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, μ. = 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≲M ≲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 flat 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.