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 Lépine-Shara Proper Motion-North catalog of stars with proper motion μ>0.15^'' yr^-1. Our survey uncovers a total of 521 systems with angular separations 3^''<Δθ<1500^'', with 15 triples and 1 quadruple. Our new list of wide systems with Hipparcos primaries includes 130 systems identified here for the first time, including 44 in which the secondary star has V>15.0. Our census is statistically complete for secondaries with angular separations 20^''<Δθ<300^'' and apparent magnitudes V<19.0. Overall, we find that at least 9.5% of nearby (d<100 pc) Hipparcos stars have distant stellar companions with projected orbital separations s>1000 AU. We observe that the distribution in orbital separations is consistent with Öpik's law, f(s)ds~s^-1ds, only up to a separation s~4000 AU, beyond which it follows a more steeply decreasing power law f(s)ds~s^-lds 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.