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A.J. Burgasser (California Institute of Technology)
T dwarfs are brown dwarfs similar to Gl 229B, exhibiting methane absorption bands at 1.6 and 2.2 \micron. These objects make up the coolest spectral class currently known, with effective temperatures ranging from about 1300 K down to at least 750 K. Sky surveys, such as 2MASS, SDSS, and the NTT Deep Field, have identified over two dozen T dwarfs in the past eighteen months, due to increased sky coverage and sensitivity at longer wavelengths over previous brown dwarf searches. I present one of the main results of my dissertation, the classification of T dwarfs in the near-infrared, based on variations in observed spectral morphology. I begin by characterizing the spectral differences between L and T dwarfs in the 6300--10100 {Å} regime, where Kirkpatrick et al.\ has established the L dwarf classification. I then present a sequence of near-infrared spectra obtained using the Ohio State Infrared Imager/Spectrometer mounted on the CTIO Blanco 4m Telescope, along with data from the literature, and discuss spectral variations. Based on these data, I define a classification grid for T dwarfs in the near-infrared, and present spectral indices useful for subtyping. Finally, I discuss spectral features sensitive to specific gravity and/or metallicity, which complicates the derived temperature scheme and may prompt the definition of a second classification dimension based on brown dwarf mass, in analogy to the MK luminosity class.
This contribution makes use of data from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by NASA and NSF. Observations made at CTIO were made with the support of the NOAO Ph.D. thesis program.
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