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 latest L dwarfs .
Of the 67 new discoveries , 17 have types of L6 or later .
Analysis of these new discoveries shows that H \alpha emission has yet to be convincingly detected in any L dwarf later than type L4.5 , indicating a decline or absence of chromospheric activity in the latest L dwarfs .
Further analysis shows that 16 ( and possibly 4 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 \sim L6.5 V then declines for later types .
This disappearance may be the first sign of depletion of atomic lithium 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 absolute magnitudes for 17 L dwarf systems with trigonometric parallax measurements , we develop spectrophotometric relations to estimate distances to the other L dwarfs .
Of the 67 new discoveries , 21 have photometric distances placing them within 25 parsecs of the Sun .
A table of all known L and T dwarfs believed to lie within 25 parsecs – 53 in total — is also presented .
Using the distance measurement of the coolest L dwarf known , we calculate that the gap in temperature between L8 and the warmest known T dwarfs is less than 350K and probably much less .
If the transition region between the two classes spans a very small temperature interval , this would explain why no transition objects have yet been uncovered .
This evidence , combined with model fits to low-resolution spectra of late-M and early-L dwarfs , indicates that L-class objects span the range 1300K \lesssim T _ { eff } \lesssim 2000K .
The near-infrared color-color diagram shows that L dwarfs fall along a natural , redder extension of the well known M dwarf track .
These near-infrared colors get progressively redder for later spectral types , with the L dwarf sequence abruptly ending near ( J - H , H - K _ { s } , J - K _ { s } ) \approx ( 1.3 , 0.8 , 2.1 ) .