© Ingo Thies, Pavel Kroupai and Simon P. Goodwin et alA snapshot of a circumstellar disc around a Sun-type star being perturbed by a close star-star encounter.
Brown dwarfs are balls of gas whose mass is generally dozens of times that of Jupiter. Like stars, brown dwarfs are capable of fusing hydrogen atoms, but they are too lightweight to sustain the process.
The origin of these failed stars is a mystery. Brown dwarfs might form like their larger cousins, collapsing directly from turbulent gas clouds, or they might form in a similar way to planets, condensing out of the discs of gas girdling young stars.
Simulations had shown that instabilities in the disc of gas around an isolated young star can trigger the formation of brown dwarfs. Now Ingo Thies and Pavel Kroupa of the University of Bonn in Germany, and colleagues, have shown that the process can also take place in a more commonplace scenario, involving a crowded cluster of newborn stars.
In new simulations, the team found that young stars that zoom close enough to their siblings can destabilise the surrounding gas discs, allowing denser areas to collapse rapidly and form brown dwarf-sized objects.
Gas pulled away in such encounters might also form planets with a wide range of orbital inclinations. This could explain the origin of some of the exoplanets that have been detected with orbits significantly tilted with respect to their star's equatorial plane, Thies says.
It is not clear how often brown dwarfs might be created in such stellar hit-and-runs, says Mark Krumholz of the University of California, Santa Cruz. "But this is a nice idea in that it does make some testable predictions." Since there is a higher chance of such close encounters in dense star clusters, we might expect to detect more brown dwarfs there, he says.
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