Near-infrared spectroscopy of brown dwarfs: Methane and the transition between the L and T spectral types

We report the possible identification of weak methane spectral features in the near-infrared K band in two late L dwarfs, DENIS 0205-11 (L7) and 2MASS 1523+30 (L8). New, high signal-to-noise ratio, flux-calibrated spectra, spanning the wavelength interval 1.10-2.35 μm with an average resolving power R = 1800, were obtained using NIRSPEC on the Keck II telescope. Results are reported and compared for three late L dwarfs (L5, L7, and L8) and two T dwarfs (T1 and T6). The spectra, which are continuous through the atmospheric absorption bands, show the development of deep steam bands and the weakening of iron hydride features through the L dwarfs and the emergence of strong methane bands in the T dwarfs. A detailed comparison of the K-band regions with synthetic spectra suggests that the weak features seen in the L7 and L8 dwarfs at 2.20 μm are likely to be methane. We see no evidence for methane in the H band. At the R = 1800 resolution, significant differences are evident between the spectral signatures of L8 and T1, leaving room for additional transition objects (L9 or T0).