We present infrared spectra of brown dwarfs with spectral types from mid-L to T. The 0.9-2.5 $\mu$m spectra of three dwarfs found by the Sloan Digital Sky Survey contain absorption bands of both methane and carbon monoxide and bridge the gap between late L and previously observed T dwarfs. These dwarfs form a clear spectral sequence, with CH$_{4}$ absorption increasing as the CO absorption decreases. Water vapor band strengths increase in parallel with the methane bands and thus also link the L and T types. We suggest that objects with detectable CO and CH$_{4}$ in the H and K bands should define the earliest T subclasses. From observations of bright (K~$\leq$~13 mag) L dwarfs found by 2MASS, we find that the onset of detectable amounts of CH$_{4}$ occurs near spectral type L5. For this spectral type methane is observable in the 3.3~$\mu$m $u$$_{3}$ band only, and not in the overtone and combination bands at H and K.
CITATION STYLE
Geballe, T. R., Noll, K. S., Leggett, S. K., Knapp, G. R., Fan, X., & Golimowski, D. (2001). Infrared Spectroscopy of Brown Dwarfs: the Onset of CH4 Absorption in L Dwarfs and the L/T Transition. In Ultracool Dwarfs (pp. 83–91). Springer Berlin Heidelberg. https://doi.org/10.1007/978-3-642-56672-1_8
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