You should feel for the brown dwarfs. Not only has their failure to ignite like normal stars earned them the unfortunate nickname (fail stars), but new findings from the Hubble Space Telescope have revealed that they can’t even maintain a relationship together.
Brown dwarfs are celestial bodies formed when giant clouds of gas and dust, called molecular clouds, collapse to form extremely dense patches. However, unlike your ordinary old star, a brown dwarf cannot accumulate enough material from the remnants of that cloud to accumulate sufficient mass and initiate the fusion of hydrogen into helium in its core. The brown dwarf fuses some elements, but specifically the fusing of hydrogen into helium is what defines a “main sequence” star; hence the nickname “failed star”.
Brown dwarfs, like many stars, are believed to often be born in binary pairs. However, there is a big gap in the literature on this subject. While approximately 75% of massive stars in the universe are known to have a companion star, and approximately 50% of Sun-sized stars occur in such binary configurations, the number of brown dwarf binaries detected is almost zero. Why might this happen?
Hubble observations may have an answer. It seems that the older the brown dwarf, the less likely it is to have a companion. This means that the gravitational binding binaries of brown dwarfs could be so weak that the two objects could drift apart within a few hundred million years. What could be driving them away from each other? Perhaps the much stronger gravitational pull of other stars passing by.
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“Our research confirms that, although binary brown dwarfs have been observed at younger ages, widely separated companions are extremely rare among the lowest-mass and coldest isolated brown dwarfs,” said Clémence Fontanive, lead author of the study and a scientist at the Trottier Research Institute. The outer planets said in a statement. “This suggests that such systems cannot survive over time. When they are young, they become part of a molecular cloud, and as they age, the cloud disperses. As this happens, objects begin to move and stars move past each other.”
Fontanive added: “Because brown dwarfs are so light, the gravitational effect binding large binaries together is very weak, and skipping stars can easily tear these binaries apart.”
In a way (sorry in advance) but it kind of makes brown dwarfs look like the cosmic equivalent of the distracted boyfriend meme. You know that.
It grows apart as you get older
Hubble allows astronomers to detect binaries with components as close as 298 million miles (480 million kilometers) apart. This is equivalent to about three times the distance between the Earth and the Sun, which is quite small in cosmic terms.
The team first selected a brown dwarf sample previously detected by NASA’s Wide Field Infrared Survey Explorer (WISE). The researchers then narrowed this sample until they had some of the coldest and lowest-mass failed stars in the relative vicinity of the solar system. Because brown dwarfs cannot sustain nuclear fusion in their cores, they exhibit cold temperatures, several hundred degrees warmer than Jupiter, which has a temperature of about minus 166 degrees Fahrenheit (minus 110 degrees Celsius). Such cold temperatures allow them to live for quite a long time.
To hunt for the coolest brown dwarf companions, the team relied on the fact that these cold failed stars may have condensed water in their atmospheres. Fontanive and his colleagues used two different near-infrared filters to examine this water content. One filter imaged cool brown dwarfs brightly, while the other covered certain wavelengths that made failed stars appear very dim as a result of water absorption in their atmospheres.
Fontanive and his colleagues actually conducted a similar study with Hubble several years ago, focusing on extremely young brown dwarfs. Some of these failed stars had binary companions, confirming the existence of brown dwarfs. can that it exists as binaries, and that there are mechanisms by which natal stars can form low-mass binaries—even if these cases are vanishingly rare.
Scientists theorized that the lack of observations of binary brown dwarfs suggests that they struggle to remain gravitationally bound for long periods of time. This new Hubble finding provides further support for this concept.
“Most stars have companions, whether binary companions or exoplanets,” said team member and scientist Beth Biller from the University of Edinburgh. “This research actually shows that the same is not true for brown dwarfs. After a brief period early in life, most brown dwarfs remain single for the remainder of their very long existence.”
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Fontanive added that the motivation for this study was to determine how low the masses of stellar objects need to be to preserve the trends seen in multiple star systems.
“Our Hubble study provides direct evidence that these binaries we observe when young are unlikely to survive to older ages, likely to decay,” he concluded. “This is the best observational evidence to date that pairs of brown dwarfs are moving away from each other over time. Without Hubble’s sharp vision and sensitivity, we would not be able to do this kind of research and confirm previous models.”
The team’s research was published in the journal Monthly Notices of the Royal Astronomical Society.