Astronomers have observed a "vampire" star feeding on a brown dwarf companion for the first time. It is a slow and gruesome death that could last for billions of years, despite the fact that the star is snacking on the equivalent of 5.2 trillion quarter-pounder burgers every second.
Brown dwarfs straddle the line between planets and stars, with masses from 13 to 80 times that of Jupiter or 0.013 to 0.08 times that of the sun. They get their unfortunate nickname "failed stars" because, though they form from a collapsing cloud of dense gas, they fail to gather enough mass to trigger the nuclear fusion of hydrogen to helium in their cores, the process that defines what a star is.
This newly discovered brown dwarf, found to be a snack for a small companion star, is located in the system ZTF J0440+2325, which dwells around 300 light-years from Earth. This represents the first time astronomers have seen a low-mass star acting as a stellar vampire, stealing material from a low-mass stellar companion. The slow rate at which this is occurring indicates this cosmic vampirism will last for hundreds of thousands to billions of years.
"When we think of stars interacting with planets or brown dwarfs, the picture is always that the star eventually swallows the other thing," team member Kevin Burdge of the Massachusetts Institute of Technology (MIT) said in a statement. "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."
A strange cosmic triangle
This new system was first observed in observations of the sky conducted by the Zwicky Transient Facility (ZTF) in the form of a strange light triangle that repeatedly appeared in the data.
"I remember first looking at this and thinking: Stars don't make triangular waveforms like this," Burdge explained.
At the time, Burdge and colleagues were hunting for so-called "black widow pulsars" that contain a stellar remnant called a neutron star feasting on a companion star much like how the arachnid of the same name devours its prey. However, because these systems contain a low-mass object gravitationally tugging on a high-mass companion, there is usually a "wobble" in their light signature, and the team found no "wobble" in this particular light signature.
"We weren't seeing that whipping back and forth here," said Burdge. "It didn't make any sense. We couldn't explain what this was."
Ruling out a black widow pulsar as the origin of this strange repeating light signal in ZTF data, Burdge and colleagues followed another hunch; proposing that this is a more balanced system with objects of similar masses.
"If you have less mass in the system overall, things can gently orbit each other without whipping back and forth," Burdge said. "That was the idea. But we never had any proof. And this weird triangle just sat for years."
That was until Burdge and colleagues revisited it, determining it originated from within the boundary of the Milky Way. Designating the source ZTF J0440+2325, the team followed up with multiple telescopes. With further observations, they found a slight wobble, less than seen for black widow pulsars but not insignificant.
"That was the real clincher for this system," team member Aaron Householder of MIT 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."
What else do we know about ZTF J0440+2325
The researchers were able to figure out that the brown dwarf takes around 87 minutes to orbit the star feeding upon it. That means that this entire system could fit within the sun.
The team was also able to determine the mass of both bodies in ZTF J0440+2325. The star has a mass around 85 times the mass of Jupiter, with the brown dwarf has a mass around 25 times the mass of the gas giant.
Though the star is stripping and accreting matter like black holes sometimes do with stars, the huge difference in size between the star in ZTF J0440+2325 and a typical black hole means this proceeds differently. That is because, despite having many times more mass than the sun, black holes are smaller than the sun with radii ranging from 6 miles to 190 miles (9 kilometers to 300 kilometers).
When a black hole strips matter from a star, this stolen material forms an accretion disk around the black hole that then gradually feeds it. In the case of this system, the stolen brown dwarf matter is falling directly to the star.
"The difference here is: The thing absorbing matter is not a tiny black hole but a star, which is relatively big in size," Burdge said. "So matter just pummels directly onto the surface, at very high speeds, like an asteroid hitting the moon."
The team estimated the rate at which matter falls from the brown dwarf to the star, finding it to be the equivalent of 1/100,000 of Earth's mass per year. That's about 5.2 billion quarter-pounder burgers every second, or maybe 1.2 trillion 1-pound burritos if burgers aren't your speed.
That seems like a lot, but it is just a tiny fraction of the brown dwarf's total mass, making this a slow and steady transfer of material that could go on for billions of years.
This neatly loops back to the initial puzzle that began the investigation into this system in the first place: that weird triangle light signature. Viewed from a distance, the steady feeding process would proceed like a triangle as the victim brown dwarf orbits the star that is eating it.
"It's like you've got this continuous fireball on 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 said.
The team now intends to hunt for more similar systems within the Milky Way.
"It's inspiring a lot of new searches on our part,” Householder concluded. "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 team's findings were published on Monday (Oct. 5) in the journal Nature Astronomy.
