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The Spectra of T Dwarfs. II. Red Optical Data
We present 6300-10100 Å spectra for a sample of 13 T dwarfsobserved using LRIS mountedtpdel 99 on the Keck I 10 m Telescope. Avariety of features are identified and analyzed, includingpressure-broadened K I and Na I doublets; narrow Cs I and Rb I lines;weak CaH, CrH, and FeH bands; strong H2O absorption; and apossible weak CH4 band. Hα emission is detected inthree of the T dwarfs, strong in the previously reported active T dwarf2MASS 1237+6526 and weak in SDSS 1254-0122 and 2MASS 1047+2124. None ofthe T dwarfs exhibit Li I absorption. Guided by the evolution of opticalspectral features with near-infrared spectral type, we derive a paralleloptical classification scheme, focusing on spectral types T5 to T8,anchored to select spectral standards. We find general agreement betweenoptical and near-infrared types for nearly all the T dwarfs so farobserved, including two earlier type T dwarfs, within our classificationuncertainties (~1 subtype). These results suggest that competing gravityand temperature effects compensate for each other over the 0.6-2.5 μmspectral region. We identify one possible means of disentangling theseeffects by comparing the strength of the K I red wing to the 9250Å H2O band. One of our objects, 2MASS 0937+2931,exhibits a peculiar spectrum with a substantial red slope and relativelystrong FeH absorption, both consequences of a metal-deficientatmosphere. On the basis of its near-infrared properties and substantialspace motion, this object may be a thick disk or halo brown dwarf.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
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