Brown-Dwarf Origins

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Science  06 Jul 2012:
Vol. 337, Issue 6090, pp. 43-44
DOI: 10.1126/science.1224342

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Brown dwarfs occupy an intermediate mass range, with too low a mass to initiate hydrogen fusion and become a star and too much mass to be called a planet (see the figure). Their existence was inferred theoretically nearly 50 years ago (1, 2), when it was realized that objects of mass less than about 0.08 solar mass (M) would initially contract due to gravity but then settle into a stable equilibrium due to quantum mechanical effects, before they became hot and dense enough to initiate hydrogen fusion. The first observational detections of a brown dwarf came in 1995 (3, 4), and there are now several hundred identified from infrared surveys ( 5, 6). The origin of brown dwarfs is an important astrophysical mystery because its solution can also determine how a broad range of objects, from high- to low-mass stars to planets, are formed and gain their masses. On page 69 of this issue, André et al. (7) report high-resolution observations with the Institut de Radioastronomie Millimétrique Plateau de Bure Interferometer of the cloud core Oph B-11 in the Ophiuchus molecular cloud. Its mass is estimated to lie in the range ∼0.02 to 0.03 M, so that even a 100% efficient collapse into a central object would yield a brown dwarf.