Poincaré sections shown in the (ω_1; e₁) space for mass ratio at α_m = 1.1. Gray points stand for Poincaré sections produced under the quadrupole-order model and red points stand for the ones produced under the octupole-order Hamiltonian model. The section is defined by g₂ = π/2. The island centers and their corresponding Hamiltonians are marked in the left panel of Fig. 3. In panel (b), the centers of libration islands arising in the Poincaré section under the octupole-order model are marked by black stars and they are as examples to analyze the arguments of secondary resonance, as shown in Fig. 7.