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Methane Brown Dwarfs: Two's Company, Seven's a Crowd.

A half decade ago brown dwarfs were mere conjecture. The term was coined to describe objects that form like stars but have too little mass to sustain nuclear fusion in their cores. Theory suggests that gas balls with less than 8 percent of our Sun's mass should qualify.

In the last few years some compelling brown-dwarf candidates surfaced in spectroscopic surveys; the atmospheres of those objects contain lithium, which is eventually consumed by nuclear reactions in bona fide stars. But skeptics awaited a not-quite-star with the spectral signature of methane (CH4), a compound that cannot survive at temperatures much above 1,200[degree sign] Kelvin. (True stars, by contrast, have surface temperatures of 2,500[degree sign] K or more.) Methane is conspicuous in the near-infrared spectrum of Jupiter, and its presence would be proof of a failed nuclear furnace.

The wait ended in late 1995 with the discovery of a methane brown dwarf in orbit about a nearby star called Gliese 229 (S&T: April 1996, page 24). For nearly four years Gliese 229B, as the substellar companion is known, stood alone. But no longer. Two ambitious sky surveys now under way - the Sloan Digital Sky Survey and the Two Micron All Sky Survey - have turned up a half dozen more methane dwarfs. The objects were culled from millions of others on the basis of their unusual colors. The presence of methane in their atmospheres was then confirmed with near- infrared spectra, some of which were taken only days before the surveys' discoveries were announced on May 31st.

Because Gliese 229B orbits a star, it may have formed when a rotating gas cloud fragmented. By contrast, the newfound methane dwarfs are free- floating, so they probably formed in isolation. An accurate census of such solitary lightweights may provide unique insights into the physics of star formation. The surveys, when complete, should also be able to say whether brown dwarfs are a significant constituent of our Milky Way's dark-matter halo. According to Gliese 229B codiscoverer David A. Golimowski (Johns Hopkins University), the new findings provisionally suggest that "free-floating brown dwarfs exist in significant numbers," possibly as many as one or two for every bona fide star in the solar neighborhood. However, because of their low masses, even this many would fall far short of accounting for all of our galaxy's mystery matter.

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Title Annotation:ongoing surveys should show whether brown dwarfs are significant
Publication:Sky & Telescope
Article Type:Brief Article
Date:Sep 1, 1999
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