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Low-metallicity L and T Dwarfs
J. Davy Kirkpatrick (IPAC/Caltech)
Dagny L. Looper (IfA/UHawaii)
Adam J. Burgasser (MIT)
Travis R. Barman (Lowell Obs.)
The 2MASS Proper Motion Survey
2MASS includes multiple-epoch coverage
of ~4700 sq. deg. We have used these data
to look for near-IR proper motion objects.
We have uncovered ~150 motion objects
with no optical counterparts in B- or Rband DSS data. We have followed up most
of these using SpeX at the IRTF.
The goal of this survey is to uncover halo
(low-metallicity) brown dwarfs whose
colors are poorly known, along with other
cool, nearby exotica.
I-band DSS image from 1989 (blue) and
J-band 2MASS image from 1999 (red).
Motion object is marked.
image showing
motion of
source over 10
Example subdwarfs from our
SpeX prism spectra of two of our
subdwarf discoveries (red)
compared to standard L dwarfs of
the same spectral type (black).
The most noticeable difference is
the suppressed H- and K-bands in
the subdwarfs. This is due to the
dominance of collision-induced
absorption by H2 in these lowmetallicity atmospheres.
Why are subdwarf brown
dwarfs important?
Star Formation: These discoveries show further evidence that lowmetallicity clouds still produce very low-mass objects. Although more of
these very cool subdwarfs are needed before that formation efficiency can be
compared to brown dwarf formation at current epochs, this survey has
nonetheless provided additional, very important clues on searching for other
such objects using photometry alone.
Extrasolar Giant Planets: These subdwarfs allow us to study cool
atmospheres with low values of [M/H], which is an important test of
theoretical models. These objects have temperatures comparable to some of
the newly found extrasolar planets and thus provide low-metallicity fiducials
for the study of gas giant atmospheres.