A formation mechanism for narrow rings around minor bodies

Context. The recent discovery of narrow rings around minor bodies has raised many questions regarding their origin and current dynamics. Sharp ring boundaries seem indicative of shepherding moonlets, but none have been found. All rings lie close to spin-orbit resonances (SORs) with the central body, particularly the 1/3, even though it is not clear how these may be related. Furthermore, in at least one case the location of the ring is exterior to the Roche radius, adding to the striking differences with respect to giant planets. Aims. We study the dynamical evolution of a particle disk around a minor body, perturbed by the non-spherical component of the gravity field, particle collisions, and spin changes of the central mass linked to angular momentum conservation. By varying key parameters, we search for cases where the combined effects may lead to resonance capture and orbital configurations similar to those that have been observed. Methods. We performed N-body simulations of massless particles orbiting a central spherical body with a co-rotating mass anomaly. Collisions were modeled by adopting a simple radial damping force. Angular momentum conservation links the body’s spin to the disk’s orbital evolution. Since the gravitational effect of the test particles is neglected, this back-reaction is introduced externally assuming ad hoc spin-down rates and disk mass. Results. Interaction between non-sphericity and collisions leads to the formation of a narrow ring that slowly recedes from the central mass. Spin-down of the minor planet shifts the SORs outward, enabling resonant capture. For suitable parameters, a portion – or all – of the initial disk can become trapped in the 1/3 SOR with the mass anomaly, in dynamically stable low-eccentricity orbits. Although the required disk mass is high (≳1% of the central body), long-term collisional erosion could reduce it to values that are consistent with observed ringlets.