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

Michael C. Cushing, Thomas L. Roellig, Mark S. Marley, D. Saumon, S. K. Leggett, J. Davy Kirkpatrick, John C. Wilson, G. C. Sloan, Amy K. Mainzer, Jeff E. Van Cleve, James R. Houck

Research output: Contribution to journalArticlepeer-review

114 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)614-628
Number of pages15
JournalAstrophysical Journal
Issue number1 I
StatePublished - Sep 1 2006
Externally publishedYes


  • Infrared: stars
  • Stars: late-type
  • Stars: low-mass, brown dwarfs

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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