Twenty-three new ultracool subdwarfs from the Sloan Digital Sky Survey

A search of the Sloan Digital Sky Survey spectroscopic database has turned up 23 new ultracool subdwarfs, low-mass metal-poor stars of spectral subtype M 7.0 or later. Spectra from these red objects all show very strong molecular bands of CaH but relatively weak bands of TiO, indicative of a cool, metal-poor atmosphere. Five of the stars are formally classified as M subdwarfs (sdM7.0-sdM8.5), 13 as more metal-poor extreme subdwarfs (esdM7.0-esdM8.0), and five as extremely metal-poor ultra subdwarfs (usdM7.0-usdM7.5). In the [H(r), r-z reduced proper motion diagram, these subdwarfs clearly populate the locus of low-luminosity stars with halo kinematics. It is argued that the objects are all very low mass, metal-poor stars from the Galactic halo ( Population II). These new discoveries more than double the census of spectroscopically confirmed ultracool subdwarfs. We show that the stars stand out remarkably in the [g-r, r-i] color-color diagram; a proposed color and proper motion selection scheme is expected efficient in identifying more of these old, very low mass stars in the vicinity of the Sun.