Moderate-resolution spitzer infrared spectrograph observations of M, L, and T dwarfs

We present 10-19 μn moderate-resolution spectra of 10 M dwarfs, one L dwarf, and two T dwarf systems obtained with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. The IRS allows us to examine molecular spectroscopic features/lines at moderate spectral resolution in a heretofore untapped wavelength regime. These R = λ/△λ ∼ 600 spectra allow for a more detailed examination of clouds and nonequilibrium chemistry, as well as the molecular features of H₂O, NH₃, and other trace molecular species that are the hallmarks of these objects. A cloud-free model best fits our mid-infrared spectrum of the T1 dwarf ε Indi Ba, and we find that the NH3 feature in e Indi Bb is best explained by a nonequilibrium abundance due to vertical transport in its atmosphere. We examined a set of objects (mostly M dwarfs) in multiple systems to look for evidence of emission features, which might indicate an atmospheric temperature inversion, as well as trace molecular species; however, we found no evidence of either.