Twenty-three new ultra-cool subdwarfs from the Sloan Digital Sky Survey
Abstract
A search of the Sloan Digital Sky Survey spectroscopic database has turned up 23 new ultra-cool subdwarfs, low-mass metal-poor stars of spectral subtype M 7.0 or later. Spectra from these red objects all show 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), thirteen as more metal-poor extreme subdwarfs (esdM7.0-esdM8.5), and five as extremely metal-poor ultra subdwarfs (usdM7.0-usdM8.0). In the [H_r,r-z] reduced proper motion diagram, the subdwarfs clearly populate the locus of low-luminosity stars with halo kinematics (Population II). These new discoveries more than double the census of spectroscopically confirmed ultra-cool subdwarfs. It is shown that the stars stand out remarkably in the [g-r,r-i] color-color space. A proposed color and proper motion selection scheme is expected to be extremely efficient in identifying more of these old, very low-mass stars in the vicinity of the Sun.Keywords
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