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The all sky surveys done by the Palomar Observatory Schmidt, the European Southern Observatory Schmidt, and the United Kingdom Schmidt, the InfraRed Astronomical Satellite, and the Two Micron All Sky Survey have proven to be extremely useful tools for astronomy with value that lasts for decades. The Wide-field Infrared Survey Explorer (WISE) is mapping the whole sky following its launch on 2009 December 14. WISE began surveying the sky on 2010 January 14 and completed its first full coverage of the sky on July 17. The survey will continue to cover the sky a second time until the cryogen is exhausted (anticipated in 2010 November). WISE is achieving 5σ point source sensitivities better than 0.08, 0.11, 1, and 6 mJy in unconfused regions on the ecliptic in bands centered at wavelengths of 3.4, 4.6, 12, and 22 μm. Sensitivity improves toward the ecliptic poles due to denser coverage and lower zodiacal background. The angular resolution is 6. 1, 6. 4, 6. 5, and 12. 0 at 3.4, 4.6, 12, and 22 μm, and the astrometric precision for high signal-to-noise sources is better than 0. 15.

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).

A revised near infrared classification scheme for T dwarfs is presented, based on and superseding prior schemes developed by Burgasser et al. and Geballe et al., and defined following the precepts of the MK Process. Drawing from two large spectroscopic libraries of T dwarfs identified largely in the Sloan Digital Sky Survey and the Two Micron All Sky Survey, nine primary spectral standards and five alternate standards spanning spectral types T0 to T8 are identified that match criteria of spectral character, brightness, absence of a resolved companion and accessibility from both northern and southern hemispheres. The classification of T dwarfs is formally made by the direct comparison of near infrared spectral data of equivalent resolution to the spectra of these standards. Alternately, we have redefined five key spectral indices measuring the strengths of the major H 2 O and CH 4 bands in the 1-2.5 µm region that may be used as a proxy to direct spectral comparison. Two methods of determining T spectral type using these indices are outlined and yield equivalent results. These classifications are also equivalent to those from prior schemes, implying that no revision of existing spectral type trends is required. The one-dimensional scheme presented here provides a first step toward the observational characterization of the lowest luminosity brown dwarfs currently known. Future extensions to incorporate spectral variations arising from differences in photospheric dust content, gravity and metallicity are briefly discussed. A compendium of all currently known T dwarfs with updated classifications is presented.

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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.

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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).