On the inclinations of the Jupiter Trojans

TL;DR

This paper investigates the high inclinations of Jupiter Trojans within a model of Jupiter's inward migration, exploring various scenarios including secular evolution, planetary embryos, and pre-stirred populations, to explain their inclination distribution and asymmetry.

Contribution

The study introduces a comprehensive analysis of the origin of Trojan inclinations, emphasizing the role of embryo-induced stirring and the impact of Saturn's perturbations during Jupiter's migration.

Findings

Embryos can stir Trojan inclinations and deplete swarms but are hard to eliminate entirely.

Secular evolution of Trojans and Saturn does not significantly alter inclinations.

High inclinations are preserved through the gas phase and are affected by Saturn's perturbations.

Abstract

Jupiter Trojans are are characterized by dark photometric colors, high inclinations and an asymmetry in number of bodies between the two swarms. Different models have been proposed to explain the high inclination of the Trojans and to interpret their relation with the TNOs, but none of them can also satisfactorily explain the asymmetry ratio. Recently it has been found that the asymmetry can arise if Jupiter has migrated inwards by at least a few au during its growth. The asymmetry and the dark colors of the Trojans are natural outcomes of this model, but simulations with massless unperturbed disc particles led to a flat distribution of the Trojan inclinations and a final total mass that was 3-4 orders of magnitude larger than the current one. In our work, we investigate the possible origin of the peculiar inclination distribution of the Trojans in the scenario where Jupiter migrates inwards. We analyze different possibilities: (a) the secular evolution of an initially flat Trojan population, (b) the presence of planetary embryos among the Trojans and (c) capture of the Trojans from a pre-stirred planetesimal population. We find that the secular evolution of the Trojans and Saturn do not affect the Trojan inclinations appreciably, nor is there any significant mass depletion. Embryos embedded in the swarms, in contrast, can stir the Trojan inclinations and can also deplete the swarms efficiently, but it turns out that it is very difficult to get rid of all of the massive bodies. We propose that the disc where Jupiter's core was forming was already excited by the presence of other embryos competing in Jupiter's core's feeding zone. We show that the trapped Trojans preserve their high inclination through the gas phase of the disc and that Saturn's perturbations are more effective on highly inclined Trojans, leading to a lower capture efficiency and to a substantial depletion of the swarms.