Reaching the boundary between stellar kinematic groups and very wide
  binaries. II. alpha Lib + KU Lib: a common proper motion system in Castor
  separated by 1.0 pc
  Aims: I investigate the gravitational binding of a nearby common proper
motion system in the young Castor moving group (tau ~ 200 Ma), formed by the
bright quadruple star alpha Lib (Zubenelgenubi) and the young solar analog KU
Lib. The system has an exceptionally wide angular separation, of about 2.6 deg,
which corresponds to a projected physical separation of about 1.0 pc. Methods:
I compile basic information of the system and compare its binding energy with
those of other weakly bound systems in the field, and study the physical
separations of resolved multiple systems in Castor. Results: KU Lib has roughly
the same proper motion, parallactic distance, radial velocity, and metallicity
than the young hierarchical quadruple system alpha Lib. Besides, KU Lib also
displays youth features. The resemblance between these basic parameters and the
relatively large estimated binding energy point out that the five stars are
gravitationally bound. KU Lib and alpha Lib constitute the widest known
multiple system at all mass domains, and likely represent the most extreme
example of young wide binaries on the point of being disrupted. Besides, I make
a comprehensive compilation of star candidates in Castor, including new ones.
