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The Spectra of T Dwarfs. I. Near-Infrared Data and Spectral Classification
We present near-infrared spectra for a sample of T dwarfs, including 11new discoveries made using the 2 Micron All Sky Survey. These objectsare distinguished from warmer (L-type) brown dwarfs by the presence ofmethane absorption bands in the 1-2.5 μm spectral region. A firstattempt at a near-infrared classification scheme for T dwarfs is made,based on the strengths of CH4 and H2O bands andthe shapes of the 1.25, 1.6, and 2.1 μm flux peaks. Subtypes T1 V-T8V are defined, and spectral indices useful for classification arepresented. The subclasses appear to follow a decreasing Teffscale, based on the evolution of CH4 and H2O bandsand the properties of L and T dwarfs with known distances. However, wespeculate that this scale is not linear with spectral type for cooldwarfs, due to the settling of dust layers below the photosphere andsubsequent rapid evolution of spectral morphology aroundTeff~1300-1500 K. Similarities in near-infrared colors andcontinuity of spectral features suggest that the gap between the latestL dwarfs and earliest T dwarfs has been nearly bridged. This argument isstrengthened by the possible role of CH4 as a minor absorber,shaping the K-band spectra of the latest L dwarfs. Finally, we discussone peculiar T dwarf, 2MASS 0937+2931, which has very blue near-infraredcolors (J-Ks=-0.89+/-0.24) due to suppression of the 2.1μm peak. The feature is likely caused by enhanced collision-inducedH2 absorption in a high-pressure or low-metallicityphotosphere.
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| A homogeneous catalog of new UBV and H-beta photometry of B- and A-type stars in and around the Scorpius-Centaurus OB association
B- and A-type stars in and near the Sco-Cen OB association areinvestigated with UBV and H-beta photometry to acquire data relevant tothe luminosity function of Sco-Cen. The measurements generally consistof two 10-s integrations of U, B, V, (W, N) filters, and theobservations are corrected iteratively for atmospheric extinction andinstrumental response. The data presented give the mean V magnitude,mean B-V, mean U-B, and the estimated uncertainties for these values.The catalog provides a homogeneous catalog of data for a large fieldwith stellar objects delineating membership to the association Sco-Cenand that affect the luminosity function of the aggregate.
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