A Spitzer infrared spectrograph spectral sequence of M, L, and T dwarfs

We present a low-resolution (R lambda/Delta lambda approximate to 90), 5.5-38 mu m 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 mu m, CH4 at 7.65 mu m, and NH3 at 10.5 mu m and are relatively featureless at lambda >= 15 mu m. 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 mu m. 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 mu m 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 mu m 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 lambda > 2.5 mu m.