New Distant Companions to Known Nearby Stars. II. Faint companions of Hipparcos stars and the frequency of wide binary systems

Abstract

We perform a search for faint, common proper motion companions of Hipparcos stars using the recently published LSPM-north catalog of stars with proper motion mu>0.15 arcsec/yr. Our survey uncovers a total of 521 systems with angular separations 3" 15.0. Our census is statistically complete for secondaries with angular separations 20" 1,000 AU. We observe that the distribution in orbital separations is consistent with Opik's law f(s) ds ~ s^{-1} ds only up to separation s~4,000 AU, beyond which it follows a more steeply decreasing power law f(s) ds ~ s^{-l} ds with l=1.6+/-0.1. We also find that the luminosity function of the secondaries is significantly different from that of the single stars field population, showing a relative deficiency in low-luminosity (8<M_V<14) objects. The observed trends suggest either a formation mechanism biased against low-mass companions, or a disruption over time of systems with low gravitational binding energy