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

TL;DR

This paper investigates whether the wide star system alpha Lib and KU Lib in the Castor group are gravitationally bound, providing evidence that they form the widest known young multiple system, which may be on the verge of disruption.

Contribution

It presents the first detailed analysis confirming that alpha Lib and KU Lib form a gravitationally bound system separated by 1.0 pc, expanding understanding of wide binaries in young stellar groups.

Findings

alpha Lib and KU Lib share similar proper motion, distance, and metallicity

The system is likely gravitationally bound despite its extreme separation

It is the widest known young multiple star system

Abstract

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.