The initiation of the DENIS and 2MASS surveys are resulting in the discoveries of dozens of field brown dwarf candidates, and the need to develop a new spectral class of L dwarfs. The L dwarfs are perhaps only a few hundred degrees cooler than the latest M dwarfs (M9–9.5 V), and are many hundreds of degrees warmer than the brown dwarf Gliese 229B. However, the formation of dust removes TiO and VO from the atmospheres, resulting in qualitatively different red spectra, with for example a strong, pressure-broadened K I resonance doublet. In contrast, the infrared spectra show the same (H2O and CO) molecular features as in late M dwarfs, with no evidence of methane. The detection of the Li I 6707 Å resonance doublet shows that many L dwarfs, at least, have substellar masses. Based on analysis of point sources from the first 1% of sky, well over 1,000 L dwarfs should be detected in the 2MASS survey. Our results suggest that several might exist within 5 pc of the Sun.
After waiting three decades since Kumar (1963) proposed their existence, we are gratified to see literally dozens of candidates probably or definitely below the stellar mass limit being found in young clusters and associations. Here one has the big advantages that the age, the distance and luminosity of a cluster member are generally known. In this presentation, complementary to the topic of this meeting, we report the finding of a large number of candidates in one of the first infrared surveys of the field population.