Reaching the boundary between stellar kinematic groups and very wide binaries

Abstract

Aims. With the latest Gaia DR3 data, we analyse the widest pairs in the Washington Double Star (WDS) catalogue with angular separations, ρ, greater than 1000 arcsec. Methods. We confirmed the pairs’ membership to stellar systems based on common proper motions, parallaxes, and (when available) radial velocities, together with the locii of the individual components in colour-magnitude diagrams. We also looked for additional closer companions to the ultrawide pairs, either reported by WDS or found by us with a new Gaia astrometric search. In addition, we determined masses for each star (and white dwarf) and, with the projected physical separation, computed the gravitational potential energy, |Ug*|, of the systems. Results. Of the 155 159 pairs currently catalogued by WDS, there are 504 with ρ > 1000 arcsec. Of these, only 2 ultrawide pairs have not been identified, 10 do not have any available astrometry, 339 have not passed a conservative filtering in proper motion or parallax, 59 are members of young stellar kinematic groups, associations or open clusters, and only 94 remain as bona fide ultrawide pairs in the galactic field. Accounting for the additional members at shorter separations identified in a complementary astrometric and bibliographic search, we found 79 new stars (39 reported, plus 40 not reported by WDS) in 94 ultrawide stellar systems. This sample is expanded when including new close binary candidates with large Gaia DR3 RUWE, σVr, or a proper motion anomaly. Furthermore, the large fraction of subsystems and the non-hierarchical configurations of many wide systems with three or more stars is remarkable. In particular, we found 14 quadruple, 2 quintuple, 3 sextuple, and 2 septuple systems. The minimum computed binding energies, |Ug*| ~ 1033 J, are in line with theoretical predictions of tidal destruction by the Galactic gravitational potential. The most fragile and massive systems have huge projected physical separations of well over 1 pc. Therefore, they are either in the process of disruption or they are part of unidentified juvenile stellar kinematic groups.