Sixty - seven Additional L Dwarfs Discovered by the Two Micron All Sky Survey ( 2 MASS )

We present J H K , 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 tha t Ha 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 ~ L 6 . 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 a spectrophotometric relation 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 52 in total is also presented. Using the distance measurement of 'Portions of the data presented herein were obtained at the W.M. Keck Observatory which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. 'Infrared Processing and Analysis Center, MS 100-22, California Institute of Technology, Pasadena, CA 91125; [email protected], [email protected], [email protected] 3Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104-6396; [email protected] 4Steward Observatory, University of Arizona, Tucson, AZ 85721; [email protected] 5Department of Physics, MS 103-33, California Institute of Technology, Pasadena, CA 91125; [email protected] 6U.S. Naval Observatory, P.O. Box 1149, Flagstaff, AZ 86002; [email protected], [email protected] 'Department of Astronomy, MSC 152, California Institute of Technology, Pasadena, CA 91126-0152