2006ApJ...648..614C

We present a low-resolution (R≡λ/Δλ~90), 5.5-38 µm spectral sequence of a sample of M, L, and T dwarfs obtained with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. The spectra exhibit prominent absorption bands of H₂O at 6.27 µm, CH₄ at 7.65 µm, and NH₃ at 10.5 µm and are relatively featureless at λ≳15 µm. Three spectral indices that measure the strengths of these bands are presented; H₂O absorption features are present throughout the MLT sequence, while the CH₄ and NH₃ bands first appear at roughly the L/T transition. Although the spectra are, in general, qualitatively well matched by synthetic spectra that include the formation of spatially homogeneous silicate and iron condensate clouds, the spectra of the mid-type L dwarfs show an unexpected flattening from roughly 9 to 11 µm. We hypothesize that this may be a result of a population of small silicate grains that are not predicted in the cloud models. The spectrum of the peculiar T6 dwarf 2MASS J0937+2931 is suppressed from 5.5 to 7.5 µm relative to typical T6 dwarfs and may be a consequence of its mildly metal-poor/high surface gravity atmosphere. Finally, we compute bolometric luminosities of a subsample of the M, L, and T dwarfs by combining the IRS spectra with previously published 0.6-4.1 µm spectra and find good agreement with the values of Golimowski et al., who use L'- and M'-band photometry to account for the flux emitted at λ>2.5 µm.