Astronomy and Astrophysics – Astrophysics
Scientific paper
2001-02-03
Astronomy and Astrophysics
Astrophysics
9 pages (Latex) and 5 figures (postscript). Proc of Mtg on Ultracool Dwarfs, Manchester, 12 August 2000, to be published in Le
Scientific paper
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 $\nu$$_{3}$ band only, and not in the overtone and combination bands at H and K.
Fan Xiaohui
Geballe Thomas Ronald
Golimowski Dave
Knapp Gillan R.
Leggett Sandy K.
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