Brown Dwarf Pairs Drift Apart in Old Age

The only thing worse than drifting through space for an eternity is doing it alone. Observations with the Hubble Space Telescope show that brown dwarfs that once had companions suffer that fate. Binary brown dwarfs that were once bound to each other tend to drift apart as time passes.

Brown dwarfs are one of Nature’s genre-busters. They refuse to be pigeonholed into our definitions. They’re neither stars nor planets and are sometimes referred to as failed stars. They gathered too much mass to be called planets but not enough to be called stars. They live in a kind of twilight zone, where they go about their business fusing only deuterium. This fusion is enough to emit some light and warmth but nothing that rivals an actual main sequence star.

Brown dwarfs are not necessarily brown in colour. Their name comes from their size. They’re in between white dwarf stars and “dark” planets, if that makes sense. Brown dwarfs fade over time as they deplete their deuterium. The warmest ones are red or orange, and the cooler ones are magenta or even black to our eyes. Astronomers think brown dwarfs will cool down forever.

Most stars are in binary pairs, and brown dwarfs are no exception. Up to 85% of stars in the Milky Way are in binary pairs, according to some research. But the Hubble shows that when it comes to brown dwarfs, divorce is more common than in Hollywood.

In a survey of stars in our solar neighbourhood, the HST didn’t find any binary brown dwarfs with widely separated companions. That implies that brown dwarfs can’t maintain their binary relationships, probably because they’re simply not massive enough.

“This is the best observational evidence to date that brown dwarf pairs drift apart over time,” said Clémence Fontanive, the lead author of a new paper. “We could not have done this kind of survey and confirmed earlier models without Hubble’s sharp vision and sensitivity.”

The new paper is in the Monthly Notices of the Royal Astronomical Society. Its title is “An HST survey of 33 T8 to Y1 brown dwarfs: NIR photometry and multiplicity of the coldest isolated objects.” The lead author is Clémence Fontanive from the Trottier Institute for Research on Exoplanets, Université de Montréal, Canada. Brown dwarfs occupy spectral types M, L, T, and Y, and the numbers in the title are sub-types.

“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. This suggests that such systems do not survive over time,” said lead author Fontanive.

The researchers worked with a set of 33 nearby ultracool brown dwarfs, a sample large enough to be statistically significant. The survey was designed to be deeply sensitive to low-mass objects that could be companions. Though the survey unearthed some potential companions for some of the brown dwarfs, further analysis showed they’re background objects.

The fact that they detected no binary companions allowed the researchers to “place stringent upper limits on the occurrence of binary companions,” according to the paper. But the lack of detection also means they can’t place any constraints or limits on the binary orbital separation or mass ratio distributions of this population.

This survey only examined older, dimmer brown dwarfs. Younger brown dwarfs can still have their binary partners. Studies of younger brown dwarfs show that around eight percent of them have binary partners. In fact, the younger the brown dwarf, the more likely it is to have a binary partner. “These findings marginally confirm the idea that the decrease in binary frequencies with later type observed across the stellar and substellar regimes for the field population might continue throughout the substellar mass range down to the very lowest masses, as illustrated in Fig. 12,” the authors explain.

In a press release, lead author Fontanive explained why brown dwarfs lose their binary partners over time.

“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. 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, the gravitational hold tying wide binary pairs is very weak, and bypassing stars can easily tear these binaries apart,” said Fontanive.

The authors point out that there’s an inevitable weakness in their results. Since brown dwarfs are so small and dim, the usual methods of detecting companions don’t work. Astronomers rely on the transit method and the radial velocity method to detect companion objects, whether planets orbiting stars or other objects in relationships with one another.

But their inherent dimness makes detecting transits very difficult. Their inherent low masses likewise make the radial velocity ineffective. That leaves them with the direct optical detection method the researchers in this study relied on.

There could be a better way.

“Astrometry might provide a more viable alternative approach to search for companions to faint brown dwarfs, although very little work has been carried out on this side, and no systems have been reported this way so far,” the authors write in the conclusion.

When it comes to astrometry, the ESA’s Gaia spacecraft is the standard-bearer. It has the power to detect Jupiter-mass companions when they’re orbiting main sequence stars, but detecting binary brown dwarfs is still difficult, even for Gaia. Gaia has detected many brown dwarfs, but for now, it’s up to direct imaging to detect brown dwarf binary pairs. In this study, direct imaging found no widely separated binary companions despite the HST’s effectiveness.

“With an excellent sensitivity and completeness to companions on wide orbital separations, our survey robustly confirms that wide companions are extremely rare in the Galactic field around the lowest mass systems,” the authors write. Any companions would need to be inside the 1 to 5 AU limit of this work.

“Our results, with no detection of wide companions out of 33 observed objects, reinforce the idea that the widely separated binaries with very low-mass primaries identified in young associations have no counterparts among isolated objects in the Solar neighbourhood,” the authors conclude.