An infrared spectroscopic sequence of M, L, and T dwarfs

We present a 0.6 - 4.1 mu m spectroscopic sequence of M, L, and T dwarfs. The spectra have R = lambda/Delta lambda approximate to 2000 from 0.9 to 2.4 mu m and R = 2500 200 from 2.9 to 4.1 mu m. 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 similar to 0.6 mu m. Prominent atomic and molecular absorption features are identified including neutral lines of Al, Fe, Mg, Ca, Ti, Na, and K and 19 new weak CH(4) absorption 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 (4)Pi - X(4)Sigma(-) transition of VO at similar to 1.06 mu m in the spectra of L dwarfs and the P- and R-branches of the nu(3) band of CH4 in 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 similar to L0, except for the 1.189 mu mFe I line, which persists to at least similar to 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 mu m. Finally, 2MASS J2224381 - 0158521 (L4.5) has an anomalously red spectrum and the strongest Delta nu = +2 CO bands in our sample. This may be indicative of unusually thick condensate clouds and/or low surface gravity.