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 μm 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 μm to that at 1.27 μm 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 deg² 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~1000 K in the 40 pc³ volume of our search. This estimate is based on small-sample statistics and should be treated with appropriate caution. Based on observations obtained with the Sloan Digital Sky Survey by the SDSS Collaboration and the Apache Point Observatory 3.5 m telescope, which are owned and operated by the Astrophysical Research Consortium, and with the United Kingdom Infrared Telescope (UKIRT).