2008 LC18: a potentially unstable Neptune Trojan

• Published in: Monthly notices of the Royal Astronomical Society Vol. 422; no. 3; pp. 2145 - 2151
• Main Authors: Horner, J., Lykawka, P. S., Bannister, M. T., Francis, P.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.05.2012; Oxford University Press
• Subjects: Astrophysics; comets: general; Evolution; Kuiper belt: general; minor planets, asteroids: general; Orbits; Planets; planets and satellites: individual: Neptune; planets and satellites: individual: Neptune; minor planets, asteroids: general; comets: general; Kuiper belt: general
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2012.20757.x

The recent discovery of the first Neptune Trojan at the planet's trailing (L5) Lagrange point, 2008 LC18, offers an opportunity to confirm the formation mechanism of a member of this important tracer population for the Solar system's dynamical history. We tested the stability of 2008 LC18's orbit through a detailed dynamical study, using test particles spread across the ±3σ range of orbital uncertainties in a, e, i and Ω. This showed that the wide uncertainties of the published orbit span regions of both extreme dynamical instability, with lifetimes <100 Myr, and significant stability, with lifetimes >1 Gyr. The stability of 2008 LC18's clones is greatly dependent on their semimajor axis and only weakly correlated with their orbital eccentricity. Test particles on orbits with an initial semimajor axis of less than 29.91 au have dynamical half-lives shorter than 100 Myr; in contrast, particles with an initial semimajor axis of greater than 29.91 au exhibit such strong dynamical stability that almost all are retained over the 1 Gyr of our simulations. More observations of this object are necessary to improve the orbit. If 2008 LC18 is in the unstable region, then our simulations imply that it is either a temporary Trojan capture or a representative of a slowly decaying Trojan population (like its sibling the L4 Neptunian Trojan 2001 QR322), and that it may not be primordial. Alternatively, if the orbit falls into the larger, stable region, then 2008 LC18 is a primordial member of the highly stable and highly inclined component of the Neptune Trojan population, joining 2005 TN53 and 2007 VL305. We attempted to recover 2008 LC18 using the 2.3-m telescope at Siding Spring Observatory to provide this astrometry, but were unsuccessful due to the high stellar density of its current sky location near the Galactic centre. The recovery of this object will require a telescope in the 8-m class.