Stability, chaos and entrapment of stars in very wide pairs

TL;DR

This paper investigates the long-term stability and chaotic behavior of very wide star pairs in the Galaxy, showing that some can remain bound for billions of years despite tidal forces, with implications for understanding stellar binarity and gravity.

Contribution

It demonstrates the existence of long-lived, stable, yet chaotic orbits of wide star pairs under galactic tidal influences, and explores the possibility of star entrapment.

Findings

Stable orbits can last over 10 Gyr outside the Jacobi radius.

The boundary of stable phase space is fractal and exhibits chaos.

Wide star pairs are observed in significant numbers in the Galaxy.

Abstract

The relative motion of stars and other celestial objects in very wide pairs, separated by distances of the order of 1 pc, is strongly influenced by the tidal gravitational potential of the Galaxy. The Coriolis component of the horizontal tidal force in the rotating reference frame tends to disrupt such marginally bound pairs. However, even extremely wide pairs of bodies can be bound over intervals of time comparable to the Hubble time, under appropriate initial conditions. Here we show that for arbitrary chosen initial coordinates of a pair of stars, there exists a volume of the space of initial velocity components where the orbits remain bound in the planar tidal field for longer than 10 Gyr, even though the initial separation is well outside the Jacobi radius. The boundary of this phase space of stable orbits is fractal, and the motion at the boundary conditions is clearly chaotic. We found that the pairs may remain confined for several Gyr, and then suddenly disintegrate due to a particularly close rendezvous. By reversing such long-term stable orbits, we find that entrapment of unrelated stars into wide pairs is possible, but should be quite rare. Careful analysis of precision astrometry surveys revealed that extremely wide pairs of stars are present in significant numbers in the Galaxy. These results are expected to help discriminating the cases of genuine binarity and chance entrapment, and to make inroads in testing the limits of Newtonian gravitation.