Spitzer Space Telescope - Theoretical Research Proposal #40140


Understanding L/T Transition Binaries and Y Dwarfs

Principal Investigator: Mark Marley
           Institution: NASA Ames Research Center

     Technical Contact: Mark Marley, NASA Ames Research Center

Co-Investigators: 
Didier Saumon, Los Alamos National Laboratory                         

Science Category: brown dwarfs/very low mass stars 
Dollars Approved: 73653 

Abstract:
We propose two investigations that are directly relevant to understanding
Spitzer observations of L- and T-type brown dwarfs. First, we will provide
important new constraintson the currently unknown mechanism responsible for the
rapid L to T dwarf transition. To do so we will study L/T binary pairs, for
which there are abundant Spitzer and groundbased datasets. Each member of a pair
has the same age and metallicity, but very different spectral properties owing
to their vastly different iron and silicate cloud decks. We will find
self-consistent pairs of models that fit the unresolved Spitzer IRAC and IRS
data and the available resolved near-infrared photometry. These pairs of models,
each with the same age and metallicity, but differing mass, effective
temperature, and cloud properties, will provide new constraints on the nature of
the L to T transition, which is the major outstanding issue in studies of brown
dwarf atmospheres. Secondly, we will produce new models of very cool (Teff < 600
K) brown dwarfs to better understand the spectral signatures associated with the
yet to be discovered Y dwarfs. Existing models in the literature have used
outdated ammonia opacity datasets and do not account for the role of
non-equilibrium chemistry, which we have demonstrated is crucial for
understanding the slightly warmer T dwarfs. Our work will provide new
predictions for the expected IRAC photometric signatures of Y dwarfs (which
critically are fainter in Band 2 than generally expected) and will aid in the
characterization of these objects when they are eventually discovered by various
ongoing Spitzer (as foreground objects) or ground based surveys.