A Unified Near‐Infrared Spectral Classification Scheme for T Dwarfs

A revised near infrared classification scheme for T dwarfs is presented,based on and superseding prior schemes developed by Burgasser et al. andGeballe et al., and defined following the precepts of the MK Process. Drawingfrom two large spectroscopic libraries of T dwarfs identified largely in theSloan Digital Sky Survey and the Two Micron All Sky Survey, nine primaryspectral standards and five alternate standards spanning spectral types T0 toT8 are identified that match criteria of spectral character, brightness,absence of a resolved companion and accessibility from both northern andsouthern hemispheres. The classification of T dwarfs is formally made by thedirect comparison of near infrared spectral data of equivalent resolution tothe spectra of these standards. Alternately, we have redefined five keyspectral indices measuring the strengths of the major H$_2$O and CH$_4$ bandsin the 1-2.5 micron region that may be used as a proxy to direct spectralcomparison. Two methods of determining T spectral type using these indices areoutlined and yield equivalent results. These classifications are alsoequivalent to those from prior schemes, implying that no revision of existingspectral type trends is required. The one-dimensional scheme presented hereprovides a first step toward the observational characterization of the lowestluminosity brown dwarfs currently known. Future extensions to incorporatespectral variations arising from differences in photospheric dust content,gravity and metallicity are briefly discussed. A compendium of all currentlyknown T dwarfs with updated classifications is presented.Comment: 52 pages, 11 figures; accepted for publication to Ap