Origin and dynamical evolution of Neptune Trojans - II. Long-term evolution

Abstract

We present results examining the fate of the Trojan clouds produced in our\nprevious work. We find that the stability of Neptunian Trojans seems to be\nstrongly correlated to their initial post-migration orbital elements, with\nthose objects that survive as Trojans for billions of years displaying\nnegligible orbital evolution. The great majority of these survivors began the\nintegrations with small eccentricities (e < 0.2) and small libration amplitudes\n(A < 30 - 40{\\deg}). The survival rate of "pre-formed" Neptunian Trojans (which\nin general survived on dynamically cold orbits (e < 0.1, i < 5 - 10{\\deg}))\nvaried between ~5 and 70%. By contrast, the survival rate of "captured" Trojans\n(on final orbits spread across a larger region of e-i element space) were\nmarkedly lower, ranging between 1 and 10% after 4 Gyr. Taken in concert with\nour earlier work, we note that planetary formation scenarios which involve the\nslow migration (a few tens of millions of years) of Neptune from an initial\nplanetary architecture that is both resonant and compact (aN < 18 AU) provide\nthe most promising fit of those we considered to the observed Trojan\npopulation. In such scenarios, we find that the current day Trojan population\nwould number ~1% of that which was present at the end of the planet's\nmigration, with the bulk being sourced from captured, rather than pre-formed\nobjects. We note, however, that even those scenarios still fail to reproduce\nthe currently observed portion of the Neptune Trojan population moving on\norbits with e < 0.1 but i > 20{\\deg}. Dynamical integrations of the currently\nobserved Trojans show that five out of the seven are dynamically stable on 4\nGyr timescales, while 2001 QR322, exhibits significant dynamical instability.\nThe seventh Trojan object, 2008 LC18, has such large orbital uncertainties that\nonly future studies will be able to determine its stability.\n