At brown dwarfs they are not exactly stars or planets. And a new study suggests there may be more of them lurking in our galaxy than scientists previously thought. the work was published in the magazine The Astrophysical Journal Letters.
The research offers a tantalizing explanation of how a peculiar cosmic object called WISEA J153429.75-104303.3 – nicknamed “The Accident” – came to be. Accident is a brown dwarf. Although they form like stars, these objects are not massive enough to initiate nuclear fusion, the process that makes stars glow. And while brown dwarfs sometimes defy characterization, astronomers have a good understanding of their general characteristics. Or had, until they found this object.
Challenge to expectations
The Accident got its name after being discovered by pure luck. It escaped normal searches because it doesn’t look like any of the little over 2,000 brown dwarfs found in our galaxy so far.
As they age, brown dwarfs cool and their brightness at different wavelengths of light changes. It’s no different from how some metals, when heated, go from bright white to deep red as they cool. The Accident baffled scientists because it was weak in some important wavelengths, suggesting it was very cold (and old), but light in others, indicating a higher temperature.
“This object defied all our expectations,” said Davy Kirkpatrick, an astrophysicist at the Center for Infrared Processing and Analysis (Ipac) at the California Institute of Technology (Caltech) in Pasadena (USA). He and his co-authors postulate in their new study that The Accident may be 10 billion to 13 billion years old — at least twice the average age of other known brown dwarfs. This means that it would have formed when the Milky Way was much younger and had a different chemical composition. If that’s the case, there are probably many more of these ancient brown dwarfs lurking in our galactic neighborhood.
The crash was first detected by NASA’s Near-Earth Object Wide-Field Infrared Survey Explorer (Neowise) telescope, launched in 2009 under the moniker Wise and managed by NASA’s Jet Propulsion Laboratory (JPL) in southern Japan. California. Because they are relatively cool objects, brown dwarfs radiate mostly infrared light, or longer wavelengths than the human eye can see.
To figure out how The Accident could have such seemingly contradictory properties – some suggesting it’s too cold, others suggesting it’s much warmer – scientists needed more information. They then observed it at additional infrared wavelengths with a ground-based telescope at the WM Keck Observatory in Hawaii. But the brown dwarf looked so weak at those wavelengths that they couldn’t detect it, apparently confirming their suggestion that it was too cold.
Next, they decided to determine whether the dimness resulted from The Accident being further away from Earth than expected. But that was not the case, according to accurate distance measurements by the Hubble (Nasa/ESA) and Spitzer (Nasa) space telescopes. Having determined the object’s distance – about 50 light-years from Earth – the team realized that it is moving rapidly – about 800,000 kilometers per hour. In other words, it is much faster than all other known brown dwarfs located at this distance from Earth. This means that the object has probably been rotating through the galaxy for a long time, encountering massive objects that accelerate it with their gravity.
With a lot of evidence suggesting that The Accident is extremely old, researchers propose that its strange properties are not strange and that it could be a clue to its age.
When the Milky Way formed, about 13.6 billion years ago, it was composed almost entirely of hydrogen and helium. Other elements, like carbon, were formed inside stars; when the most massive stars exploded like supernovae, they spread the elements across the galaxy.
Methane, composed of hydrogen and carbon, is common in most brown dwarfs that have a temperature similar to The Accident. But The Accident’s light profile suggests that it contains very little methane. Like all molecules, methane absorbs specific wavelengths of light, so a methane-rich brown dwarf would be weak at those wavelengths. Accident, on the other hand, is bright at these wavelengths, which could indicate low levels of methane.
Thus, The Accident’s light profile could be similar to that of a very old brown dwarf that formed when the galaxy was still poor in carbon. Too little carbon in the formation means too little methane in your atmosphere today.
“It’s not a surprise to find such an old brown dwarf, but it’s a surprise to find one in our backyard,” said Federico Marocco, an astrophysicist at Ipac at Caltech who conducted the new observations using the Keck and Hubble telescopes. “We expected brown dwarfs that old to exist, but we also expected them to be incredibly rare. The chance of finding one so close to the Solar System might be a happy coincidence, or it tells us that they are more common than we thought.”
To find older brown dwarfs like The Accident — if they’re out there — researchers may have to change the way they look for these objects.
The Accident was discovered by citizen scientist Dan Caselden, who was using an online program he built to find brown dwarfs in Neowise data. The sky is full of objects that radiate infrared light; in general, these objects appear to remain fixed in the sky due to their great distance from Earth. But because brown dwarfs are so faint, they’re only visible when they’re relatively close to Earth, and that means scientists can watch them move across the sky for months or years. (Neowise maps the entire sky once every six months.)
Caselden’s program attempted to remove stationary infrared objects (such as distant stars) from Neowise’s maps and highlight moving objects that had characteristics similar to known brown dwarfs. He was looking at one of these brown dwarf candidates when he spotted another much weaker object moving quickly across the screen. It was WISEA J153429.75-104303.3, which had not been highlighted because it did not match the program profile of a brown dwarf. Caselden caught him by accident.
“This finding is telling us that there is more variety in brown dwarf compositions than we’ve seen so far,” Kirkpatrick said. “There are probably more weird (compositions) out there, and we need to think about how to look for them.”
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