Brown dwarfs are chemically similar to stars, but are not massive enough to trigger nuclear fusion in their cores. However, the similarity in composition begs the question: do they form the same way as stars? A new study published in Science argues that least some brown dwarfs form from hydrogen cores, similar to protostars, but much less massive. This means that brown dwarfs and stars may fall along the same continuum in terms of how they begin, helping to clarify formation models and providing hints about the population of brown dwarfs in the galaxy.
Brown dwarfs are astronomical objects lying between planets and stars in terms of mass and status. Like stars, they are comprised mostly of hydrogen and helium, but they have insufficient mass to shine via nuclear fusion. However, unlike planets, they are massive enough that their cores are sustained by quantum degeneracy (where the Pauli exclusion principle prevents electrons from occupying the same space). Brown dwarf masses range from about 15 to 75 times Jupiter's mass; that upper limit is roughly 7.5 percent of the Sun's mass.
Philippe André, Derek Ward-Thompson, and Jane Greaves observed a proto-brown dwarf candidate using the James Clerk Maxwell Telescope (JCMT) in Hawaii and the Plateau de Bure Interferometer (PdBI) in France, both of which operate in the submillimeter portion of the spectrum. They measured the candidate proto-brown dwarf's mass to lie between 1.5 and two percent of the Sun's mass, based on a spectral analysis. More massive bodies have fatter spectral lines due to greater motion of molecules, while less massive objects exhibit very narrow lines. The researchers determined the object was firmly in the brown dwarf mass range, but lacked the telltale features of a mature brown dwarf (such as the specific infrared emissions expected).
No stars or protostars were found in the same region. That means the object probably formed on its own, instead of as part of a larger cluster of bodies. This is evidence against the other major formation model, in which brown dwarfs form alongside stars, but in some cases are flung out of the system before they can accrete enough mass to become stars. An isolated proto-brown dwarf, on the other hand, likely formed the same way stars do (via gravitational collapse of a cloud of gas). While a single proto-brown dwarf is insufficient to cement a starlike formation model as the sole mechanism for origins, it demonstrates some brown dwarfs may form that way.