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 a n unresolved object with extremely red colors from commissioning imaging da ta of the Sloan Digital Sky Survey. Its spectrum from 0.8 to 2.5 mu m is do minated by strong bands of H2O and CH4. Its spectrum and colors over this r ange 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 tha t it lies at a distance of similar to 10 pc. Such a cool object must have a mass well below the hydrogen-burning limit of 0.08 M. and therefore is a g enuine brown dwarf, with a probable mass in the range 0.015-0.06 M. for an age range of 0.3-5 Gyr.