Twenty-three new ultra-cool subdwarfs from the Sloan Digital Sky Survey

TL;DR

This paper reports the discovery of 23 new ultra-cool subdwarfs from SDSS data, significantly increasing the known population of these old, low-mass, metal-poor stars with halo kinematics.

Contribution

It introduces a new sample of ultra-cool subdwarfs, expanding the census and demonstrating effective color and proper motion criteria for future identification.

Findings

23 new ultra-cool subdwarfs identified from SDSS

Subdwarfs show strong CaH and weak TiO bands indicating low metallicity

Color and proper motion selection schemes are highly efficient

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 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 ultra-cool 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 to be extremely efficient in identifying more of these old, very low mass stars in the vicinity of the Sun.