Energy levels of the Hamiltonian function (3) on the (X = Rcosϕ, Y = Rsinϕ)-plane, for p_R and J_ϕ fixed at their stationary values given in Eq. (12) and with the parameters from Table 1, except ζ₀ = 6300 km² s^-2 kpc^-1 and i = + 14°. Black lines correspond to the azimuthal minima of the Hamiltonian and are identified with the physical spiral arms. They correspond to the choice ϕ₀ = 0, 2π in the second condition of Eq. (12). Red lines correspond to azimuthal maxima of ℋ; they have ϕ₀ = ± π in the second condition of Eq. (12). Dots represent the stationary solutions of the Hamiltonian system, elliptic (red) and hyperbolic (black). The grey region is of escaping orbits. The escape radius is obtained as R_esc = 21.16 kpc.