SDSS J125637-022452: a high proper motion L subdwarf

Abstract

We report the discovery of a high proper motion L subdwarf (μ = 0.617" yr^-1) in the Sloan Digital Sky Survey (SDSS) spectral database. The optical spectrum from the star SDSS J125637-022452 has mixed spectral features of both late-M spectral subtype (strong TiO and CaH at 7000 Å) and mid-L spectral subtype (strong wings of KI at 7700 Å, CrH, and FeH), which is interpreted as the signature of a very low-mass, metal-poor star (ultracool subdwarf) of spectral-type sdL. The near-infrared (NIR) (J - K_s) colors from 2MASS show the object to be significantly bluer compared to normal L dwarfs, which is probably due to a strong collision-induced absorption (CIA) from the H₂ molecule. This is consistent with the idea that CIA from H₂ is more pronounced at low metallicities. Proper motion and radial velocity measurements also indicate that the star is kinematically "hot" and probably associated with the Galactic halo population.