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Before the 2-Micron All-Sky Survey (2MASS) began, only six objects were known with spectral types later than M9.5 V. In the Ðrst 371 deg2 of actual 2MASS survey data, we have identiÐed another 20 such objects spectroscopically conÐrmed using the Low Resolution Imaging Spectrograph (LRIS) at the W. M. Keck Observatory. Because the TiO and VO bands, which dominate the far-optical portions of late-M spectra, disappear in these cooler dwarfs, we deÐne a new spectral class "" L ÏÏ in which metallic oxides are replaced by metallic hydrides and neutral alkali metals as the major spectroscopic signatures. We establish classiÐcation indices and type all 25 L dwarfs. The 26th "" post-M9.5 ÏÏ objectÈGl 229BÈis the prototype of a methane-dominated spectral class, which we propose as class "" T.ÏÏ At least Ðve of the 20 2MASS L dwarfs show the 6708 lithium doublet at low resolution, the strongest having an equiva-A lent width of 18.5For objects this cool, the presence of lithium proves that they are substellar. Two A . other 2MASS objects appear to have lithium lines at the limit of our detectability, which if veriÐed means that at least one-third of our L dwarfs are bona Ðde brown dwarfs. All of the 2MASS brown dwarfs discovered so far haveWe have not yet, despite deliberately searching for them, J[K s [1.30. found any brown dwarfs with colors resembling Gl 229B (J[K s B[0.1).

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We have combined 2MASS and POSS II data in a search for nearby ultracool (later than M6.5) dwarfs with Spectroscopic follow-up observations identify 53 M7ÈM9.5 dwarfs and seven L K s \ 12. dwarfs. The observed space density is 0.0045^0.0008 M8ÈM9.5 dwarfs per cubic parsec, without accounting for biases, consistent with a mass function that is smooth across the stellar/substellar limit. We show the observed frequency of Ha emission peaks at D100% for M7 dwarfs and then decreases for cooler dwarfs. In absolute terms, however, as measured by the ratio of Ha to bolometric luminosity, none of the ultracool M dwarfs can be considered very active compared to earlier M dwarfs, and we show that the decrease that begins at spectral type M6 continues to the latest L dwarfs. We Ðnd that Ñaring is common among the coolest M dwarfs and estimate the frequency of Ñares at 7% or higher. We show that the kinematics of relatively active (EW [ 6 ultracool M dwarfs are consistent with an ordi-A ) nary old disk stellar population, while the kinematics of inactive ultracool M dwarfs are more typical of a 0.5 Gyr old population. The early L dwarfs in the sample have kinematics consistent with old ages, suggesting that the hydrogen-burning limit is near spectral types L2ÈL4. We use the available data on M and L dwarfs to show that chromospheric activity drops with decreasing mass and temperature and that at a given (M8 or later) spectral type, the younger Ðeld (brown) dwarfs are less active than many of the older, more massive Ðeld stellar dwarfs. Thus, contrary to the well-known stellar age-activity relationship, low activity in Ðeld ultracool dwarfs can be an indication of comparative youth and substellar mass.

We present a spectroscopic classification system for M-dwarfs and M-subdwarfs based on quantitative measures of TiO and CaH features in the region 6200 - 7400 Angstroms. Our sample of cool stars covers the range from solar metallicity stars to the most extreme subdwarfs known. Using synthetic spectra computed by Allard and Hauschildt (1995), we derive metallicities for the stars. Stars are classified as dwarfs (M V), subdwarfs (sdM), or extreme subdwarfs (esdM). These classifications correspond to [m/H] ~ 0.0, -1.2, and -2.0 respectively. Our metallicity scale agrees with theoretical HR diagrams and HST globular cluster measurements. We discuss some nearby subdwarfs of particular interest in light of our metallicity scale.Comment: AASTeX, 43 pages, 11 figures, A.J. accepte

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 Hα 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 ∼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 -2of 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 ∼ < T ef f ∼ < 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 ) ≈ (1.3, 0.8, 2.1).

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We present the discovery of two T dwarf binaries, 2MASS 1225−2739AB and 2MASS 1534−2952AB, identified in a sample of ten T dwarfs imaged with the Hubble Space Telescope Wide Field Planetary Camera 2. Companionship is established by the uniquely red F814W−F1042M colors of the binary components, caused by heavily pressure-broadened K I absorption centered at 7665 & 7699Å. The separations of the two binary systems are 0. ′′ 282±0. ′′ 005 and 0. ′′ 065±0. ′′ 007, implying projected separations of 3.17±0.14 and 1.0±0.3 AU, respectively. These close separations are similar to those found in previous brown dwarf binary searches, and permit orbital mapping over the coming decade. 2MASS 1225−2739AB has a substantially fainter secondary, with ∆M F 814W = 1.59±0.04 and ∆M F 1042M = 1.05±0.03; this system is likely composed of a T6 primary and T8 secondary with mass ratio 0.7-0.8. The observed binary fraction of our HST sample, 20 +17 −7 %, is consistent with results obtained for late-M and L field dwarfs, and implies a bias-corrected binary fraction of 9 +15 −4 % for a 1 AU and q 0.4, significantly lower than the binary fractions of F-G and early-type M dwarf stars. Neither of the T binaries have separations a 10 AU, consistent with results from other brown dwarf binary searches. Using the statistical models of Weinberg, Shapiro, & Wasserman, we conclude that tidal disruption by passing stars or Giant Molecular Clouds, which limits the extent of wide stellar binaries, plays no role in eliminating wide brown dwarf binaries, implying either disruption very early in the formation process (ages 1−10 Myr) or a formation mechanism which precludes such systems. We find that the maximum binary separation in the brown dwarf regime appears to scale as M 2 total , a possible clue to the physical mechanism which restricts wide substellar systems.