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 present 0.6È2.5 km, spectra of 27 cool, low-luminosity stars and substellar objects. Based R Z 400 on these and previously published spectra, we develop a preliminary spectral classiÐcation system for L and T dwarfs. For late L and T types the classiÐcation system is based entirely on four spectral indices in the 1È2.5 km interval. Two of these indices are derived from water absorption bands at 1.15 and 1.4 km, the latter of which shows a smooth increase in depth through the L and T sequences and can be used to classify both spectral types. The other two indices make use of methane absorption features in the H and K bands, with the K-band index also applicable to mid-to-late L dwarfs. Continuum indices shortward of 1 km used by previous authors to classify L dwarfs are found to be useful only through mid-L subclasses. We employ the 1.5 km water index and the 2.2 km methane index to complete the L classiÐcation through L9.5 and to link the new system with a modiÐed version of the 2MASS "" color-d ÏÏ index. By correlating the depths of the methane and water absorption features, we establish a T spectral sequence from T0 to T8, based on all four indices, that is a smooth continuation of the L sequence. We reclassify two 2MASS L8 dwarfs as L9 and L9.5 and identify one SDSS object as L9. In the proposed system methane absorption appears in the K band approximately at L8, two subclasses earlier than its appearance in the H band. The L and T spectral classes are distinguished by the absence and presence, respectively, of H-band methane absorption.

We discuss measurements of the properties of ∼10,000 asteroids detected in 500 deg 2 of sky in the Sloan Digital Sky Survey (SDSS) commissioning data. The moving objects are detected in the magnitude range 14 < r * < 21.5, with a baseline of ∼5 minutes, resulting in typical velocity errors of ∼3%. Extensive tests show that the sample is at least 98% complete, with the contamination rate of less than 3%.We find that the size distribution of asteroids resembles a broken power-law, independent of the heliocentric distance: D −2.3 for 0.4 km ∼ < D ∼ < 5 km, and D −4 for 5 km ∼ < D ∼ < 40 km. As a consequence of this break, the number of 1 Based on observations obtained with the Sloan Digital Sky Survey.-3asteroids with r * < 21.5 is ten times smaller than predicted by extrapolating the power-law relation observed for brighter asteroids (r * ∼ < 18). The observed counts imply that there are about 530,000 objects with D > 1 km in the asteroid belt, or about four times less than previous estimates. We predict that by its completion SDSS will obtain about 100,000 near simultaneous five-band measurements for a subset drawn from 280,000 asteroids brighter than r * < 21.5 at opposition. Only about a third of these asteroids have been previously observed, and usually in just one band.The distribution of main belt asteroids in the 4-dimensional SDSS color space is bimodal, and the two groups can be associated with S (rocky) and C (carbonaceous) asteroids. A strong bimodality is also seen in the heliocentric distribution of asteroids and suggests the existence of two distinct belts: the inner rocky belt, about 1 AU wide (FWHM) and centered at R ∼2.8 AU, and the outer carbonaceous belt, about 0.5 AU wide and centered at R ∼3.2 AU. The median color of each class becomes bluer by about 0.03 mag AU −1 as the heliocentric distance increases. The observed number ratio of S and C asteroids in a sample with r * < 21.5 is 1.5:1, while in a sample limited by absolute magnitude it changes from 4:1 at 2 AU, to 1:3 at 3.5 AU. In a size-limited sample with D > 1 km, the number ratio of S and C asteroids in the entire main belt is 1:2.3.The colors of Hungarias, Mars crossers, and near-Earth objects, selected by their velocity vectors, are more similar to the C-type than to S-type asteroids, suggesting that they originate in the outer belt. In about 100 deg 2 of sky along the Celestial Equator observed twice two days apart, we find one plausible Kuiper Belt Object (KBO) candidate, in agreement with the expected KBO surface density. The colors of the KBO candidate are significantly redder than the asteroid colors, in agreement with colors of known KBOs. We explore the possibility that SDSS data can be used to search for very red, previously uncatalogued asteroids observed by 2MASS, by extracting objects without SDSS counterparts. We do not find evidence for a significant population of such objects; their contribution is no more than 10% of the asteroid population.

We present near-infrared observations of 71 newly discovered L and T dwarfs, selected from imaging data of the Sloan Digital Sky Survey (SDSS) using the i-dropout technique. Sixty-five of these dwarfs have been classified spectroscopically according to the near-infrared L dwarf classification scheme of Geballe et al. and the unified T dwarf classification scheme of Burgasser et al. The spectral types of these dwarfs range from L3 to T7, and include the latest types yet found in the SDSS. Six of the newly identified dwarfs are classified as early- to mid-L dwarfs according to their photometric near-infrared colors, and two others are classified photometrically as M dwarfs. We also present new near-infrared spectra for five previously published SDSS L and T dwarfs, and one L dwarf and one T dwarf discovered by Burgasser et al. from the Two Micron All Sky Survey. The new SDSS sample includes 27 T dwarfs and 30 dwarfs with spectral types spanning the complex L-T transition (L7-T3). We continue to see a large (~0.5 mag) spread in J-H for L3 to T1 types, and a similar spread in H-K for all dwarfs later than L3. This color dispersion is probably due to a range of grain sedimentation properties, metallicity, and gravity. We also find L and T dwarfs with unusual colors and spectral properties that may eventually help to disentangle these effects.Comment: accepted by AJ, 18 pages, 10 figures, 5 tables, emulateapj layou

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules. In dense interstellar clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density and temperature. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons (e-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.

We present the results from a survey of i-dropout objects selected from D1550 deg2 of multicolor imaging data from the Sloan Digital Sky Survey to search for luminous quasars at Objects with z Z 5.8. i*[z* [ 2.2 and z* \ 20.2 are selected, and follow-up J-band photometry is used to separate L-and T-type cool dwarfs from high-redshift quasars. We describe the discovery of three new quasars, SDSSp J083643.85]005453.3 (z \ 5.82), J130608.26]035626.3 (z \ 5.99), and J103027.10]052455.0 (z \ 6.28). The quasar SDSSp J083643.85]005453.3 is a radio source with Ñux of 1.1 mJy at 20 cm. The spectra of all three quasars show strong and broad Lya ] N V emission lines and very strong Lya forest absorption, with a mean continuum decrementThe ARC 3.5 m spectrum of SDSSp D A[ 0.90. J103027.10]052455.0 shows that over a range of D300 immediately blueward of the Lya emission, Ó the average transmitted Ñux is only 0.003^0.020 times that of the continuum level, consistent with zero Ñux over a D300 range of the Lya forest region and suggesting a tentative detection of the complete Ó Gunn-Peterson trough. The existence of strong metal lines in the quasar spectra suggests early metal enrichment in the quasar environment. The three new objects, together with the previously published z \ 5.8 quasar SDSSp J104433.04[012502.2, form a complete color-selected Ñux-limited sample at z Z 5.8. We estimate the selection function of this sample, taking into account the estimated variations in the quasar spectral energy distribution, as well as observational photometric errors. We Ðnd that at z \ 6, 2 This paper is dedicated to the memory of Arthur Davidsen, who was a pioneer of the study of the intergalactic medium and who showed great leadership as chairman of the Advisory Council of the Sloan Digital Sky Survey.3 Institute for Advanced Study, Olden Lane, Princeton, NJ 08540. 4 Princeton University Observatory, Princeton, NJ 08544. . This is a factor of D2 lower than that at z D 5 and is consistent with an extrapolation of the observed quasar evolution at z \ 5. Using the current sample, we discuss the constraint on the shape of the quasar luminosity function and the implications for the contribution of quasars to the ionizing background at z D 6. The luminous quasars discussed in the paper have central black hole masses of several times 109 by the Eddington argument, with likely dark halo masses on the order M _ of 1013Their observed space density provides a sensitive test of models of quasar and galaxy for-M _ . mation at high redshift.

We have obtained a good-quality R D 400, 0.8È2.5 km spectrum as well as accurate photometry of Gliese 570D, one of the coolest and least-luminous brown dwarfs currently known. The spectrum shows that Gl 570D has deeper absorptions in the strong water and methane bands at 1.12È1.17, 1.33È1.45, 1.62È1.88, and 2.20È2.45 km and is both bluer at J [ K and redder at K [ L@ than previously observed T dwarfs. Data analysis using model spectra coupled with knowledge of the well-understood primary implies that for the same surface gravity, Gl 570D is about 160 K cooler than Gl 229B. For an age range of 2È5 Gyr, Gl 570D has an e †ective temperature in the range 784È824 K, a surface gravity log g in the range 5.00È5.27 (cm s~2), and a luminosity in the range (2.88È2.98) ] 10~6 L _ .

The molecular ion H3+ is considered the cornerstone of interstellar chemistry because it initiates the reactions responsible for the production of many larger molecules. Recently discovered in dense molecular clouds, H3+ has now been observed in the diffuse interstellar medium toward Cygnus OB2 No. 12. Analysis of H3+ chemistry suggests that the high H3+ column density (3.8 x 10(14) per square centimeter) is due not to a high H3+ concentration but to a long absorption path. This and other work demonstrate the ubiquity of H3+ and its potential as a probe of the physical and chemical conditions in the interstellar medium.