The 2MASS wide-field T dwarf search. I. Discovery of a bright T dwarf within 10 parsecs of the Sun

We present the discovery of a bright (J = 13.94 +/- 0.03) T dwarf, 2MASS 1503+2525, identified in a new, wide-field search for T dwarfs using the recently completed Two Micron All Sky Survey (2MASS). The 1-2.5 mum spectrum of this object exhibits the strong H2O and CH4 bands characteristic of mid- and late-type T dwarfs, and we derive a spectral type of T5.5 using both the Burgasser et al. and the Geballe et al. classification schemes. Based on its spectral type and the absolute magnitudes of known T dwarfs, we estimate the distance of this object as 8 +/- 3 pc if it is single, likely within 10 pc of the Sun. Our new 2MASS search, which covers 74% of the sky and greatly expands on earlier color constraints, should identify 15-25 new T dwarfs with J less than or equal to 16. Combined with the 20 known members of this class that already fall within our search criteria, our new sample will provide improved statistics for such key quantities as the binary fraction and the field substellar mass function. Furthermore, multiple detections from overlapping 2MASS scans provide multiple-epoch astrometry and photometry, and we present proper motions for five T dwarfs in our sample.