Discovery of Four Field Methane (T-type) Dwarfs with 2MASS

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We report the discovery of four field methane (T-type) brown dwarfs using 2MASS survey data. One additional methane dwarf, previously discovered by SDSS, was also identified. Near-infrared spectra clearly show the 1.6 and 2.2 um CH4 absorption bands characteristic of objects with Tₑff < 1300 K, as well as broadened H2O bands at 1.4 and 1.9 um. Comparing the spectra of these objects with Gl 229B, we propose that all new 2MASS T dwarfs are warmer than 950 K, in order from warmest to coolest: 2MASS J1217-03, J1225-27, J1047+21 and J1237+65. Based on this preliminary sample, we find a warm T dwarf surface density of 0.0022 T dwarfs/sq. deg., or 90 warm T dwarfs over the whole sky detectable to J < 16. The resulting space density upper limit, 0.01 T dwarfs/pc³, is comparable to that of the L dwarf sample from Kirkpatrick et al. (1999).