Temporary capture of planetesimals by a giant planet and implication for the origin of irregular satellites

Performing global three-body orbital integration, we investigate temporary capture of planetesimals by a giant planet from their heliocentric orbits. We confirm four types of capture orbits, which were found in our previous local simulation. Shapes of relatively small temporary capture orbits in the vicinity of the planet's Hill sphere are similar to those found in local simulations, while large capture orbits outside of the Hill sphere in the case of a giant planet are found to be elongated in the azimuthal direction, reflecting the curvature of the planet's orbit. We find that basic features of temporary capture rates are similar to previous results of local simulation, i.e., they increase nearly monotonically with increasing eccentricity and that prograde temporary capture rates have a peak at a certain value of planetesimal orbital eccentricity. Contribution to the capture rates from orbits interior to the planet's orbit is equal to that from exterior orbits under the local approximation. We find that this symmetry breaks down when the planet is as massive as giant planets. In this case, long-lived prograde temporary capture with low energy is dominated by planetesimals initially on interior orbits, since potential energy at the L-1 point is lower than that at the L-2 point. We also examine source regions for long-lived prograde capture based on our numerical results and analytic consideration, because such low-velocity encounters with a planet may facilitate capture of irregular satellites. We find that the source region interior to the planet's orbit in the case of Jupiter corresponds to the Hilda region in the outer main belt, indicating that some of the prograde irregular satellites of Jupiter may have been delivered from this region.