The Discovery of a Field Methane Dwarf from Sloan Digital Sky Survey Commissioning Data

We report the discovery of the coolest field dwarf yet known, selected as an unresolved object with extremely red colors from commissioning imaging data of the Sloan Digital Sky Survey. Its spectrum from 0.8 to 2.5 μm is dominated by strong bands of H2O and CH4. Its spectrum and colors over this range are very similar to those of Gl 229B, the only other known example of a methane dwarf. It is roughly 1.2 mag fainter than Gl 229B, suggesting that it lies at a distance of ~10 pc. Such a cool object must have a mass well below the hydrogen-burning limit of 0.08 M☉ and therefore is a genuine brown dwarf, with a probable mass in the range 0.015-0.06 M☉ for an age range of 0.3-5 Gyr.

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