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The body of a newly discovered rogue star may be a “failed star,” but it’s certainly not a velocity failure!
The potential brown dwarf is racing through our Milky Way galaxy at 1.2 million mph (1.9 million kph). That’s about 1,500 times faster than the speed of sound! Thank God, this cosmic run is heading towards the center of the Milky Way and not towards us. However, the object is traveling so fast that it may eventually escape our galaxy altogether.
The incredible speed of this newly revealed stellar body, named CWISE J1249+3621, is not the only interesting thing about the object, which is currently about 400 light years from Earth.
The stellar body has a mass that is about 8% that of the Sun, or 80 times the mass of Jupiter, which puts it right on the dividing line between a star and a group of supermassive objects often called “brown matter,” ( somewhat unfairly) labeled “failed stars.”
CWISE J1249+3621, first discovered by citizen scientists working with the Backyard Worlds: Planet 9 project, which uses data from NASA’s Wide Field Infrared Survey Explorer (WISE) to detect faint objects moving relatively close to the sun.
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After several citizen scientists pointed out the object, a team of astronomers continued using the Keck I Telescope, one of two twin 10-meter telescopes located on the dormant Maunakea volcano, in Hawaii.
“We discovered a very low-mass object, just at the stellar/mass boundary, with a very large velocity, moving fast enough that it could be unbound to the Milky Way galaxy,” lead study team Adam Burgasser, from the University of California San Diego, said Space.com. “It joins a collection of ‘hypervelocity’ stars found over the past few decades, most of them thousands of light-years from the sun, but this source is only 400 light-years away.”
Burgasser also said analysis of the atmosphere of CWISE J1249+3621 was among the team’s observations. This showed that the possible brown dwarf also has an unusual chemical composition. The team aimed to use the information they gathered about the motion and composition of CWISE J1249+3621 to speculate on its possible origin.
“This discovery opens a fundamentally new way to study brown dwarfs in remote regions of the Milky Way, including its center, halo and globular clusters and various satellites,” Burgasser said. “All these systems are too far away to study brown dwarfs directly, but if they are thrown to us, it is much easier!”
Where is this rogue star running from?
Brown dwarfs form just as stars do — from huge clouds of gas and dust, called molecular clouds, that develop dense patches that collapse under their own gravity. However, unlike regular stars such as the Sun, brown dwarfs do not collect enough material from the remnants of the cloud that gave rise to them to reach the mass needed to generate the pressures and temperatures in their cores that initiate fusion of hydrogen to helium. This is the process that defines a “main sequence” star. Thus, the moniker “failed star” was thrust upon a brown dwarf.
Brown dwarfs have masses that range from about four times that of Jupiter to about 80 times that of the 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 right at the hypothetical boundary between a star and a brown dwarf.
“The low mass is significant because it is the lowest mass and highest velocity ‘star’ found to date. The original hypervelocity stars discovered about 20 years ago were O giant stars. [around 50 times as massive as the sun] and B stars [up to 16 times as massive as the sun]a selection bias is likely because these stars are rare and would have to be found at great distances,” Burgasser said. “Our discovery shows that whatever process (or processes) put these stars away must operate at high masses and both low. “
The UC San Diego researcher explained that the team is really excited to try to answer what sent this star body careening through the Milky Way.
“Our supermassive black hole, Sagittarius A*, could have kicked the star out of the center of the Milky Way, a process commonly used to explain the origin of other hypervelocity stars,” Burgasser said. “In particular, our star is moving into the center, not away, but it could be on a return trip after being evicted before.”
He also said that it is also possible that the brown dwarf is on the run from a “cosmic vampire.” The rogue stellar body may have been part of a binary system that had a white dwarf star body that was ripping material from it. This terrible feeding eventually causes the white dwarf to erupt in a cosmic explosion known as a Type Ia supernova. This would destroy the white dwarf and provide the “kick” that sent this runaway racing through the Milky Way at incredible speed.
“Another possibility is that the star was launched from a globular cluster through dynamical interactions with black holes at the center of the cluster; recent simulations show that this should happen repeatedly over the age of the Milky Way,” Burgasser said. “Any of the above processes, given a quick enough kick, could send it out, or in the case of an ‘extragalactic’ star, it happens to be passing through.”
He added that, for now, the team cannot rule out that this possible brown dwarf could be an intruder in our galaxy that came from outside the Milky Way. But because it is passing through the plane of our Milky Way it is less likely.
“The orbit is definitely the most surprising aspect of this object; it’s moving radially in and out of the center of the Milky Way and almost perfectly in the plane,” Burgasser said. “Most of the high-velocity stars we see orbit much more chaotic or tilted. I think this is a real clue to their true origin.”
Related: Brown dwarfs: Coolest stars or hottest planets?
Runaway brown dwarfs, if that is indeed what CWISE J1249+3621 is, appear to be rare, but this could be due to their cool and faint nature, making it is difficult to detect them. This means that the population of rogue browns could be much larger than current detection rates indicate.
“These types of stars are extremely rare; only a few dozen have been found out of the billions of stars that have been examined, and, as noted, this is the first low-mass one. And this object in particular is difficult to see how beautiful it is. A cool and dim star, almost 10,000 times smaller than the sun and emitting most of its light at infrared wavelengths,” Burgasser said. “It’s hard to say how common these bodies are, with only one found so far, but since this is so close, we’re speculating that there could be many more.
“That speculation is informed in part by the fact that the majority of stars in the Milky Way are low-mass, and about one in five are brown dwarfs, and that these objects are the easiest to ‘throw around’ because that they have such a low mass. “
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The team now plans to follow up the investigation of CWISE J1249+3621’s atmosphere in more detail to see if its chemical abundances reveal something about its origin. They will also try to find more of these large, low-mass escape routes, a hunt in which citizen scientists will play a key role.
“We definitely want to find more of these things, and our citizen scientists have identified more high-speed candidates to follow up on,” Burgasser said. “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 have hundreds of thousands of small weak points to sort out.”
The team’s research is discussed in a preliminary peer-reviewed paper that can be seen on the arXiv repository site.
