The Orbits of Neptune Trojans
|Course||Astrometry and celestial mechanics|
|Keywords||Neptune Trojans Numerical Simulation Solar System Dynamics|
The Neptune Trojan(NT for abb. hereafter) cloud is the newly found and po-tentially the fourth asteroids reservior besides the Main Belt, the Jupiter Trojan and the Kuiper Belt. Though it has only8members from2001QR322’s announcement in2003till now, it has received great attention by the astronomers, considering their origin, colors, the size or their orbital stability. The so called4:1enigma-the high inclination orbits is3times more than the low inclination orbits—is still not explained. It is widely accepted that the high inclination orbits are captured during the planetary migration rather than excited during the peaceful evolvement period after the planets are settled. According to this mechanism, the planet capture the planetesimals into the1:1mean motion resonance(MMR) during their migration. Due to the chaotic property of migration, planetesimals are permitted to enter the high inclination zone, and when the migration anneals, the transiently captured NTs are frozen on high inclination or-bits. But there are two questions about this scenario:First, the transiently captured NTs always get high inclination as well as high eccentricity,but dynamical astronomy results show that those who get eccentricity larger than0.1would not be stable to sur-vive till today. And this is the main problem this scenario should face. Second, the number of NTs excited by this mechanism seems not bigger than those not excited. So from the quantitative aspect, this mechanism did’t resolve the4:1enigma.We propose a different ideaf about this problem in Chapter2in this thesis. Dy-namical analysis show that the Trojan region is sculpt by lots of resonances, containing a series of secondary resonance related to the quasi-2:1MMR between Uranus and Neptune, the Lidov-Kozai resonance as well as a series of secular resonances. Among them the first ones have a great impact on the dynamics of NTs. It is possible that there will be slow diffusion of orbits driven by these resonances. On the other hand, from the end of the planetary migration till now, the configuration of the outer Solar System may still have changed a little bit, by the interaction with the residual debris disk, or by the stochastic effect such as the potential collision of Uranus which is implied by the direction of its rotation axis. Thus the frequency and the strength of the quasi-2:1resonance of Uranus and Neptune is changeable. Using numerical simulation, we investigated the effect of the quasi-2:1resonance and its slightly change on the dynam-ical property of the NTs, as well as on the diffusion of the asteroids, especially the inclination excitation. Results show that with the enhancement of the quasi-2:1reso-nance, the stable zone of the NTs shrinks and when the2:1resonance exactly happens, NTs will be clear out in a short period. The existance of orbital diffusion is universal a-mong all the configuration when quasi-2:1MMR exists, but we found that the stronger the quasi-2:1MMR gets, the more unstable asteroids appear, and the orbital diffusion is suppressed more. When the quasi-2:1MMR is week, there may be a considerable number of asteroids which got an increase in inclination, especially when the quasi-2:1MMR is so week that it permits the existance of the horseshoe orbit of Neptune, there will be much more orbits that get their orbital inclination excited. Following this re-sult, we advance to dip into the possibility of the capture of horseshoe orbits to tadpole orbits, to make it an completed scenario.In chapter3, we simulated two individuals—the only two objects in L5found very recently—2004KV18and2008LC18in detail. We generated1000clones inside the6dimensional orbital element space with a length of3times the nominal error in each dimension. According to the statistical results, we conclude:·2004KV18is a transient NT, with an age not longer than2x105yr, and it won’t last for another1.65×105yr. Its orbital property can not be considered as the typical property of the NTs. It will probably become a TNO after leaving the Trojan territory.·2008LC18is more typical than2004KV18, and it is more like a primordial NT. Among all the possible orbits according to astrometry results, the region in initial orbital element space:σ∈(-90°,-40°),a>29.95AU(σ is the1:1resonance angle and a is the semimajor axis) hosts most stable clones. The stability has no relationship with eccentricity and inclination. The stable orbits show common-ly very regular motion in both temporally forward and backward simulations. Particularly, the high inclination of the stable orbits does not change much in both time directions, implying that the orbit was not excited on the tadpole orbit. But their inclination may vary significantly when they are outside of the Trojan phase. From this aspect, it may be a captured object.In chapter4, after summarizing all the main conclusions of this thesis, we also discussed two issues. One is about how NTs will evolve when the giant planets are undergoing migration:captured or expelled. The other is about the potential bias in the value of4:1in the high inclination excess enigma, if there should be a relationship between mass distribution and inclination distribution due to collision evolvement in the low inclination zone.