fall right onto the surface of the star," Householder said. "This is the first time we've caught a low-mass star actively accreting from another low-mass object."

A fireball on the star

As matter streams from the brown dwarf and strikes the star at high speed, it creates an intensely heated region — a persistent fireball. As the brown dwarf orbits, that hotspot rotates into and out of view, producing the triangular brightening and dimming that first caught Burdge's eye.

"It's like you've got this continuous fireball onto one of the objects, and as one orbits the other, that hotspot comes in and out of view, and the peak of the triangle signal is when you're looking right at the fireball," Burdge explained.

Billions of years of slow consumption

The star is consuming about one hundred-thousandth of Earth's mass each year. That’s vast — roughly 40 million dump trucks of matter annually — but the brown dwarf is so massive that it represents only a tiny fraction of its total mass. The feeding could continue for hundreds of thousands to billions of years.

Opening a new window

The discovery shows that slow, steady accretion is possible between low-mass objects, not just in systems with black holes or neutron stars. The team plans to search for more examples.

"It's inspiring a lot of new searches on our part," Householder said. "I think we're going to learn a lot about a different kind of way that planets and brown dwarfs interact with their host stars."

The research was supported in part by the National Science Foundation.

• accretion
• brown dwarf
• Kevin Burdge
• MIT
• Nature Astronomy
• ZTF J0440+2325