You have got to feel for brown dwarfs. Not only has their failure to ignite like normal stars earned them an unfortunate nickname — failed stars — but new findings from the Hubble Space Telescope have revealed they can't even hold a relationship together.
Brown dwarfs are celestial objects 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 quite gather enough material from the remains of that cloud to pile on enough mass and kickstart the fusion of hydrogen to helium in its core. A brown dwarf does fuse some elements, but the fusion of hydrogen to helium in particular is what defines a "main sequence" star — hence the "failed star" moniker.
Like many stars, brown dwarfs are believed to often be born in binary pairs. However, there's a major gap in the literature on this subject. While around 75% of massive stars across the universe are known to have a companion star, and around 50% of stars the size of the sun are seen in such binary configurations, the number of detected brown dwarf binaries is nearly zero. Why would that be?
Well, Hubble observations may have an answer. The older the brown dwarf is, it would appear, the less likely it is to have a companion. This implies that the gravity binding binary pairs of brown dwarfs could be so weak that the two bodies drift apart over a few hundred million years. What might make them drift apart? Perhaps the much stronger gravitational pull of any other stars passing by.
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"Our survey confirms that widely separated companions are extremely rare among the lowest-mass and coldest isolated brown dwarfs, even though binary brown dwarfs are observed at younger ages," Clémence Fontanive, research lead author and a scientist at the Trottier Institute for Research on Exoplanets, said in a statement. "This suggests that such systems do not survive over time. When they're young, they're part of a molecular cloud, and then, as they age, the cloud disperses. As that happens, things start moving around, and stars pass by each other."
"Because brown dwarfs are so light," Fontanive added, "the gravitational hold tying wide binary pairs is very weak, and bypassing stars can easily tear these binaries apart."
In a way (apologies in advance) but this makes brown dwarfs a bit like the cosmic equivalent of the distracted boyfriend meme. You know the one.
Growing apart with age
Hubble allows astronomers to detect binaries with components that sit as close as 298 million miles (480 million kilometers) from one another. This is equivalent to around three times the distance between Earth and the sun, which is quite small in cosmic terms.
The team first selected a sample of brown dwarfs that had been spotted previously by NASA’s Wide-Field Infrared Survey Explorer (WISE). The researchers then narrowed this sample down until they obtained some of the coldest and lowest mass failed stars in the relative vicinity of the solar system. Because brown dwarfs can't sustain nuclear fusion in their cores, they exhibit cool temperatures equivalent to a few hundred degrees hotter than Jupiter, which has a temperature of around minus 166 degrees Fahrenheit (minus 110 degrees Celsius). Such cool temperatures ensure they can live for a rather long time.
To hunt the coldest brown dwarf companions, the team relied on the fact that these frigid failed stars would have condensed water in their atmospheres. Fontanive and colleagues used two different near-infrared filters to study this water content. One filter displayed the cold brown dwarfs brightly, while the other covered specific wavelengths that made the failed stars appear very faint as a result of water absorption in their atmospheres.
Fontanive and colleagues actually conducted a similar study with Hubble several years ago, which focused on extremely young brown dwarfs. Some of these infant failed stars had binary companions, which had confirmed that brown dwarfs could exist in binaries, and that the mechanisms that birth stars can create low-mass binaries — even if these cases are vanishingly rare.
Scientists theorized that the lack of observed binary brown dwarfs suggested that they struggle to stay gravitationally bound over long periods of time. This new Hubble find adds further support to this concept.
"Most stars have friends — whether that is a binary companion or exoplanets," Beth Biller, team member and a scientist at the University of Edinburgh, said in the statement. "This survey really demonstrates that the same is not true for brown dwarfs. After a brief period early in their lifespans, most brown dwarfs remain single for the rest of their very long existence."
Fontanive added that the motivation for this study was to determine how low in mass stellar objects have to be to maintain trends seen in multi-star systems.
"Our Hubble survey offers direct evidence that these binaries that we observe when they're young are unlikely to survive to old ages, they're likely going to get disrupted," he concluded. "This is the best observational evidence to date that brown dwarf pairs drift apart over time. We could not have done this kind of survey and confirmed earlier models without Hubble’s sharp vision and sensitivity."
The team's research is published in the journal Monthly Notices of the Royal Astronomical Society.
