The Onset of Methane in L Dwarfs

We have detected weak absorption features produced by the strong ν₃ methane band at 3.3 μm in two L dwarfs, 2MASSW J1507476-162738 and 2MASSI J0825196+211552, classified as spectral types L5 and L7.5, respectively. These absorptions occur in objects warmer than any in which methane previously has been detected and mark the first appearance of methane in the ultracool star-to-brown dwarf spectral sequence.