13 mag) L dwarfs found by 2MASS, we
find that the onset of detectable amounts of CH4 occurs
near spectral type L5. For this spectral type methane is observable
in the 3.3 µm 
3
band only, and not in the overtone and combination bands at H and K.
T. R. Geballe
K. S. Noll
S. K. Leggett
G. R. Knapp and X. Fan
D. Golimowski
Gemini Observatory Northern Operations Center, 670 N. A'ohoku Place, Hilo HI 96720
Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218
Joint Astronomy Centre, 660 N. A'ohoku Pl., Hilo, HI 96720
Princeton University Observatory, Princeton, NJ 08544
Dept. of Physics and Astronomy, Johns Hopkins Univ., 3701 San Martin Dr., Baltimore, MD 21218
We present infrared spectra of brown dwarfs with spectral types from
mid-L to T. The 0.9-2.5 µm 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
CH4 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 CH4 in the H and K bands should
define the earliest T subclasses. From observations of bright (K 13 mag) L dwarfs found by 2MASS, we
find that the onset of detectable amounts of CH4 occurs
near spectral type L5. For this spectral type methane is observable
in the 3.3 µm 3
band only, and not in the overtone and combination bands at H and K.
To appear in the Proceedings of the IAU Conference on Ultracool Dwarfs, to be published in Lecture Notes in Physics.
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