Reaching the boundary between stellar kinematic groups and very wide binaries. The Washington Double Stars with the widest angular separations

TL;DR

This study investigates extremely wide stellar systems beyond the traditional separation limit, finding several systems with separations larger than 0.1 parsecs and analyzing their properties and binding energies.

Contribution

It provides a detailed analysis of very wide binaries with separations exceeding 0.1 pc, challenging the assumed upper limit of wide binary separations.

Findings

Six systems have separations larger than 0.1 pc.

Four systems have separations greater than 0.2 pc.

No clear correlation between system age and wide separation.

Abstract

Aims. I look for and characterise very wide binaries and multiple systems with projected physical separations larger than s = 0.1 pc, which is generally believed to be a sharp upper limit to the distribution of wide binay semimajor axes. Methods. I investigated in detail 30 Washington Double Stars with angular separations rho > 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 young kinematic group). Results. Of the seven very wide systems, six have projected physical separations larger than the hypothetical cutoff at s = 0.1 pc; four of them have separations s > 0.2 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 beta Pictoris moving group), there is not a clear prevalence of young systems (tau < 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.