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S.J. Kortenkamp (Univ.\ Arizona & Planetary Science Inst., Tucson), R. Malhotra (Univ.\ Arizona), T. Michtchenko (Univ.\ São Paulo)
We investigate the survivability of Trojan-type companions of Neptune during primordial radial migration of the four giant planets. We adopt the usual planet migration model in which the migration speed decreases exponentially with a characteristic time scale \tau. We perform a series of numerical simulations, each involving the migrating giant planets plus ~1000 test particle Neptune Trojans with initial orbital distributions similar to those of the known jovian Trojans asteroids. Migration with \tau = 106 years allows about 35% of pre-existing Neptune Trojans to survive to 5\tau, by which time the giant planets have essentially reached their final orbits. In contrast, migration with \tau = 107 years yields only a ~5% probability of Trojan survival to a time of 5\tau. Interestingly, the loss of Neptune Trojans during planetary migration is not a random diffusion process. Rather, losses occur almost exclusively during discrete episodes when Trojan particles are swept by secondary resonances associated with mean-motion commensurabilities of Uranus with Neptune. These secondary resonances arise when the circulation frequencies, f, of critical arguments for Uranus-Neptune mean-motion near-resonances (e.g., f\mbox{\tiny{UN}}\mbox{\tiny{1:2}}, f\mbox{\tiny{UN}}\mbox{\tiny{4:7}}) are commensurate with harmonics of the libration frequency of the critical argument for the Neptune-Trojan 1:1 mean-motion resonance (f\mbox{\tiny{NT}}\mbox{\tiny{1:1}}). Even tightly bound Neptune Trojans can be lost when they become trapped in 1:3 or 1:2 secondary resonances between f\mbox{\tiny{UN}}\mbox{\tiny{1:2}} and f\mbox{\tiny{NT}}\mbox{\tiny{1:1}}. With \tau = 107 years the latter 1:2 secondary resonance is responsible for the single greatest episode of loss, ejecting nearly 75% of pre-existing Neptune Trojans. This episode occurs during the late stages of planetary migration when the remnant planetesimal disk would have been largely cleared. We speculate that if the number of bodies liberated during this event was sufficiently high it could have caused a spike in the impact rate throughout the solar system.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.