The first substellar subdwarf? Discovery of a metal-poor L dwarf with halo kinematics

We present the discovery of the first L-type subdwarf, 2MASS J05325346+8246465. This object exhibits enhanced collision-induced H-2 absorption, resulting in blue near-infrared (NIR) colors (J-K-s = 0.26 +/- 0.16). In addition, strong hydride bands in the red optical and NIR, weak TiO absorption, and an optical/J-band spectral morphology similar to the L7 DENIS 0205-1159AB imply a cool, metal-deficient atmosphere. We find that 2MASS 0532+8246 has both a high proper motion, mu = 2.60 +/- 0.15 yr(-1), and a substantial radial velocity, v(rad) = -195 +/- 11 km s(-1), and its probable proximity to the Sun ( d = 10 30 pc) is consistent with halo membership. Comparison to subsolar-metallicity evolutionary models strongly suggests that 2MASS 0532+8246 is substellar, with a mass of 0.077 less than or similar to M less than or similar to 0.085 M. for ages 10-15 Gyr and metallicities Z = 0.1-0.01 Z. The discovery of this object clearly indicates that star formation occurred below the hydrogen burning mass limit at early times, consistent with prior results indicating a. at or slightly rising mass function for the lowest mass stellar subdwarfs. Furthermore, 2MASS 0532+8246 serves as a prototype for a new spectral class of subdwarfs, additional examples of which could be found in NIR proper-motion surveys.