The Spectra of T Dwarfs. I. Near-Infrared Data and Spectral Classification

Creators: Burgasser, Adam J.; Kirkpatrick, J. Davy; Brown, Michael E.; Reid, I. Neill; Burrows, Adam; Liebert, James; Matthews, Keith; Gizis, John E.; Dahn, Conard C.; Monet, David G.; Cutri, Roc M.; Skrutskie, Michael F.

Abstract

We present near-infrared spectra for a sample of T dwarfs, including 11 new discoveries made using the 2 Micron All Sky Survey. These objects are distinguished from warmer (L-type) brown dwarfs by the presence of methane absorption bands in the 1-2.5 μm spectral region. A first attempt at a near-infrared classification scheme for T dwarfs is made, based on the strengths of CH_4 and H_2O bands and the shapes of the 1.25, 1.6, and 2.1 μm flux peaks. Subtypes T1 V-T8 V are defined, and spectral indices useful for classification are presented. The subclasses appear to follow a decreasing T_(eff) scale, based on the evolution of CH_4 and H_2O bands and the properties of L and T dwarfs with known distances. However, we speculate that this scale is not linear with spectral type for cool dwarfs, due to the settling of dust layers below the photosphere and subsequent rapid evolution of spectral morphology around T_(eff) ~ 1300-1500 K. Similarities in near-infrared colors and continuity of spectral features suggest that the gap between the latest L dwarfs and earliest T dwarfs has been nearly bridged. This argument is strengthened by the possible role of CH_4 as a minor absorber, shaping the K-band spectra of the latest L dwarfs. Finally, we discuss one peculiar T dwarf, 2MASS 0937+2931, which has very blue near-infrared colors (J - K_s = -0.89 ± 0.24) due to suppression of the 2.1 μm peak. The feature is likely caused by enhanced collision-induced H_2 absorption in a high-pressure or low-metallicity photosphere.

Additional Information

© 2002 American Astronomical Society. Received 2001 May 4; accepted 2001 August 27. A. J. B. would like to thank J. Cuby, X. Delfosse, T. Geballe, S. Leggett, M. Strauss, A. Tokunaga, and Z. Tsvetanov for providing electronic versions of their published spectra, and acknowledges useful discussions with R. Blum, D. Golimowski, M. Marley, G. Neugebauer, D. Saumon, and M. R. Zapatero Osorio in the preparation of the manuscript. Observations described in this article would not have been possible without the assistance of our knowledgeable telescope operators and instrument specialists : Mike Doyle, Karl Dunscombe, Jean Mueller, Kevin Rykowski, Barrett "Skip" Staples, and Merle Sweet at Palomar; Teresa Chelminiak, Bob Goodrich, Chuck Sorenson, and Meg Whittle at Keck; and Maria Theresa Acevedo, Alberto Alvarez, Robert Blum, Mauricio Fernandez, Angel Guerra, and Patricio Ugarte at CTIO. A. J. B., J. D. K., and J. E. G. acknowledge the support of the Jet Propulsion Laboratory, California Institute of Technology, which is operated under contract with the National Aeronautics and Space Administration. A. B. acknowledges support from NASA grants NAG 5-7073, NAG 5-7499, and NAG 5-10629. Portions of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The Digitized Sky Survey was produced at the Space Telescope Science Institute under US Government grant NAG W-2166. DSS images were obtained from the Canadian Astronomy Data Centre, which is operated by the Herzberg Institute of Astrophysics, National Research Council of Canada. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This publication makes use of data from the 2 Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation. Finally, we would like to thank our anonymous referee for his or her constructive criticisms.

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