OSSOS: The Eccentricity and Inclination Distributions of the Stable Neptunian Trojans

TL;DR

This study analyzes the orbital distribution of stable Neptunian Trojans, revealing a bimodal 'hot' and 'cold' population structure with implications for Neptune's migration history.

Contribution

It introduces a model of Neptunian Trojan populations with distinct hot and cold components, based on survey data and orbital distribution analysis.

Findings

Neptunian Trojan libration amplitudes follow a Rayleigh distribution with σ_Aphi of 15°.

The eccentricity and inclination distributions are best modeled with bimodal 'hot' and 'cold' components.

The 'hot' population exceeds the 'cold' in number, especially among brighter Trojans.

Abstract

The minor planets on orbits that are dynamically stable in Neptune's 1:1 resonance on Gyr timescales were likely emplaced by Neptune's outward migration. We explore the intrinsic libration amplitude, eccentricity, and inclination distribution of Neptune's stable Trojans, using the detections and survey efficiency of the Outer Solar System Origins Survey (OSSOS) and Pan-STARRS1. We find that the libration amplitude of the stable Neptunian Trojan population can be well modeled as a Rayleigh distribution with a libration amplitude width $\sigma_{A_phi}$ of 15$^\circ$. When taken as a whole, the Neptune Trojan population can be acceptably modeled with a Rayleigh eccentricity distribution of width $\sigma_e$ of 0.045 and a typical sin(i) x Gaussian inclination distribution with a width $\sigma_i$ of 14 +/- 2 degrees. However, these distributions are only marginally acceptable. This is likely because, even after accounting for survey detection biases, the known large Hr < 8 and small Hr >= 8 Neptune Trojans appear to have markedly different eccentricities and inclinations. We propose that like the classical Kuiper belt, the stable intrinsic Neptunian Trojan population have dynamically `hot' and dynamically `cold' components to its eccentricity/inclination distribution, with $\sigma_{e-cold}$ ~ 0.02 / $\sigma_{i-cold}$ ~ 6$^\circ$ and $\sigma_{e-hot}$~ 0.05 / $\sigma_{i-hot}$ ~ 18$^\circ$. In this scenario, the `cold' L4 Neptunian Trojan population lacks the Hr >= 8 members and has 13 +11/-6 `cold' Trojans with Hr < 8. On the other hand, the `hot' L4 Neptunian Trojan population has 136 +57/-48 Trojans with Hr < 10 -- a population 2.4 times greater than that of the L4 Jovian Trojans in the same luminosity range.