The discovery of a second field methane brown dwarf from Sloan Digital SkySurvey commissioning data

We report the discovery of a second field methane brown dwarf from the comm issioning data of the Sloan Digital Sky Survey (SDSS). The object, SDSS J13 4646.45-003150.4 (hereafter SDSS 1346-00), was selected because of its very red color and stellar appearance. Its spectrum between 0.8 and 2.5 mu m is dominated by strong absorption bands of H2O and CH4 and closely mimics tho se of Gliese 229B and SDSS 162414.37+002915.6 (hereafter SDSS 1624+00), two other known methane brown dwarfs. SDSS 1346-00 is approximately 1.5 mag fa inter than Gliese 229B, suggesting that it lies about 11 pc from the Sun. T he ratio of flux at 2.1 mu m to that at 1.27 mu m is larger for SDSS 1346-0 0 than for Gliese 229B and SDSS 1624+00, which suggests that SDSS 1346-00 h as a slightly higher effective temperature than the others. Based on a sear ch area of 130 deg(2) and a detection limit of z* = 19.8, we estimate a spa ce, density of 0.05 pc(-3) for methane brown dwarfs with T-eff similar to 1 000 K in the 40 pc(3) volume of our search. This estimate is based on small -sample statistics and should be treated with appropriate caution.