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HD 205178


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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.

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Osservazione e dati astrometrici

Costellazione:Acquario
Ascensione retta:21h33m46.32s
Declinazione:-05°25'07.0"
Magnitudine apparente:7.764
Distanza:110.497 parsec
Moto proprio RA:7.6
Moto proprio Dec:-52.5
B-T magnitude:8.132
V-T magnitude:7.795

Cataloghi e designazioni:
Nomi esatti   (Edit)
HD 1989HD 205178
TYCHO-2 2000TYC 5216-1468-1
USNO-A2.0USNO-A2 0825-19282900
HIPHIP 106464

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