Brown dwarfs and cool stars: An overview

The first unambiguous discoveries of cluster brown dwarfs (Teide 1, PPL 15) and of planetary (51 Peg B) and brown dwarf (Gliese 229B) companions occurred five years ago. Yet while extrasolar planets are now being discovered at a breathtaking rate, brown dwarf companions to ordinary stars are apparently rare. 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. A similar conclusion may apply to the field of the solar neighborhood. The infrared sky surveys DENIS and especially 2MASS, and now also the optical Sloan Digital Sky Survey (SDSS), show that brown dwarfs exist in significant numbers. 2MASS alone has discovered nearly 100 of the newly-defined spectroscopic class of L dwarfs, many of which are certifiably substellar due to the appearance of strong Li I lines. In 1999 the first field methane or T dwarfs like Gl 229B were reported by the SDSS and 2MASS science teams. Modelling of a complete sample of 2MASS L dwarfs, and the relative fraction showing Li I, predicts that the luminosity function of substellar objects peaks at a T-eff below 1,000 K - that is, most of the Sun's brown dwarf neighbors may be cooler than Gl 229B, and the coolest objects may still await discovery.