Distribution of ZLK center determined under the quadrupole-order model (dashed lines) and characteristic curves of periodic orbits determined under the octupole-order model (solid lines) in the (i_tot, e₁) space for mass ratios at α_m=1.1,25,40. To determine periodic orbits associated with ZLK resonance, the initial angles are fixed at g_1,0 = g_2,0 = π/2. The points marked by hollow circles in panels a and c are taken as examples to produce Poincaré sections, as shown in Figs. 1 and 2.