Characterization of the Nearby L/T Binary Brown Dwarf WISE
  J104915.57-531906.1 at 2 Parsecs from the Sun
  WISE J104915.57$-$531906.1 is a L/T brown dwarf binary located 2pc from the
Sun. The pair contains the closest known brown dwarfs and is the third closest
known system, stellar or sub-stellar. We report comprehensive follow-up
observations of this newly uncovered system. We have determined the spectral
types of both components (L8+/-1, for the primary, agreeing with the discovery
paper; T1.5+/-2 for the secondary, which was lacking spectroscopic type
determination in the discovery paper) and, for the first time, their radial
velocities (V_rad~23.1, 19.5 km/s) using optical spectra obtained at the
Southern African Large Telescope (SALT) and other facilities located at the
South African Astronomical Observatory (SAAO). The relative radial velocity of
the two components is smaller than the range of orbital velocities for
theoretically predicted masses, implying that they form a gravitationally bound
system. We report resolved near-infrared JHK_S photometry from the IRSF
telescope at the SAAO which yields colors consistent with the spectroscopically
derived spectral types. The available kinematic and photometric information
excludes the possibility that the object belongs to any of the known nearby
young moving groups or associations. Simultaneous optical polarimetry
observations taken at the SAAO 1.9-m give a non-detection with an upper limit
of 0.07%. For the given spectral types and absolute magnitudes, 1Gyr
theoretical models predict masses of 0.04--0.05 M_odot for the primary, and
0.03--0.05 M_odot for the secondary.
