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 H[subscript 2]O, and enhanced collision induced absorption of H[subscript 2]. 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 = 1farcs405 ± 0.008 yr[superscript –1]), large radial velocity (V rad = 454 ± 15 km s[superscript –1]), estimated u, v, w velocities (94, –573, 125) km s[superscript –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.