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A newly discovered stray stellar body may be a “failed star,” but it’s certainly not a failure when it comes to speed!
The potential brown dwarf is racing through our Milky Way galaxy at 1.2 million mph (1.9 million km/h). That’s about 1,500 times faster than the speed of sound! Fortunately, this cosmic fugitive is heading toward the center of the Milky Way, not toward us. However, the object is moving so fast that it could eventually escape our galaxy entirely.
The incredible speed of this newly discovered stellar body, called CWISE J1249+3621, isn’t the only fascinating thing about the object, currently located about 400 light-years from Earth.
The mass of the stellar body is about 8% that of the Sun, or 80 times that of Jupiter, which puts it right in the middle of the boundary between a star and a fascinating group of objects called “brown dwarfs”, often (somewhat unfairly) referred to as “failed stars”.
CWISE J1249+3621 was first discovered by citizen scientists working as part of the Backyard Worlds: Planet 9 project, which detected faint, moving objects relatively close to the sun using data from NASA’s Wide-field Infrared Survey Explorer (WISE).
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After several citizen scientists noticed the object, a team of astronomers followed up on the event using the Keck I Telescope, one of two twin 10-meter telescopes located on the dormant volcano Maunakea in Hawaii.
“We’ve discovered a very low-mass object just above the stellar/brown dwarf mass limit, moving at such a high velocity that it may not be connected to the Milky Way galaxy,” study leader Adam Burgasser of the University of California, San Diego, told Space.com. “It joins a collection of ‘extreme velocity’ stars found over the past few decades, most of which are thousands of light-years from the Sun, whereas this source is ‘only’ 400 light-years away.”
Burgasser added that the team’s observations included an analysis of CWISE J1249+3621’s atmosphere. This showed that the possible brown dwarf also has an unusual chemical composition. The team used the information they gathered about CWISE J1249+3621’s motion and composition to speculate about its possible origins.
“This discovery opens up a new way to study brown dwarfs, which are located primarily in distant regions of the Milky Way, including its center, halo, and various globular clusters and moons,” Burgasser said. “All of these systems are too far away to study brown dwarfs directly in detail, but if they are launched at us, it’s much easier!”
What is this rogue star running from?
Brown dwarfs form just like stars — from giant clouds of gas and dust that develop into extremely dense patches called molecular clouds that collapse under their own gravity. But unlike a normal star like the Sun, brown dwarfs can’t gather enough material from the remnants of the cloud that gave them birth to reach the mass needed to generate the pressures and temperatures needed to start the hydrogen-to-helium conversion process in their cores. This is the process that defines a “main sequence” star. For this reason, brown dwarfs have been nicknamed “failed stars.”
Brown dwarfs range in mass from about four times that of Jupiter to about 80 times that of a gas giant. (For comparison, the Sun is 1,000 times more massive than Jupiter.) The mass of CWISE J1249+3621 is exciting because it places it on the hypothetical boundary between a star and a brown dwarf.
“The low mass is important because it is the lowest-mass, high-velocity ‘star’ found to date. The original hypervelocity stars found about 20 years ago were massive O stars [around 50 times as massive as the sun] and B stars [up to 16 times as massive as the sun]”It’s a possible selection bias, as these stars are rare and should be found at large distances,” Burgasser said. “Our discovery suggests that whatever process (or processes) is causing these stars to escape, it must operate at both high and low masses.”
The UC San Diego researcher explained that the team is very excited to try to answer what causes this stellar body to drift through the Milky Way.
“The star may have been ejected from the center of the Milky Way by our supermassive black hole, Sagittarius A*, a process commonly used to explain the origins of other hypervelocity stars,” Burgasser said. “The important thing is that our star may be moving toward the center, not away, but on a return journey after having been ejected earlier.”
It’s also possible, he added, that the brown dwarf is running from a “cosmic vampire.” The rogue stellar body could be part of a binary system in which a white dwarf corpse is tearing material away from itself. This nasty feeding eventually causes the white dwarf to explode in a cosmic explosion called a Type Ia supernova. This destroys the white dwarf and provides the “kick” that sends the runaway star hurtling through the Milky Way at incredible speeds.
“Another possibility is that the star was ejected from the globular cluster by dynamical interactions with black holes at the cluster’s center; recent simulations suggest that this should have happened several times over the age of the Milky Way,” Burgasser said. “Any of the above processes, given a fast enough kick, could have ejected it, or in the case of an ‘extragalactic’ star, it could simply be passing through.”
He added that at this time, the team cannot rule out the possibility that this possible brown dwarf is an intruder into our galaxy from outside the Milky Way. However, the fact that it passes through the plane of our Milky Way makes this less likely.
“The orbit is definitely the most surprising aspect of this object; it moves radially toward the center of the Milky Way and is almost perfectly plane,” Burgasser said. “Most of the high-velocity stars we see are in much more chaotic or inclined orbits. I think this is a real clue to its true origin.”
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Runaway brown dwarfs, if CWISE J1249+3621 is indeed what it is, appear to be rare, but this may be due to their cool, dim nature, which makes them difficult to detect. This means the population of runaway brown dwarfs may be much larger than current detection rates suggest.
“These types of stars are extremely rare; only a few dozen of the billions of stars studied have been found, and as mentioned, this is the first low-mass one. And this object in particular is difficult to see because it is a very cold and dim star, about 10,000 times fainter than the sun, and emits most of its light in infrared wavelengths,” Burgasser said. “It’s hard to say how common these objects are; only one has been found so far, but since this is so close, we predict there could be many more.
“This speculation is partly driven by the fact that most stars in the Milky Way are low-mass, with one-fifth being brown dwarfs, and these objects are the easiest to ‘throw around’ because they are so low-mass.”
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The team now aims to investigate the atmosphere of CWISE J1249+3621 in more detail to see if its chemical abundances reveal anything about its origin. They will also attempt to discover more of these low-mass stellar runaways, a hunt in which citizen scientists will play a key role.
“We definitely want to find more of these objects, and our citizen scientists have identified several more high-velocity candidates to pursue,” Burgasser concluded. “Citizen scientists were absolutely essential to this study! They were the ones who identified this source as an interesting target worth investigating. Without them, we would still have hundreds of thousands of faint little dots to sort through.”
The team’s research is discussed in a peer-reviewed paper on the arXiv repository.