2009ApJ...696..986C

We present the serendipitous discovery of an L subdwarf in the Two Micron All Sky Survey (2MASS) J06164006-6407194, in a search of the 2MASS for T dwarfs. Its spectrum exhibits features indicative of both a cool and metal poor atmosphere including a heavily pressure-broadened K I resonant doublet, Cs I and Rb I lines, molecular bands of CaH, TiO, CrH, FeH, and H₂O, and enhanced collision induced absorption of H₂. We assign 2MASS J0616-6407 a spectral type of sdL5 based on a comparison of its red optical spectrum to that of near solar-metallicity L dwarfs. Its high proper motion (µ = 1".405±0.008/yr), large radial velocity (V_rad= 454±15 km/s), estimated u, v, w velocities (94, -573, 125) km/s and Galactic orbit with an apogalacticon at ∼29 kpc are indicative of membership in the outer halo making 2MASS J0616-6407 the first ultracool member of this population.