2008 LC18: a potentially unstable Neptune Trojan

Abstract

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 orbital uncertainties in a, e, i and {\Omega}. This showed that the wide uncertainties of the published orbit span regions of both extreme dynamical instability, with lifetimes < 100 Myr, and with significant stability (> 1 Gyr lifetimes). The stability of 2008 LC18's clones is greatly dependent on their semi-major axis and only weakly correlated with their eccentricity. Test particles on orbits with an initial semi-major axis less than 29.91 AU have dynamical half-lives shorter than 100 Myr; in contrast, particles with an initial semi-major axis 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 2.3m 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 8m class.Comment: 11 pages, 3 figures, 1 table, accepted for publication in Monthly Notices of the Royal Astronomical Societ

The Dynamical History of Chariklo and Its Rings

“…Instead, they must be continually replenished. Over the years, a number of other solar system small body populations have been suggested as potential sources for the Centaurs, including captured Oort Cloud comets (e.g., Emel'yanenko et al 2005;Brasser et al 2012;Fouchard et al 2014), the Jovian Trojans (e.g., Horner & Evans 2006;Horner et al 2012b), and the Neptune Trojans (Horner & Lykawka 2010a, 2010bHorner et al 2012a).…”

The intrinsic Neptune Trojan orbit distribution: Implications for the primordial disk and planet migration

“…The Minor Planet Center (MPC) lists nine Neptune Trojans (six L4, three L5), but one of the L5 Trojans is unstable and likely a recently captured Centaur (Gladman et al 2012, Horner et al 2012). This object is therefore not considered to be reflective of the intrinsic inclination distribution of the (putatively primordial) Neptune Trojans.…”

2004 KV18: a visitor from the scattered disc to the Neptune Trojan population

“…In our studies of the other potentially unstable Neptune Trojans 2001 QR 322 (Horner & Lykawka ) and 2008 LC 18 (Horner et al ), we found that their stability was a strong function of their initial orbital parameters, and that both objects had orbital uncertainties that spanned regions of both great dynamical stability and significant dynamical instability. As such, it is clearly interesting to consider whether the stability of 2004 KV 18 is similarly influenced by the choice of initial orbital elements used.…”

“…Such a range of capture outcomes is clearly seen in theoretical work modelling the origin of both the Neptunian and Jovian Trojan populations (e.g. Morbidelli et al ; Lykawka et al , ; Lykawka & Horner ), and so it is unsurprising that recent dynamical studies have revealed that both 2001 QR 322 and 2008 LC 18 might be such dynamically unstable Neptunian Trojans (Horner & Lykawka , ; Horner et al ). Indeed, recent work has revealed that one of the Jovian Trojans, (1173) Anchises, is definitively dynamically unstable (Horner, Müller & Lykawka ) – adding further weight to the capture hypothesis.…”

“…In order to study the dynamical behaviour of 2004 KV 18 , we followed the method used in our previous investigations of the Jovian and Neptunian Trojans (Horner & Lykawka , ; Horner et al ). We created a large population of ‘clones’ of 2004 KV 18 , spread around the nominal best‐fitting orbit for the object (Table ), such that values ranging up to ±3σ from the nominal values were tested.…”