Perturbers. Stars with γ ≥ γ_pert are considered as perturbers of a binary system. Those are the stars represented as empty (red) circles in the above image. The filled (black) circles represent the members of the binary or multiple sub-system. If the dimensionless perturbation γ, given by Eq. (30), becomes greater than γ_crit, then the star is included into the binary or multiple sub-system. More stars could be added forming a multiple system with up to  ~10 stars. Those systems are usually dispersed quickly. On the other hand, a star with γ becoming smaller than γ_pert is no longer a perturber. Those are the stars represented with filled (blue) squares. For an equal mass system, we typically set γ_pert ~ 10^-7 and γ_crit = 0.125 if the inner binary is considered as “soft”, while γ_crit = 0.015625 is assumed if it is considered as “hard”.