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Brown Dwarfs: From Mythical to Ubiquitous

Astrophysical objects below the stellar mass limit but well above the mass of Jupiter eluded discovery for nearly three decades after Kumar first proposed their existence, and for two decades after Tarter proposed the name "brown dwarfs." The first unambiguous discoveries of planetary (51 Peg B) and brown dwarf (Gliese 229B) companions occurred about three years ago. Yet while extrasolar planets are now being discovered at a breathtaking rate, brown dwarf companions to ordinary stars are apparently rare; likewise imaging surveys show that GL229B is still unique as a distant companion to a low mass star. On the other hand, the deep imaging studies of the Pleiades and several imbedded young clusters show that the mass function (ie. of single objects) extends in substantial numbers down to at least 40 Jupiter masses. The high mass / stellar density Orion Nebula Cluster may have relatively fewer low mass objects. In the field of the solar neighborhood, the infrared sky surveys DENIS and especially 2MASS show that brown dwarfs, certified by the lithium test, exist in significant numbers. These appear to include most of the newly-defined spectroscopic class of L dwarfs. These objects are cooler than, and with different atomic and molecular absorption features than the late M dwarfs. If the first 1% of sky analyzed is not atypical, over a thousand L dwarfs should be detected in the 2MASS survey.