Very wide binaries and other comoving stellar companions: A Bayesian analysis of the Hipparcos Catalogue

Abstract

We develop Bayesian statistical methods for discovering and assigning probabilities to non-random (e.g., physical) stellar companions. These companions are either presently bound or were previously bound. The probabilities depend on similarities in corrected proper motion parallel and perpendicular to the brighter component's motion, parallax, and the local phase-space density of field stars. Control experiments are conducted to understand the behavior of false positives. The technique is applied to the Hipparcos Catalogue within 100 pc. This is the first all-sky survey to locate escaped companions still drifting along with each other. In the < 100 pc distance range, ∼220 high probability companions with separations between 0.01 and 1 pc are found. The first evidence for a population (∼300) of companions separated by 1-8 pc is found. We find these previously unnoticed naked-eye companions (both with V < 6th mag): Capella & 50 Per, 5 Vel & HIP 43797, Alioth (∈ UMa), Megrez (Δ UMa) & Alcor, γ & x Cen, Φ Eri & η Hor, 62 & 63 Cnc, γ & x Per, ζ, & Δ Hya, β01, β02 & β03 Tuc, N Vel & HIP 47479, HIP 98174 & HIP 97646, and s Eri & HIP 14913. High probability fainter companions (>6th mag) of primaries with V < 4 are found for: Fomalhaut (α PsA), γ UMa, α Lib, Alvahet (l Cephi), Δ Ara, β Ser, l Peg, β Pic, κ Phe, and γ Tuc. © 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.