2005ApJ...623.1115C

We present a 0.6-4.1 µm spectroscopic sequence of M, L, and T dwarfs. The spectra have R≡λ/Δλ~2000 from 0.9 to 2.4 µm and R=2500-200 from 2.9 to 4.1 µm. These new data nearly double the number of L and T dwarfs that have reported L-band spectra. The near-infrared spectra are combined with previously published red-optical spectra to extend the wavelength coverage to ∼0.6 µm. Prominent atomic and molecular absorption features are identified including neutral lines of Al, Fe, Mg, Ca, Ti, Na, and K and 19 new weak CH₄absorption features in the H-band spectra of mid- to late-type T dwarfs. In addition, we detect for the first time the 0-0 band of the A ⁴Π-X ⁴Σ^– transition of VO at ∼1.06 µm in the spectra of L dwarfs and the P- and R-branches of the ν₃band of CH₄in the spectrum of a T dwarf. The equivalent widths of the refractory atomic features all decrease with increasing spectral type and are absent by a spectral type of ∼L0, except for the 1.189 µm Fe I line, which persists to at least ∼L3. We compute the bolometric luminosities of the dwarfs in our sample with measured parallaxes and find good agreement with previously published results that use L'-band photometry to account for the flux emitted from 2.5 to 3.6 µm. Finally, 2MASS J2224381-0158521 (L4.5) has an anomalously red spectrum and the strongest Δν=+2 CO bands in our sample. This may be indicative of unusually thick condensate clouds and/or low surface gravity.