Spitzer Space Telescope - Theoretical Research Proposal #30140


The Role of Dust and Non-Equilibrium Chemistry in the Atmospheres of L
and T Dwarfs as Constrained by Spitzer IRS and IRAC Observations

Principal Investigator: Mark Marley
           Institution: NASA Ames Research Center

Co-Investigators: 
Didier Saumon, Los Alamos National Laboratory                         

Science Category: brown dwarfs/very low mass stars 
Dollars Approved: 67246.0 

Abstract:
We propose to instensively study the physical processes that can
affect the mid-infrared spectra and colors of L- and T-type brown
dwarfs as observed by Spitzer.  In particular we will modify our
current atmosphere structure and spectral models of these objects to
understand the origin of the unexpected flattening seen at 8 to 10
microns in IRS spectra of mid L-type objects.  This flattening is
likely due to a larger abundance of small silicate grains in the L
dwarf clouds than predicted by our model.  By producing a large number
of models with a variety of grain sizes and compositions (enstatite,
forsterite, and possibly quartz, including both crystalline and
amorphous phases) we expect to constrain the composition and vertical
structure of L dwarf cloud layers.  We will also explore the role of
non-equilibrium chemistry driven by atmospheric mixing in affecting
the abundances of CO and ammonia in late L and T dwarf atmospheres.
Excess CO lowers the flux in IRAC band 2 below that predicted by
models that assume equilibrium chemistry and makes cool brown dwarfs
more difficult to detect than predicted by equilibriium models (an
important point for surveys aiming at detecting T and Y dwarfs).
Lower than expected ammonia abundances are apparent in the shape of
the ammonia absorption feature seen in IRS spectra at 10  microns.