2MASS J06164006-6407194: The first outer halo L subdwarf

Abstract

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 solarmetallicity L dwarfs. Its high proper motion (μ = 1″.405 ± 0.008 yr-1), large radial velocity (V rad = 454 ± 15 km s-1), estimated u, v, w velocities (94,-573,125) km s-1 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. © 2009. The American Astronomical Society.