USNO-B is an all-sky catalog that presents positions, proper motions, magnitudes in various optical passbands, and star/galaxy estimators for 1,042,618,261 objects derived from 3,643,201,733 separate observations. The data were obtained from scans of 7,435 Schmidt plates taken for the various sky surveys during the last 50 years. USNO-B1.0 is believed to provide all-sky coverage, completeness down to V = 21, 0.2 arcsecond astrometric accuracy at J2000, 0.3 magnitude photometric accuracy in up to five colors, and 85% accuracy for distinguishing stars from non-stellar objects. A brief discussion of various issues is given here, but the actual data are available from http://www.nofs.navy.mil and other sites.Comment: Accepted by Astronomical Journa

The Sloan Digital Sky Survey (SDSS) is an imaging and spectroscopic survey that will eventually cover approximately one-quarter of the celestial sphere and collect spectra of %10 6 galaxies, 100,000 quasars, 30,000 stars, and 30,000 serendipity targets. In 2001 June, the SDSS released to the general astronomical community its early data release, roughly 462 deg 2 of imaging data including almost 14 million detected objects and 54,008 follow-up spectra. The imaging data were collected in drift-scan mode in five bandpasses (u, g, r, i, and z); our 95% completeness limits for stars are 22.0, 22.2, 22.2, 21.3, and 20.5, respectively. The photometric calibration is reproducible to 5%, 3%, 3%, 3%, and 5%, respectively. The spectra are flux-and wavelength-calibrated, with 4096 pixels from 3800 to 9200 Å at R % 1800. We present the means by which these data are distributed to the astronomical community, descriptions of the hardware used to obtain the data, the software used for processing the data, the measured quantities for each observed object, and an overview of the properties of this data set.

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.