Paper: astro-ph/0508082
Date: Tue, 2 Aug 2005 22:52:23 GMT (128kb)

Title: Kelu-1 is a Binary L Dwarf: First Brown Dwarf Science from Laser Guide
Star Adaptive Optics

Authors: Michael C. Liu (IfA/Hawaii), Sandy K. Leggett (UKIRT/JAC)

Comments: Astrophysical Journal, in press. 24 pages. (Note that Figure 1 of the
PDF version is degraded by arxiv.org. The Postscript version is fine.)
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(Abridged) We present near-IR imaging of the nearby L dwarf Kelu-1 obtained
with the Keck sodium laser guide star adaptive optics (LGS AO) system as part
of a high angular resolution survey for substellar binaries. Kelu-1 was one of
the first free-floating L dwarfs identified, and the origin of its
overluminosity compared to other similar objects has been a long-standing
question. Our images clearly resolve Kelu-1 into a 0.29'' (5.4 AU) binary, and
a previous non-detection by HST demonstrates that the system is a true physical
pair. Binarity explains the properties of Kelu-1 that were previously noted to
be anomalous compared to other early-L dwarfs. We estimate spectral types of
L1.5-L3 and L3-L4.5 for the two components, giving model-derived masses of
0.05-0.07 Msun and 0.045-0.065 Msun for an estimated age of 0.3-0.8 Gyr. More
distant companions are not detected to a limit of 5-9 Mjup. The presence of
lithium absorption indicates that both components are substellar, but the
weakness of this feature relative to other L dwarfs may arise from the fact
that only Kelu-1B is Li-bearing. Determining if both or if only one of the
components possesses lithium could constrain the age of Kelu-1 (and other
Li-bearing L binaries) with higher precision than is possible for most
ultracool field objects. These results are the first LGS AO observations of
brown dwarfs and demonstrate the potential of this new instrumental capability
for substellar astronomy.

\\ ( http://arXiv.org/abs/astro-ph/0508082 , 128kb)