The discovery of a group of brown dwarfs gives further credence to the idea that they form through disk fragmentation.

Despite the profusion of brown dwarfs, discovered in the local solar neighborhood over the past decade, their origins remain under debate. Systems containing more than one of these failed stars can provide clues as to how they formed. In a paper accepted for publication to the Astrophysical Journal in December of 2010, Jacqueline Faherty of the American Museum of Natural History (AMNH) reported the discovery of a low mass, comoving system containing at least two brown dwarfs and two low mass stars. This system provides observational evidence for the model predictions of brown dwarfs forming through the fragmentation of a massive disk surrounding newborn stars.

Brown dwarfs share the dubious honor of falling somewhere between stars and planets on the stellar scale. Their masses tend to be fifteen or more times that of Jupiter, while their radii remain similar. As large as they are, they still lack the necessary mass to sustain hydrogen fusion, a process necessary for all stars. Their in-between status has led to three different theories regarding their origin.

In 1999, an object known as 2MASS J0850359+105716 (hereafter 2MASS J0850+1057) was identified as a single brown dwarf in the Two Micro All Sky Survey (2MASS). In 2001, it was resolved into a binary system, with the secondary also a low-mass brown dwarf. Nine years later, a paper led by Adam Burgasser of the University of California San Diego speculated that the primary may itself be a closely separated unresolved pair, which would categorize the system as one of only a few brown dwarf triples.

At the same time, Faherty, along with.Burgasser, John J. Bochanski (Penn State University), Dagny L. Looper (University of Hawai’i), Andrew A. West (Boston University), and Nicole S. Van Der Bliek (National Optical Observatory, Chile) found that the brown dwarf system was in a very wide orbit (7700 times the orbit of the Earth-Sun system) around a higher mass fast moving star, NLTT 20346. Moreover, in analyzing images of NLTT 20346, Faherty and her colleagues realized that it was also a close binary. Further images were taken to confirm the once-believed single star was, in fact, two low-mass stars separated by approximately 50-80 AU.

“It was one of those one-off objects that looks boring as an individual object,” Faherty explained. “It wasn’t until it was associated with a brown dwarf potential triple system that it becomes interesting.”

Ultimately, the commoving system contains at least four, and possibly as many as five objects, making it a rarity. “It’s kind of like finding another type of solar system out there, because it’s not what we usually think of when we imagine objects orbiting around stars.”

The characteristics of this system can be reproduced by simulations of brown dwarf formation through the gravitational fragmentation of massive extended disks around newborn stars. The process is very similar to the way planets form around stars, However, in this scenario, the brown dwarfs form much further from their parent star. Such systems have low binding energies based on the low mass and large distance between major components. In fact, the binding energy of the NLTT 20346/2MASS J0850+1057 system is among the lowest for any known multiple.

In simulations of brown dwarf formation via disk fragmentation, astronomers working at Cardiff University produced a direct analong to the NLTT 20346/2MASS J0850+1057 system. Faherty pointed out that it is very rare for theory and observation to mesh so neatly. Not so with this system. Its existence and characteristics point towards gravitational fragmentation. “It gives a little more clout to this theory of brown dwarf system formation.”

Identification of a Wide, Low-Mass Multiple System Containing the Brown Dwarf 2MASS J0850359+105716