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Published September 1, 2006 | Published + Erratum
Journal Article Open

A Spitzer Infrared Spectrograph Spectral Sequence of M, L, and T Dwarfs


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 H_2O at 6.27 μm, CH_4 at 7.65 μm, and NH_3 at 10.5 μm and are relatively featureless at λ ≳ 15 μm. Three spectral indices that measure the strengths of these bands are presented; H_2O absorption features are present throughout the MLT sequence, while the CH_4 and NH_3 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.

Additional Information

Print publication: Issue 1 (2006 September 1) Received 2006 February 21, accepted for publication 2006 May 5 An erratum for this article has been published in 2009 ApJ 706 923 (Published 2009 November 6): In the original article, the widths of the IRS H_2O, IRS CH_4, and IRS NH_3 indices were too small by a factor of 2 and should be 0.3, 0.6, and 0.6 μm, respectively. The correct version of Figure 7 is also presented here. We note that none of the conclusions of the paper are changed using the wider indices and we thank Stan Metchev and Amanda White for bringing this to our attention.


Michael C. Cushing et al 2009 ApJ 706 923 doi: 10.1088/0004-637X/706/1/923

Attached Files

Published - Cushing2006ApJ648-614.pdf

Erratum - Cushing2009p6409Astrophys_J.pdf


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