Brown dwarfs may have gained the unfortunate nickname “failed stars,” but new research suggests they can collide and merge for a second chance at success.
Brown dwarfs are cosmic objects with around 13 to 80 times the mass of Jupiter, making them around 0.013 to 0.08 times as massive as the sun. They are deemed as having “failed” because despite forming like normal stars — when vast, overly dense patches of matter collapse in interstellar clouds of gas and dust — they fail to gather enough mass from these clouds to trigger the nuclear fusion of hydrogen to helium in their cores, the process that defines a “main sequence” star, like the sun.
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“The failed stars get a second chance,” team leader Samuel Whitebook, from California Institute of Technology (Caltech), said in a statement. “Brown dwarfs don’t have internal engines like stars do, but this result shows they can exhibit very interesting dynamic physics.”
The team’s findings are extraordinary because, though similar mass transfer has been seen in binary objects before, this has occurred between stellar bodies with far greater masses.
“These are very exotic objects,” team member Tom Prince of Caltech said. “We’ve told some of our colleagues about them, and they didn’t believe such a thing exists.”
The brown dwarf pairing at the heart of this discovery, found in the ZTF Variability Archive, is designated ZTF J1239+8347 (ZTF J1239) and is located around 1,000 light-years away in the constellation Ursa Major. The two brown dwarfs, both 60 to 80 times as massive as Jupiter, orbit each other so tightly that the entire ZTF J1239 system would fit between Earth and the moon.
The researchers can’t be sure how these brown dwarfs initially came to orbit each other, but they suspect that the failed stars were pulled from separate systems and pushed together by the gravitational influence of another star. Once orbiting each other, the brown dwarfs would have gradually spiraled closer and closer together, with the gravitational influence of one brown dwarf causing its counterpart to puff out and become less dense.
“When one star’s gravity is overcome by the other’s, matter starts flowing from the less dense star to the denser star,” Whitebook said. “It’s like the matter sloughs off through a nozzle.”
This “nozzle” sprays matter from the puffy brown dwarf to one spot on its denser companion. This region is heated and begins to glow brightly. As this bright spot rotates with its parent brown dwarf, it generates a significant change in the brightness of this system every 57 seconds. It is this signal that first made this system stand out among the 2 billion objects of the ZTF Variability Archive.
This is the first mass transfer process seen in a brown dwarf pairing, but the team believes there could be many more brown dwarf pairings such as this just waiting to be uncovered.
“We expect the Vera Rubin Observatory [a major ground-based observatory in Chile] to detect dozens more of these objects,” Whitebook concluded. “We want to find more to understand the population and how common it is. We predict this happens more than you think.”
The team’s research was published on Wednesday (March 18) in The Astrophysical Journal Letters.