Reaching the boundary between stellar kinematic groups and very wide binaries^*

The Washington double stars with the widest angular separations

Departamento de Astrofísica y Ciencias de la Atmósfera, Facultad de Física, Universidad Complutense de Madrid, 28040 Madrid, Spain e-mail: caballero@astrax.fis.ucm.es

Received: 29 May 2009
Accepted: 12 August 2009

Abstract

Aims. I look for and characterise very wide binaries and multiple systems with projected physical separations larger than  pc, which is generally believed to be a sharp upper limit to the distribution of wide binary semimajor axes.

Methods. I investigated in detail 30 Washington double stars with angular separations of ρ > 1000 arcsec. I discarded 23 of them as probably unbound systems based on discordant astrometry, photometry, spectral types, and radial velocities. The remaining seven systems were subject to a comprehensive data compilation and derivation (multi-wavelength photometry, heliocentric distance, multiplicity, age, mass, metallicity, membership in a young kinematic group).

Results. Of the seven very wide systems, six have projected physical separations greater than the hypothetical cutoff at  pc and four have separations  pc. Although there are two systems in young kinematic groups (namely HD 136654 and BD+32 2572 in the Hyades Supercluster, and AU Mic and AT Mic AB in the β Pictoris moving group), there is no clear prevalence of young systems (τ < 1 Ga) among these very wide binaries. Finally, I compare the binding energies of the seven systems with those of other weakly bound systems in the field.