Cool Stars 14 - Submitted Abstract # 247
This version created on 05 October 2006

Benchmark Brown Dwarfs: Ultracool Astrophysics from Substellar Binaries
and Companions

Liu, Michael, University of Hawaii
Leggett, Sandy, Gemini Observatory
Looper, Dagny, University of Hawaii
Chiu, Kuenley, University of Exeter
Golimowski, David, JHU
Burgasser, Adam, MIT
Kirkpatrick, J. Davy, IPAC/Caltech
Geballe, Tom, Gemini
Fan, Xiaohui, University of Arizona

Wide-field sky surveys have identified hundreds of ultracool objects and
have been a great boon for substellar astrophysics.  However, such
samples are inevitably hindered by the unknown ages and metallicities of
field objects.  We address two avenues for circumventing this
limitation.

(1) Binary brown dwarfs provide a partial solution, as they constitute
systems of common (albeit unknown) age and metallicity.  Recent imaging
studies by us and others have shown that the strong J-band brightening
seen in early/mid-T spectral types partly arises from unrecognized
binarity but is also an intrinsic photospheric process.  We use the
resolved near-IR properties of known and newly discovered substellar
binaries from Keck laser guide star AO imaging to study the nature of
the L/T transition.

(2) Brown dwarfs that are resolved companions to main-sequence stars are
even more highly prized, as their ages and metallicities can be inferred
from their primary stars.  We present imaging and spectroscopic studies
of two of these rare systems.  As a system whose components possibly
span the L/T transition, we use the Gl 337CD binary (a wide companion to
a nearby K dwarf) to probe the mechanisms of the L/T transition at known
age (1-4 Gyr) and metallicity.  We also present results on a newly
discovered late-T dwarf companion around a main-sequence star, which we
find to be among the coolest and lowest luminosity brown dwarf found to
date.  We compare this object to the T7.5 dwarf Gl 570D (which itself is
a companion to a main-sequence star) in order to examine temperatures
and surface gravity diagnostics of ultracool objects.


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