The 2MASS wide-field T dwarf search. V. Discovery of a T dwarf via methane imaging

We present the discovery of a T dwarf, 2MASS 2151 - 4853, via differential imaging through methane filters. The filters are designed to highlight the strong absorption in the H band due to methane found in the atmospheres of T dwarfs and provide a very efficient means of searching for them. Subsequent J- and H-band spectroscopy confirms 2MASS 2151 - 4853 as a T dwarf of type T4.5. It has an estimated spectrophotometric distance of 18 +/- 3 pc and an estimated tangential velocity of v(t) = 50 +/- 10 km s(-1).