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

We present near-infrared (1-2.4 μm) imaging of the 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 objects of similar spectral type has been a long-standing question. Our images clearly resolve Kelu-1 into a 0.″29 (5.4 AU) binary with near-infrared flux ratios of ≈0.5 mag. A previous nondetection of binarity by the Hubble Space Telescope demonstrates that the system is a true physical pair and that its projected orbital motion has been significant over the last 7 yr. 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 and 0.045-0.065 M☉ 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 Li 6708 Å absorption indicates that both components are substellar, but the weakness of this feature relative to other L dwarfs can be explained if only Kelu-1B is Li-bearing. Determining whether both or 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.