M. A. Herrera

The Distribution of Separations of Wide Binaries

Several recent observational determinations of the distribution of separations of wide binaries are critically reviewed and compared with the distributions inferred from observational work on pre-main sequence stars and from theoretical ideas on binary formation. It is concluded that it is not very fruitful to depict the whole distribution function of separations by one law, and that it is preferable to represent separately the two main mechanisms of binary formation, namely first-collapse fragmentation (for s > 25 AU) and disk fragmentation (for s 25 AU) is given by Opik’s law ρ(s) = ks -2, but that this distribution has a cutoff that is age-dependent. This cutoff is interpreted in terms of the dynamical evolution of wide binaries.

Evolutionary Effects in the Separations of Wide Binaries

Utilizing the authors’ recent catalogue of wide double and multiple stars for the solar vicinity (Poveda et al. 1994) we investigate the distribution of separations of their components. We find that the distribution of separations, f(s), is well represented by Opik’s relation f(s) ∝ s -1 for separations smaller than 2,400 AU. We find an age-dependent critical separation S c which divides those binaries that have been affected by energy exchanges in encounters with perturbing objects from those whose separations are still unchanged from their primeval distribution. For the group of youngest systems from our catalogue S c = 7, 800 AU, and for the oldest, S c = 2, 400 AU. The velocity dispersions for the first group give a mean age of 2.5 × 109 years, while those for the oldest systems yield 7 × 109 years.

Three Very Metal-Poor, Extremely Wide Binaries

We have searched for wide binaries among the metal-poor, large proper-motion stars from the catalogues of Schuster et al. Out of 48 stars with [Fe/H] < -2.00 we have identified three common-proper-motion companions which turn out to have extreme characteristics. Of particular interest are the very large expected major semiaxes, all larger than 20 000 AU, and up to 125 000 AU. We argue that the persistence of such old wide systems is the result of the small fraction of their lifetimes that the binaries spend within the galactic disk, as well as to their large space velocities. These binaries may be remnants of long-ago disrupted globular clusters or dwarf galaxies.

The Distribution of Separations of Binaries

We examine the distribution of separations (major semiaxes) between components in wide binaries for various samples of double stars: IDS, binaries in the solar vicinity, binaries in young clusters (Hyades and Orion Nebula Cluster) and binaries among high-velocity, low-metallicity stars. For all the groups studied, we find that binaries follow Opik’s distribution of separations in the interval where sc(t) is an age-dependent critical separation.