An Astrometric Companion to the Nearby Metal‐Poor, Low‐Mass Star LHS 1589

We report the discovery of a companion to the high proper motion star LHS1589, a nearby high-velocity, low-mass subdwarf. The companion (LHS 1589B) islocated 0.224+/-0.004" to the southwest of the primary (LHS 1589A), and is 0.5magnitude fainter than the primary in the K_s band. The pair was resolved withthe IRCAL infrared camera at Lick Observatory, operating with the Laser GuideStar Adaptive Optics system. A low-resolution spectrum of the unresolved pairobtained at the MDM observatory shows the source to be consistent with a coolsubdwarf of spectral subtype sdK7.5. A photometric distance estimate places themetal-poor system at a distance d=81+/-18 parsecs from the Sun. We also measurea radial velocity V_rad=67+/-8 km/s which, together with the proper motion andestimated distance, suggests that the pair is roaming the inner Galactic haloon a highly eccentric orbit. With a projected orbital separation s=18.1+/-4.8AU, and a crude estimate of the system's total mass, we estimate the orbitalperiod of the system to be in the range 75 yr < P < 500 yr. This suggests thatthe dynamical mass of the system could be derived astrometrically, aftermonitoring the orbital motion over a decade or so. The LHS 1589AB system couldthus provide a much needed constraint to the mass-luminosity relationship ofmetal-poor, low-mass stars.Comment: 6 pages, accepted for publication in the Astrophysical Journal. This revised version includes an expanded discussion and minor correction