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

We report the discovery of a second field methane brown dwarf from the commissioning data of the Sloan Digital Sky Survey (SDSS). The object, SDSS J134646.45-003150.4 (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 those 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 fainter than Gliese 229B, suggesting that it lies about 11 pc from the Sun. The ratio of flux at 2.1 mu m to that at 1.27 mu m is larger for SDSS 1346-00 than for Gliese 229B and SDSS 1624+00, which suggests that SDSS 1346-00 has a slightly higher effective temperature than the others. Based on a search area of 130 deg(2) and a detection limit of z* = 19.8, we estimate a space, density of 0.05 pc(-3) for methane brown dwarfs with T-eff similar to 1000 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.