Cool Stars 14 - Submitted Abstract # 266
This version created on 05 October 2006

Properties of the Coolest Dwarfs


Sandy Leggett, Gemini Observatory

Eleven years after the discovery of the first T dwarf, we have a
population of ultracool L and T dwarfs that is large enough to show a
range of atmospheric properties, as well as model atmospheres advanced
enough to study these properties in detail.  Since the last Cool Stars
meeting, there have been observational developments which aid in these
studies.  In this talk I will present recent mid-infrared imaging and
spectroscopy from the Spitzer Space Telescope, and initial results
from the UKIRT Infrared Deep Sky Survey search for ultracool dwarfs.
The mid-infrared data confirms the prevalence of vertical mixing in
the atmospheres of L and T dwarfs, which has a significant impact on
chemical abundances and on the spectral energy distribution.  The 700K
to 2200K L and T dwarf photospheres require a large number of
parameters for successful modelling:  effective temperature, gravity,
metallicity, grain sedimentation and vertical mixing efficiency.  I
will explore the impact of these parameters on infrared colors and
spectra, and demonstrate methods for measuring, or constraining, these
parameters.  The coolest known T dwarfs have effective temperatures of
700-800K.  The properties of these half-dozen objects will be explored
in detail, and synthetic spectra of even cooler objects shown.  The
definition and detection of the next class of object, the Y dwarf,
will be briefly discussed.  The author acknowledges major and multiple
contributions from her collaborators;  full references will be given
during the talk and in the proceedings.

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