Stability and bifurcations in ring’s dynamics

We use perturbation theory and bifurcation theory to analyse the dynamical behaviour, associated to a model describing a particle moving within a ring around a celestial object. The central body is modelled as a homogeneous triaxial ellipsoid, rotating about its shortest physical axis at a constant angular velocity. It is assumed that the massless ring particle moves within the equatorial plane of the ellipsoid. The dynamics of the particle is studied using epicyclic variables, that lead to a straightforward definition of corotation and Lindblad resonances. These resonances are associated to a Hamiltonian function with two degrees of freedom, for which we compute appropriate expansions for the normal form and the resonant Hamiltonian. Initially, the normal form is verified to be multi-scale non-degenerate, thereby guaranteeing the existence of invariant KAM tori, providing the stability of the resonances, through their confinement in phase space. Subsequently, two test cases are examined: a nearly spherical ellipsoid and a highly aspherical ellipsoid. Furthermore, this study concentrates on three principal resonances: corotation, 1:2, and 1:3, for which we present results concerning their dynamical behaviour obtained analysing the Hamiltonian formulation of the model and the resonant normal form. Specifically, we examine the phase space structure, the amplitude of libration around the resonances, and the occurrence of bifurcations. Remarkably, in none of the two studied test cases, the resonance 1:3 presents evidence of bifurcations for relevant values of the eccentricity. Our dynamical study in the current model problem thus supports a higher probability of selecting the resonance 1:3 compared to the other resonances.