The Onset of Methane in L Dwarfs

Abstract

We have detected weak absorption features produced by the strong n3 methane band at 3.3 mm in two L dwarfs, 2MASSW J15074762162738 and 2MASSI J08251961211552, 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. Subject headings: infrared: stars�radiative transfer�stars: low-mass, brown dwarfs