arXiv subjects

Naoya Torii

Publications and source records attributed to Naoya Torii.

4 recordsLinked to original sources

arXiv2026

Dynamical Evolution of V-Shaped Collision Debris

Catastrophic collisions between proto-satellites have been proposed as a possible origin of Saturn's rings. This argument relies on the concept of the equivalent circular orbit. Here, we re-examine the post-impact dynamical evolution of collision debris using analytical arguments and $N$-body simulations with fragmentation. We focus on the long-term evolution of debris distributed in a broad V-shaped region in the $a$--$e$ plane, with two arms for particles sharing a common collision radius. Because particles on the two arms possess significantly different angular momenta, inter-arm collisions dominate the evolution and drive behavior fundamentally different from the simple circularization assumed in the equivalent circular orbit approach. As a result, the classical equivalent circular orbit concept cannot predict the long-term fate of collision debris. Both our analytical framework and $N$-body simulations show that, although some debris initially passes within the Roche limit on eccentric orbits, successive collisional evolution drives the particles approximately along the original V-shaped constraint curves toward the apex of the V-shape, i.e., the original collision radius. Instead of spreading inward to form a ring, the debris converges and reaccretes near the original collision location. We therefore conclude that catastrophic proto-satellite collisions do not produce massive Saturnian rings. Rather, the debris evolves toward reaccretion into a new generation of satellite-sized bodies near the impact radius. These results fundamentally revise the dynamical interpretation of collision-generated debris and establish a more general framework applicable beyond the Saturnian system, including other planetary ring systems and debris produced during planet formation.

astro-ph.EP↗

arXiv2026

Ring formation around giant planets by tidal disruption of a single passing large Kuiper belt object II: The dynamical fate of tidal fragments

Planetary rings are ubiquitous structure in our Solar System, but their formation mechanisms remain under debate. One of the proposed scenarios is the tidal disruption of a nearby passing body that enters within a planet's Roche limit, producing fragments that are gravitationally captured and finally form the rings. In this study, we investigate the detailed dynamical path and fate of such tidally captured fragments using direct Nbody simulations including collisional fragmentation with analytical arguments. Focusing on Saturn as a representative case, we explore how the inclination iTD and pericenter distance qTD of the orbit of the passing body control the subsequent orbital evolution, collisional grinding, and the survival of fragments mass. Our simulations show that initially highly eccentric and inclined fragments experience differential precession driven by the planet's J2 potential, followed by destructive high-velocity collisions that damp their eccentricities and inclinations. The timing and pathway of this evolution strongly depend on iTD, modifying the dynamical picture proposed in the previous work. For low to moderate iTD, a narrow, circular and equatorial rings finally form whose orbital radius is well predicted by an analytically derived equivalent circular radius based on the conservation of the vertical component of angular momentum. In contrast, for high iTD, collisional damping causes a substantial fraction of the material to fall onto the planet, preventing the formation of a massive ring. We compile our results of Nbody simulations with the analytical predictions on (qTD, iTD) parameter space and specify the parameter region where sufficient mass to form Saturn's present rings and inner satellites survives.

astro-ph.EP↗

arXiv2025

Global N-body Simulation of Gap Edge Structures Created by Perturbations from a Small Satellite Embedded in Saturn's Rings II: The Effect of Satellite's Orbital Eccentricity and Inclination

Pan and Daphnis are embedded in Saturn's rings and opening a gap with satellite wakes at the gap edges. Furthermore, in the case of Daphnis, pronounced vertical wall structures casting shadows on the rings are also observed in the satellite wakes. In this paper, we perform a global 3D N-body simulation with non-zero $e_{\rm s}$ or non-zero $i_{\rm s}$ of the satellite orbit to investigate how they affect the gap edge structures. We found that the effect of satellite eccentricity is important both in the satellite wakes and the vertical walls at the gap edges. The non-sinusoidal sawtooth-like satellite wakes and azimuthally more localized vertical walls observed by Cassini are simultaneously reproduced in the detailed structures and spatial scales. Both of them periodically vary due to the satellite excursions between the apocenter and the pericenter. The ring particles in outer (inner) rings that undergo closest encounters with the satellite near the apocenter (pericenter) are excited the most highly. Because the excited eccentricities of the ring particles are converted to the inclinations through physical collisions, the conversion is the most active for the particles that acquire the highest eccentricities, resulting in the azimuthally more localized vertical wall structures. The predicted height of the tallest vertical walls is $\sim 0.2$ times the satellite Hill radius in the case of the satellite eccentricity comparable to Daphnis when adopting Hill scaling, which is twice as much as the height obtained in the case of the circular satellite orbit and is quantitatively more consistent with the Cassini observation. These results show that the observed vertical walls are actually formed by the satellite wakes followed by their conversion to the vertical motions through inter-particle collisions, rather than by the out-of-plane perturbation from the satellite in an inclined orbit.

astro-ph.EP↗

arXiv2024

Global N-body Simulation of Gap Edge Structures Created by Perturbations from a Small Satellite Embedded in Saturn's Rings

Observations by the Voyager and Cassini spacecrafts have revealed various striking features of the gap structure in Saturn's ring, such as the density waves, sharp edge, and vertical wall structure. In order to explain these features in a single simulation, we perform a high-resolution (N~10^6-10^7) global full N-body simulation of gap formation by an embedded satellite considering gravitational interactions and inelastic collisions among all ring particles and the satellite, while these features have been mostly investigated separately with different theoretical approaches: the streamline models, 1D diffusion models, and local N-body simulation. As a first attempt of a series of papers, we here focus on the gap formation by separating satellite migration with fixing the satellite orbit in a Keplerian circular orbit. We reveal how the striking gap features - the density waves, sharp edge, and vertical wall structure - are simultaneously formed by an interplay of the satellite-ring and ring particle-particle interactions. In particular, we propose a new mechanism to quantitatively explain the creation of the vertical wall structure at the gap edge. Inelastic collisions between ring particles damp their eccentricity excited by the satellite's perturbations to enhance the surface density at the gap edge, making its sharp edges more pronounced. We find the eccentricity damping process inevitably raises the vertical wall structures the most effectively in the second epicycle waves. Particle-particle collisions generally convert their lateral epicyclic motion into vertical motion. Because the excited epicyclic motion is the greatest near the ring edge and the epicycle motions are aligned in the first waves, the conversion is the most efficient in the gap edge of the second waves and the wall height is scaled by the satellite Hill radius, which are consistent with the observations.

astro-ph.EP↗