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.

We have compiled L 0 (3.4-4.1 m) and M 0 (4.6-4.8 m) photometry of 63 single and binary M, L, and T dwarfs obtained at the United Kingdom Infrared Telescope using the Mauna Kea Observatory filter set. This compilation includes new L 0 measurements of eight L dwarfs and 13 T dwarfs and new M 0 measurements of seven L dwarfs, five T dwarfs, and the M1 dwarf Gl 229A. These new data increase by factors of 0.6 and 1.6, respectively, the numbers of ultracool dwarfs (T eff P 2400 K) for which L 0 and M 0 measurements have been reported. We compute L bol , BC K , and T eff for 42 dwarfs whose flux-calibrated JHK spectra, L 0 photometry, and trigonometric parallaxes are available, and we estimate these quantities for nine other dwarfs whose parallaxes and flux-calibrated spectra have been obtained. BC K is a well-behaved function of near-infrared spectral type with a dispersion of $0.1 mag for types M6-T5; it is significantly more scattered for types T5-T9. T eff declines steeply and monotonically for types M6-L7 and T4-T9, but it is nearly constant at $1450 K for types L7-T4 with assumed ages of $3 Gyr. This constant T eff is evidenced by nearly unchanging values of L 0 -M 0 between types L6 and T3. It also supports recent models that attribute the changing near-infrared luminosities and spectral features across the L-T transition to the rapid migration, disruption, and/or thinning of condensate clouds over a narrow range of T eff . The L 0 and M 0 luminosities of early-T dwarfs do not exhibit the pronounced humps or inflections previously noted in the I through K bands, but insufficient data exist for types L6-T5 to assert that M L 0 and M M 0 are strictly monotonic within this range of types. We compare the observed K, L 0 , and M 0 luminosities of L and T dwarfs in our sample with those predicted by precipitating-cloud and cloud-free models for varying surface gravities and sedimentation efficiencies. The models indicate that the L3-T4.5 dwarfs generally have higher gravities (log g = 5.0-5.5) than the T6-T9 dwarfs (log g = 4.5-5.0). The predicted M 0 luminosities of late-T dwarfs are 1.5-2.5 times larger than those derived empirically for the late-T dwarfs in our sample. This discrepancy is attributed to absorption at 4.5-4.9 m by CO, which is not expected under the condition of thermochemical equilibrium assumed in the models. Our photometry and bolometric calculations indicate that the L3 dwarf Kelu-1 and the T0 dwarf SDSS J042348.57À041403.5 are probable binary systems. We compute log (L bol /L ) = À5.73 AE 0.05 and T eff = 600-750 K for the T9 dwarf 2MASSI J0415195À093506, which supplants Gl 570D as the least luminous and coolest brown dwarf presently known.

We present preliminary trigonometric parallaxes and proper motions for 22 L dwarfs and 18 T dwarfs measured using the ASTROCAM infrared imager on the US Naval Observatory ( USNO) 1.55 m Strand Astrometric Reflector. The results presented here are based on observations obtained between 2000 September and 2002 November; about half of the objects have an observational time baseline of Át ¼ 1:3 yr and half Át ¼ 2:0 yr. Despite these short time baselines, the astrometric quality is sufficient to produce significant new results, especially for the nearer T dwarfs. Seven objects are in common with the USNO optical CCD parallax program for quality control and seven in common with the European Southern Observatory 3.5 m New Technology Telescope parallax program. We compare astrometric quality with both of these programs. Relative to absolute parallax corrections are made by employing Two Micron All Sky Survey and/or Sloan Digital Sky Survey photometry for reference-frame stars. We combine USNO infrared and optical parallaxes with the best available California Institute of Technology (CIT) system photometry to determine M J , M H , and M K values for 37 L dwarfs between spectral types L0 and L8 and 19 T dwarfs between spectral types T0.5 and T8 and present selected absolute magnitude versus spectral type and color diagrams, based on these results. Luminosities and temperatures are estimated for these objects. Of special interest are the distances of several objects that are at or near the L-T dwarf boundary so that this important transition can be better understood. The previously reported early to mid T dwarf luminosity excess is clearly confirmed and found to be present at J, H, and K. The large number of objects that populate this luminosity-excess region indicate that it cannot be due entirely to selection effects. The T dwarf sequence is extended to M J % 16:9 by 2MASS J041519À0935, which, at d ¼ 5:74 pc, is found to be the least luminous [log (L=L ) ¼ À5:58] and coldest (T eA % 760 K) brown dwarf known. Combining results from this paper with earlier USNO CCD results we find that, in contrast to the L dwarfs, there are no examples of lowvelocity (V tan < 20 km s À1 ) T dwarfs. This is consistent with the T dwarfs in this study being generally older than the L dwarfs. We briefly discuss future directions for the USNO infrared astrometry program.

We present new JHK photometry on the MKO-NIR system and JHK spectroscopy for a large sample of L and T dwarfs. Photometry has been obtained for 71 dwarfs and spectroscopy for 56. The sample comprises newly identified very red objects from the Sloan Digital Sky Survey (SDSS) and known dwarfs from the SDSS and the Two Micron All Sky Survey (2MASS). Spectral classification has been carried out using four previously defined indices (from Geballe et al. 2002, G02) that measure the strengths of the near infrared water and methane bands. We identify 9 new L8-9.5 dwarfs and 14 new T dwarfs from SDSS, including the latest yet found by SDSS, the T7 dwarf SDSS J175805.46+463311.9. We classify 2MASS J04151954−0935066 as T9, the latest and coolest dwarf found to date.We combine the new results with our previously published data to produce a sample of 59 L dwarfs and 42 T dwarfs with imaging data on a single photometric system -2and with uniform spectroscopic classification. We compare the near-infrared colors and absolute magnitudes of brown dwarfs near the L-T transition with predictions made by models of the distribution and evolution of photospheric condensates. There is some scatter in the G02 spectral indices for L dwarfs, suggesting that these indices are probing different levels of the atmosphere and are affected by the location of the condensate cloud layer. The near-infrared colors of the L dwarfs also show scatter within a given spectral type, which is likely due to variations in the altitudes, spatial distributions and thicknesses of the clouds. We have identified a small group of late L dwarfs that are relatively blue for their spectral type and that have enhanced FeH, H 2 O and K I absorption, possibly due to an unusually small amount of condensates.The scatter seen in the H − K color for late T dwarfs can be reproduced by models with a range in surface gravity. The variation is probably due to the effect on the K-band flux of pressure-induced H 2 opacity. The correlation of H − K color with gravity is supported by the observed strengths of the J-band K I doublet. Gravity is closely related to mass for field T dwarfs with ages > 10 8 yrs and the gravities implied by the H − K colors indicate that the T dwarfs in our sample have masses in the range 15 -75 M Jupiter . One of the SDSS dwarfs, SDSS J111010.01+011613.1, is possibly a very low mass object, with log g ∼ 4.2 -4.5 and mass ∼ 10 -15 M Jupiter .

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