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TOPIC: Neptune Trojans


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Title: The Pan-STARRS 1 Discoveries of five new Neptune Trojans
Author: Hsing Wen Lin, Ying-Tung Chen, Matthew J. Holman, Wing-Huen Ip, M. J. Payne, P. Lacerda, W. C. Fraser, D. W. Gerdes, A. Bieryla, Z.-F. Sie, W.-P. Chen, W. S. Burgett, L. Denneau, R. Jedicke, N. Kaiser, E. A. Magnier, J. L. Tonry, R. J. Wainscoat, C. Waters

In this work we report the detection of seven Neptune Trojans (NTs) in the Pan-STARRS 1 (PS1) survey. Five of these are new discoveries, consisting of four L4 Trojans and one L5 Trojan. Our orbital simulations show that the L5 Trojan stably librates for only several million years. This suggests that the L5 Trojan must be of recent capture origin. On the other hand, all four new L4 Trojans stably occupy the 1:1 resonance with Neptune for more than 1 Gyr. They can, therefore, be of primordial origin. Our survey simulation results show that the inclination width of the Neptune Trojan population should be between 7░ and 27░ at > 95% confidence, and most likely ~11░. In this paper, we describe the PS1 survey, the Outer Solar System pipeline, the confirming observations, and the orbital/physical properties of the new Neptune Trojans.

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Asteroid 2001 QR322
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Title: 2001 QR322 - an update on Neptune's first unstable Trojan companion
Author: Jonathan Horner, Patryk Sofia Lykawka

The Neptune Trojans are the most recent addition to the panoply of Solar system small body populations. The orbit of the first discovered member, 2001 QR322, was investigated shortly after its discovery, based on early observations of the object, and it was found to be dynamically stable on timescales comparable to the age of the Solar system.
As further observations were obtained of the object over the following years, the best-fit solution for its orbit changed. We therefore carried out a new study of 2001 QR322's orbit in 2010, finding that it lay on the boundary between dynamically stable and unstable regions in Neptune's Trojan cloud, and concluding that further observations were needed to determine the true stability of the object's orbit.
Here we follow up on that earlier work, and present the preliminary results of a dynamical study using an updated fit to 2001 QR322's orbit. Despite the improved precision with which the orbit of 2001 QR322 is known, we find that the best-fit solution remains balanced on a knife-edge, lying between the same regions of stability and instability noted in our earlier work. In the future, we intend to carry out new observations that should hopefully refine the orbit to an extent that its true nature can finally be disentangled.

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Title: Observation of Two New L4 Neptune Trojans in the Dark Energy Survey Supernova Fields
Author: D. W. Gerdes, R. J. Jennings, G. M. Bernstein, M. Sako, F. Adams, D. Goldstein, R. Kessler, T. Abbott, F. B. Abdalla, S. Allam, A. Benoit-LÚvy, E. Bertin, D. Brooks, E. Buckley-Geer, D. L. Burke, D. Capozzi, A. Carnero Rosell, M. Carrasco Kind, J. Carretero, C. E. Cunha, C. B. D'Andrea, L. N. da Costa, D. L. DePoy, S. Desai, J. P. Dietrich, P. Doel, T. F. Eifler, A. Fausti Neto, B. Flaugher, J. Frieman, E. Gaztanaga, D. Gruen, R. A. Gruendl, G. Gutierrez, K. Honscheid, D. J. James, K. Kuehn, N. Kuropatkin, O. Lahav, T. S. Li, M. A. G. Maia, M. March, P. Martini, C. J. Miller, R. Miquel, R. C. Nichol, B. Nord, R. Ogando, A. A. Plazas, A. K. Romer, A. Roodman, E. Sanchez, B. Santiago, M. Schubnell, I. Sevilla-Noarbe, R. C. Smith, M. Soares-Santos, F. Sobreira, E. Suchyta, M. E. C. Swanson, et al. (5 additional authors not shown)

We report the discovery of the eighth and ninth known Trojans in stable orbits around Neptune's leading Lagrange point, L4. The objects 2014 QO441 and 2014 QP441 were detected in data obtained during the 2013-14 and 2014-15 observing seasons by the Dark Energy Survey, using the Dark Energy Camera (DECam) on the 4-meter Blanco telescope at Cerro Tololo Inter-American Observatory. Both are in high-inclination orbits (18.8░ and 19.4░ respectively). With an eccentricity of 0.104, 2014 QO441 has the most eccentric orbit of the eleven known stable Neptune Trojans. Here we describe the search procedure and investigate the objects' long-term dynamical stability and physical properties.

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Title: The Intrinsic Neptune Trojan Orbit Distribution: Implications for the Primordial Disk and Planet Migration
Author: Alex H. Parker

The present-day orbit distribution of the Neptune Trojans is a powerful probe of the dynamical environment of the outer solar system during the late stages of planet migration. In this work, I conservatively debias the inclination, eccentricity, and libration amplitude distributions of the Neptune Trojans by reducing a priori unknown discovery and follow-up survey properties to nuisance parameters and using a likelihood-free Bayesian rejection sampler for parameter estimation. Using this survey-agnostic approach, I confirm that the Neptune Trojans are a dynamically excited population: at >95% confidence, the Neptune Trojans' inclination width must be sigmai>11░. For comparison and motivation purposes, I also model the Jupiter Trojan orbit distributions in the same basis and produce new estimates of their parameters (Jupiter Trojan sigmai=14.4░▒0.5░, sigmaL11=11.8░▒0.5░, and sigmae=0.061▒0.002). The debiased inclination, libration amplitude, and eccentricity distributions of the Neptune Trojans are nominally very similar to those of the Jupiter Trojans. I use these new constraints to inform a suite of simulations of Neptune Trojan capture by an eccentric, rapidly-migrating Neptune from an initially dynamically-hot disk. These simulations demonstrate that if migration and eccentricity-damping timescales were short (taua \lesssim 10 Myr, taue \lesssim 1 Myr), the disk that Neptune migrated into must have been pre-heated (prior to Neptune's appearance) to a width comparable to the Neptune Trojans' extant width to produce a captured population with an inclination distribution width consistent with that of the observed population.

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Title: Trailing (L5) Neptune Trojans: 2004 KV18 and 2008 LC18
Authors: Pu Guan, Li-Yong Zhou, Jian Li

The population of Neptune Trojans is believed to be bigger than that of Jupiter Trojans and that of asteroids in the main belt, although only eight members of this far distant asteroid swarm have been observed up to now. Six leading Neptune Trojans around the Lagrange point L4 discovered earlier have been studied in detail, but two trailing ones found recently around the L5 point, 2004 KV18 and 2008 LC18, have not been investigated yet. In this paper, we report our investigations on the dynamical behaviours of these two new Neptune Trojans. Our calculations show that the asteroid 2004 KV18 is a temporary Neptune Trojan. Most probably, it was captured into the trailing Trojan cloud no earlier than 203kyr ago, and it will not keep this identity no later than 165kyr in future. Based on the statistics on our orbital simulations, we argue that this object is more like a scattered Kuiper belt object. On the contrary, the orbit of asteroid 2008 LC18 is much more stable. Among the clone orbits spread within the orbital uncertainties, a considerable portion of clones may survive on the L5 tadpole orbits for 4Gyr. The strong dependence of the stability on the semimajor axis and resonant angle suggests that further observations are badly needed to confine the orbit in the stable region. We also discuss the implications of the existence and dynamics of these two trailing Trojans on the Solar system history.

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Title: Are Two of the Neptune Trojans Dynamically Unstable?
Authors: Jonathan Horner, Patryk Sofia Lykawka

The Neptune Trojans are the most recently discovered population of small bodies in the Solar System. To date, only eight have been discovered, though it is thought likely that the total population at least rivals that of the asteroid belt. Their origin is still the subject of some debate. Here, we detail the results of dynamical studies of two Neptune Trojans, 2001 QR322 and 2008 LC18. We find that both objects lie very close to boundaries between dynamically stable and unstable regions, with a significant probability that either or both of the objects are actually unstable on timescales of a few hundred million years. Such instability supports the idea that at least these two Neptune Trojans are dynamically captured objects, rather than objects that formed in situ. This that does not, however, rule out the possibility that these two objects were captured during Neptune's proposed post-formation migration, and have remained as Trojans ever since.

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Title: Origin and Dynamical Evolution of Neptune Trojans - II: Long Term Evolution
Authors: Patryk Sofia Lykawka, Jonathan Horner, Barrie W. Jones, Tadashi Mukai
(Version v2)

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

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Asteroid 2008 LC18
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Trojan Asteroid Found in Neptune's Trailing Gravitational Stability Zone

There are places in space where the gravitational tug between a planet and the Sun balance out, allowing other smaller bodies to remain stable. These places are called Lagrangian points. So-called Trojan asteroids have been found in some of these stable spots near Jupiter and Neptune. Trojans share their planet's orbit and help astronomers understand how the planets formed and how the solar system evolved. Now Scott Sheppard at the Carnegie Institution's Department of Terrestrial Magnetism and Chad Trujillo at the Gemini Observatory have discovered the first Trojan asteroid, 2008 LC18, in a difficult-to-detect stability region at Neptune, called the Lagrangian L5 point. They used the discovery to estimate the asteroid population there and find that it is similar to the asteroid population at Neptune's L4 point. The research is published in the August 13, 2010 issue of Science.
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An asteroid that is trapped in a 'dead zone' behind Neptune has been found for the first time. The finding suggests that the blue planet's rock collection may outnumber objects in the main asteroid belt and may provide clues to the origin of comets.
Objects can become trapped in two gravitational dead zones around Neptune, where the forces of the sun and the planet balance out. In the last decade, astronomers have identified six asteroids - called Trojans - in the zone that moves in front of the planet along its orbit. But finding Trojans in the region trailing the planet has proved more difficult, because the faint light reflected off of objects there is washed out by brighter starlight from the plane of the Milky Way.
Now, thanks to strategically located dust clouds, Scott Sheppard at the Carnegie Institution of Washington and Chadwick Trujillo at the Gemini Observatory in Hawaii have spied the first such trailing Trojan.
To find the asteroid - dubbed 2008 LC18 - Sheppard and Trujillo used existing images of the sky to identify dark clouds of dust and gas in our galaxy that fall along the path of the trailing Lagrangian point.

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Title: Dynamics of Neptune's Trojans: II. Eccentric orbits and observed ones
Authors: Li-Yong Zhou (1), Rudolf Dvorak (2), Yi-Sui Sun (1) ((1) Department of Astronomy, Nanjing University, (2) Institute for Astronomy, Vienna University)

In a previous paper, we have presented a global view of the stability of Neptune Trojan (NT hereafter) on inclined orbit. We discuss in this paper the dependence of stability of NT orbits on the eccentricity. High-resolution dynamical maps are constructed using the results of extensive numerical integrations of orbits initialised on the fine grids of initial semimajor axis (a0) versus eccentricity (e0). The extensions of regions of stable orbits on the (a0, e0) plane at different inclinations are shown. The maximum eccentricities of stable orbits in three most stable regions at low (0, 12deg.), medium (22,36deg.) and high (51, 59deg.) inclination, are found to be 0.10, 0.12 and 0.04, respectively. The fine structures in the dynamical maps are described. Via the frequency analysis method, the mechanisms that portray the dynamical maps are revealed. The secondary resonances, concerning the frequency of the librating resonant angle and the frequency of the quasi 2:1 mean motion resonance between Neptune and Uranus, are found deeply involved in the motion of NTs. Secular resonances are detected and they also contribute significantly to the triggering of chaos in the motion. Particularly, the effects of the secular resonance v8, v18 are clarified.
We also investigate the orbital stabilities of six observed NTs by checking the orbits of hundreds clones of them generated within the observing error bars. We conclude that four of them, except 2001 QR322 and 2005 TO74, are deeply inside the stable region. The 2001 QR322 is in the close vicinity of the most significant secondary resonance. The 2005 TO74 locates close to the boundary separating stable orbits from unstable ones, and it may be influenced by a secular resonance.

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