System mass – projection angle diagram of the analytic bimodal model applied to the two subclusters. Thin/red (thick/blue) lines refer to the case-1D (case-3D; see text). In particular, bound (solid curves) and incoming (collapsing; BI_a and BI_b), bound outgoing (expanding; BO), and unbound (dashed curves) outgoing (UO) solutions are shown. The horizontal lines give the computed values of the system mass for the two cases. The dotted curves separate bound and unbound regions (above and below the dotted curves, respectively).