[Paper Review] 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

J. A. Caballero|arXiv (Cornell University)|Jan 29, 2010

Stellar, planetary, and galactic studies74 references26 citations

TL;DR

This paper proposes that α Lib (Zubenelgenubi) and KU Lib form a gravitationally bound, ultra-wide common proper motion binary system in the 200 Myr-old Castor moving group, separated by 1.0 pc—making it the widest known multiple system across all mass domains. Despite its extreme separation, the system's high binding energy and youth suggest it is on the verge of disruption by Galactic tidal forces, offering a rare observational window into the dynamical evolution of young wide binaries.

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.

Motivation & Objective

• To investigate the gravitational binding of α Lib and KU Lib, a common proper motion pair separated by 2.6° (1.0 pc), in the young Castor moving group.
• To determine whether this system represents a true physical binary or a chance alignment, given its extreme projected separation.
• To assess the system's binding energy relative to other known wide binaries and test its stability against Galactic tidal forces.
• To compile a comprehensive list of stellar candidates in the Castor moving group, including new discoveries.
• To explore the boundary between kinematic groups and very wide binaries, particularly in the context of young, dynamically fragile systems.

Proposed method

• Compiled astrometric, photometric, and radial velocity data for α Lib and KU Lib from Hipparcos, SIMBAD, and other databases to assess common proper motion and physical parameters.
• Calculated the projected physical separation (s ≈ 217 kAU ≈ 1.0 pc) using parallactic distances and angular separation.
• Estimated the gravitational binding energy (|U_g*| ≈ 46.1 × 10^33 J) using the formula for binary binding energy, comparing it to other known wide binaries.
• Evaluated the system's stability against Galactic tidal forces using the critical separation limit derived from Close et al. (1990), which predicts disruption at ~6.0 pc for this mass combination.
• Applied a common proper motion filter using Aladin and USNO-B1 data to identify and validate candidate wide pairs.
• Conducted a virtual observatory survey of the Castor moving group to compile a list of 15 star candidates, including five new ones (KU Lib, VV Lyn AB, 36 UMa A, μ Dra C, V447 Lac B).

Experimental results

Research questions

• RQ1 Is the α Lib–KU Lib pair a physically bound system or a chance alignment, given their 1.0 pc projected separation?
• RQ2 What is the gravitational binding energy of the α Lib–KU Lib system, and how does it compare to other known wide binaries?
• RQ3 Can the system survive the disruptive influence of Galactic tidal forces over its lifetime in the disc?
• RQ4 What is the significance of this system in the context of the boundary between stellar kinematic groups and very wide binaries?
• RQ5 How many additional candidate wide binaries exist in the Castor moving group, and what is their statistical significance?

Key findings

• α Lib and KU Lib share nearly identical proper motions, parallaxes, radial velocities, metallicities, and youth indicators, confirming their common proper motion and physical association.
• The system has a binding energy of |U_g*| ≈ 46.1 × 10^33 J, which exceeds that of the α Cen–Proxima system (–32.1 × 10^33 J) and is an order of magnitude higher than that of fragile, very low-mass wide binaries.
• The projected physical separation of 1.0 pc corresponds to 217 kAU, making α Lib and KU Lib the widest known multiple system in all mass domains.
• The system is likely in the process of disruption by Galactic tidal forces, with a theoretical disruption limit of ~6.0 pc for its mass combination, indicating it may survive only a few more Galactic orbits.
• The study identifies five new stellar candidates in the Castor moving group: KU Lib (itself), VV Lyn AB, 36 UMa A, μ Dra C, and V447 Lac B.
• A new wide pair, V450 And AB and V451 And, is found at a projected separation of ~16 kAU (0.076 pc), further extending the known population of very wide binaries.