Rings around irregular bodies

Context . Three ring systems have been discovered to date around small irregular objects of the Solar System (Chariklo, Haumea, and Quaoar). For the three bodies, material is observed near the second-order 1/3 spin-orbit resonance (SOR) with the central object, and in the case of Quaoar, a ring is also observed near the second-order resonance 5/7 SOR. Aims . This suggests that second-order SORs may play a central role in ring confinement. This paper aims to better understand this role from a theoretical point of view. It also provides a basis to better interpret the results obtained from N -body simulations and presented in a companion paper. Methods . A Hamiltonian approach yields the topological structure of phase portraits for SORs of orders from one to five. Two cases of non-axisymmetric potentials are examined: a triaxial ellipsoid characterized by an elongation parameter, C 22 , and a body with mass anomaly µ , a dimensionless parameter that measures the dipole component of the body’s gravitational field. Results . The estimated triaxial shape of Chariklo shows that its corotation points are marginally unstable, those of Haumea are largely unstable, and those of Quaoar are safely stable. The topologies of the phase portraits show that only first- (aka Lindblad) and second-order SORs can significantly perturb a dissipative collisional ring. We calculated the widths, maximum eccentricities, and excitation timescales associated with first- and second-order SORs, as a function of C 22 and µ . Applications to Chariklo, Haumea, and Quaoar using µ ≲ 0.001 show that the first- and second-order SORs caused by their triaxial shapes excite large (≳0.1) orbital eccentricities on the particles, making the regions inside the 1/2 SOR inhospitable for rings. Conversely, the 1/3 and 5/7 SORs caused by mass anomalies excite moderate eccentricities (≲0.01), and are thus more favorable places for the presence of a ring.