You have to have a feeling for brown dwarfs. Not only has their inability to ignite like normal stars earn them an unfortunate nickname: failed stars, but new findings from the Hubble Space Telescope have revealed that they can't even sustain a relationship.
Brown dwarfs are celestial bodies that form when giant clouds of gas and dust, called molecular clouds, develop overly dense patches that collapse. Unlike your regular old star, however, a brown dwarf can't collect enough material from the remnants of that cloud to pile up enough mass and trigger the fusion of hydrogen into helium in its core. A brown dwarf fuses a number of elements, but mainly the fusion of hydrogen with helium defines a 'main sequence star' - hence the name 'failed star'.
Like many stars, brown dwarfs are believed to often be born in binary pairs. However, there is a large gap in the literature on this topic. Although about 75% of the universe's massive stars are known to have a companion star, and about 50% of Sun-sized stars are observed in such binary configurations, the number of brown dwarf binaries detected is close to zero. Why would that be?
Well, Hubble observations may have an answer. The older the brown dwarf is, it seems, the less likely it is to have a companion. This implies that the gravitationally binding binary pairs of brown dwarfs could be so weak that the two bodies drift apart in a few hundred million years. What could cause them to drift apart? Perhaps the much stronger gravity of other passing stars.
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"Our study confirms that widely separated companions are extremely rare among the least massive and coldest isolated brown dwarfs, even though binary brown dwarfs are observed at younger ages," said Clémence Fontanive, lead author of the study and scientist at the Trottier Institute for Research. Exoplanets, said in a statement. 'This suggests that such systems do not survive over time. When they are young, they are part of a molecular cloud, and as they grow older, the cloud spreads. When that happens, things start moving and stars pass by. mixed together."
"Because brown dwarfs are so light," Fontanive added, "the gravitational pull that connects wide binaries is very weak, and evading stars can easily tear these binaries apart."
In a sense (apologies in advance), but this makes brown dwarfs seem a bit like the cosmic equivalent of the derivative boyfriend meme. You know that one.
Growing apart over the years
Hubble allows astronomers to detect binary stars with components as far apart as 300 million miles. This corresponds to about three times the distance between the Earth and the Sun, which is quite small in cosmic terms.
The team first selected a sample of brown dwarfs previously spotted by NASA's Wide-Field Infrared Survey Explorer (WISE). The researchers then shrunk this sample until they obtained some of the coldest and faintest failed stars in the solar system's relative vicinity. Because brown dwarfs cannot sustain nuclear fusion in their cores, they exhibit cool temperatures equivalent to a few hundred degrees warmer than Jupiter, which has a temperature of about minus 166 degrees Fahrenheit (minus 110 degrees Celsius). Such cool temperatures allow them to live for quite a long time.
To hunt for the brown dwarfs' coldest companions, the team relied on these icy, failed stars to have condensed water in their atmospheres. Fontanive and colleagues used two different near-infrared filters to study this water content. One filter showed the cold brown dwarfs clearly, while the other covered specific wavelengths that made the failing stars appear very faint due to water absorption in their atmospheres.
Fontanive and colleagues conducted a similar study with Hubble several years ago, focusing on extremely young brown dwarfs. Some of these young, failed stars had binary companions, which had confirmed that they were brown dwarfs could be exist in binary stars, and that the mechanisms by which birth stars can create low-mass binary stars - even though these cases are extremely rare.
Scientists theorized that the lack of observed binary brown dwarfs indicated that they struggle to remain gravitationally bound for long periods. This new Hubble discovery adds further support to this concept.
"Most stars have friends - whether that's a binary companion or exoplanets," Beth Biller, team member and scientist at the University of Edinburgh, said in the statement. "This research really shows that the same is not true for brown dwarfs. After a short period early in their lives, most brown dwarfs remain solitary for the rest of their very long existence."
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Fontanive added that the motivation for this research was to determine how low the mass of stellar objects needs to be to maintain the trends we see in multi-star systems.
"Our Hubble study provides direct evidence that these binary stars that we observe when they are young are unlikely to survive into old age; they are likely to become disrupted," he concluded. "This is the best observational evidence yet that brown dwarf pairs are drifting apart over time. We couldn't have done this kind of research and confirmed previous models without Hubble's sharp vision and sensitivity."
The team's research has been published in the journal Monthly Notices of the Royal Astronomical Society.
