Cool Stars 14 - Submitted Abstract # 216
This version created on 05 October 2006

Spitzer Spectroscopy of Ultracool Dwarfs


Michael Cushing, Steward Observatory, University of Arizona
Tom Roellig, NASA Ames
Mark Marley, NASA Ames
Didier Saumon, LANL

We review the mid-infrared spectroscopic observations of M, L, and T
dwarfs obtained with the Infrared Spectrograph (IRS) onboard the
Spitzer Space Telescope.  The spectra cover from 5.5 to 38 microns and
exhibit prominent absorption bands of water at 6.27 microns, methane
at 7.65 microns, and ammonia at 10.5 microns, and are relatively
featureless at wavelength longer than 15 microns.  Both the 7.65
micron methane band and 10.5 micron ammonia band first appear in the
spectral sequence at roughly the L/T transition.  The IRS spectra are
in general well matched by atmospheric models that include the
formation of spatially homogeneous silicate and iron condensate
clouds.  However, the spectra of the mid-type L dwarfs show an
unexpected flattening from roughly 9 to 11 microns that we hypothesize
may be due to a population of small silicate grains that are not
predicted in the cloud models.  Finally we describe the effects of
non-equilibrium chemistry on the abundance of ammonia in the
atmosphere of the T7.5 dwarf GL 570D.  We find that the IRS spectrum
of Gl 570D, and in particular the 10.5 micron band of ammonia, can
only be fitted by reducing the abundance of ammonia by ~1 order of
magnitude from the value obtained with chemical-equilibrium models.

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