Accidental discovery hints at hidden population of cosmic objects

Brown dwarfs aren’t quite stars and aren’t quite planets, and a new study suggests there may be more of them in our galaxy than scientists previously thought.

A new study offers a tantalizing explanation as to how a particular cosmic object called WISEA J153429.75-104303.3 – nicknamed “The Crash” – was born. Accident is a brown dwarf. Although they form like stars, these objects do not have enough mass to trigger 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 they did, until they found this one.

The Accident got its name after it was discovered by sheer luck. It has escaped normal searches because it does not look like any of the 2,000 or so brown dwarfs that have been found in our galaxy so far.

As brown dwarfs age, they cool down and their brightness in different wavelengths of light changes. It is reminiscent of how some metals, when heated, change from bright white to dark red as they cool. The crash confused scientists because it was weak in some key wavelengths, suggesting it was very cold (and old), but glowing in others, indicating a higher temperature.

“This object has defied all of our expectations,” said Davy Kirkpatrick, astrophysicist at IPAC at Caltech in Pasadena, Calif. He and his co-authors are applying in their new study, published in Letters from the astrophysical journal, that the Accident could be between 10 and 13 billion years old, or at least twice the median age of other known brown dwarfs. This means that it would have formed when our galaxy was much younger and had a different chemical makeup. If so, there are probably many more of these ancient brown dwarfs lurking in our galactic quarter.

A particular profile

The accident was first spotted by NASA’s Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE), launched in 2009 as WISE and managed by NASA’s Jet Propulsion Laboratory in California from South. Because brown dwarfs are relatively cool objects, they mainly emit infrared light, or wavelengths longer than what the human eye can see.

To understand how the Crash could have seemingly contradictory properties – some suggesting it is very cold, others much hotter – scientists needed more information. So they observed it in additional infrared wavelengths with a ground telescope at the WM Keck Observatory in Hawaii. But the brown dwarf appeared so weak in those wavelengths that they couldn’t detect it at all, apparently confirming their suggestion that it was very cold.

They then set out to determine whether the penumbra was a result of the crash being farther from Earth than expected. But it was not, according to precise distance measurements by NASA’s Hubble and Spitzer space telescopes. After determining the object’s distance – about 50 light years from Earth – the team realized that it was moving fast – about half a million miles per hour (800,000 km / h). It’s much faster than any other brown dwarf known to be this distance from Earth, which means it has probably been circling the galaxy for a long time, encountering massive objects that speed it up with their gravity.

With a slew of evidence to suggest that the Crash is extremely old, researchers suggest that its strange properties are not at all strange and that they may be a clue to its age.

When the Milky Way was formed about 13.6 billion years ago, it was almost entirely made up of hydrogen and helium. Other elements, like carbon, have formed inside stars; when the most massive stars exploded into supernovae, they scattered the elements throughout the galaxy.

Methane, composed of hydrogen and carbon, is common in most brown dwarfs who have a temperature similar to that of 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. The crash, on the other hand, is bright at those wavelengths, which could indicate low levels of methane.

Thus, the light profile of The Accident could correspond to that of a very old brown dwarf that formed when the galaxy was still low in carbon; very little carbon in formation means very little methane in its atmosphere today.

“It’s no surprise to find such an old brown dwarf, but it’s a surprise to find one in our garden,” said Federico Marocco, astrophysicist at IPAC at Caltech, who led the news. observations using the Keck and Hubble telescopes. “We expected such ancient brown dwarfs to exist, but we also expected them to be incredibly rare. The luck of finding one so close to the solar system could be a happy coincidence, or so it tells us. that they are more common than we thought. “

A happy coincidence

To find older brown dwarfs like The Crash, if they’re there, researchers may need to change the way they search for these items.

The crash was discovered by citizen scientist Dan Caselden, who was using an online program he designed to find brown dwarfs in NEOWISE data. The sky is full of objects that emit infrared light; on the whole, 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, which means scientists can watch them move across the sky for months or years. (NEOWISE maps the entire sky about once every six months.)

Caselden’s program attempted to remove stationary infrared objects (like distant stars) from NEOWISE maps and highlight moving objects that exhibited similar characteristics to known brown dwarfs. He was looking at one of those brown dwarf candidates when he spotted another much fainter object moving rapidly across the screen. This would be WISEA J153429.75-104303.3, which had not been highlighted because it did not match the program profile of a brown dwarf. Caselden caught it by accident.

“This finding tells us that there is more variety in brown dwarf compositions than we’ve seen so far,” Kirkpatrick said. “There are probably more weird ones out there, and we need to think about how to look for them.”

Launched in 2009, the WISE spacecraft was placed into hibernation in 2011 after completing its main mission. In September 2013, NASA reactivated the spacecraft with the primary purpose of searching for near-Earth objects, or NEOs, and the mission and spacecraft were renamed NEOWISE. JPL, a division of Caltech, managed and operated WISE for NASA’s Directorate of Science Missions (SMD). The mission was competitively selected as part of NASA’s Explorers program run by the agency’s Goddard Space Flight Center in Greenbelt, Maryland. NEOWISE is a project of JPL, a division of Caltech, and the University of Arizona, supported by NASA’s Planetary Defense Coordination Office.