67 additional L dwarfs discovered by the Two Micron All Sky Survey

We present JHK(s) photometry, far red spectra, and spectral classifications for an additional 67 L dwarfs discovered by the Two Micron All Sky Survey. One of the goals of this new search was to locate more examples of the lat est L dwarfs. Of the 67 new discoveries, 17 have types of L6 or later. Anal ysis of these new discoveries shows that H alpha emission has yet to be con vincingly detected in any L dwarf later than type L4.5, indicating a declin e or absence of chromospheric activity in the latest L dwarfs. Further anal ysis shows that 16 (and possibly four more) of the new L dwarfs are lithium brown dwarfs and that the average line strength for those L dwarfs showing lithium increases until type similar to L6.5 V, then declines for later ty pes. This disappearance may be the first sign of depletion of atomic lithiu m as it begins to form into lithium-bearing molecules. Another goal of the search was to locate nearer, brighter L dwarfs of all subtypes. Using absol ute magnitudes for 17 L dwarf systems with trigonometric parallax measureme nts, we develop spectrophotometric relations to estimate distances to the o ther L dwarfs. Of the 67 new discoveries, 21 have photometric distances pla cing them within 25 pc of the Sun. A table of all known L and T dwarfs beli eved to lie within 25 pc-53 in total - is also presented. Using the distanc e measurement of the coolest L dwarf known, we calculate that the gap in te mperature between L8 and the warmest known T dwarfs is less than 350 K and probably much less. If the transition region between the two classes spans a very small temperature interval, this would explain why no transition obj ects have yet been uncovered. This evidence, combined with model fits to lo w-resolution spectra of late M and early L dwarfs, indicates that L-class o bjects span the range 1300 K less than or similar to T-eff less than or sim ilar to 2000 K. The near-infrared color-color diagram shows that L dwarfs f all along a natural, redder extension of the well-known M dwarf track. Thes e near-infrared colors get progressively redder for later spectral types, w ith the L dwarf sequence abruptly ending near (J - H, H - K-h, J - K-s) app roximate to (1.3, 0.8, 2.1).