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    Mon. Not. R. Astron. Soc. 000, 110 () Printed 24 March 2018 (MN LATEX style file v1.4)

    Simulations of the Population of Centaurs II: Individual

    Objects

    J. Horner1,2, N.W. Evans2,3 & M.E. Bailey41 Physikalisches Institut, Universitat Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland3 Theoretical Physics, Department of Physics, 1 Keble Rd, Oxford OX1 3NP3 Institute of Astronomy, Madingley Rd, Cambridge, CB3 0HA4 Armagh Observatory, College Hill, Armagh, BT61 9DG, Northern Ireland.

    ABSTRACT

    Detailed orbit integrations of clones of five Centaurs namely, 1996 AR20, 2060 Chi-ron, 1995 SN55, 2000 FZ53 and 2002 FY36 for durations of 3 Myr are presented.One of our Centaur sample starts with perihelion initially under the control of Jupiter(1996 AR20), two start under the control of Saturn (Chiron and 1995 SN55) andone each starts under the control of Uranus (2000 FZ53) and Neptune (2002 FY36)respectively. A variety of interesting pathways are illustrated with detailed examplesincluding: capture into the Jovian Trojans, repeated bursts of short-period comet be-haviour, capture into mean-motion resonances with the giant planets and into Kozairesonances, as well as traversals of the entire Solar system. For each of the Centaurs,we provide statistics on the numbers (i) ejected, (ii) showing short-period comet be-haviour and (iii) becoming Earth and Mars crossing. For example, Chiron has over 60%of its clones becoming short-period objects, whilst 1995 SN55 has over 35%. Clones ofthese two Centaurs typically make numerous close approaches to Jupiter. At the otherextreme, 2000 FZ53 has 2% of its clones becoming short-period objects. In our sim-ulations, typically 20% of the clones which become short-period comets subsequentlyevolve into Earth-crossers.

    Key words: minor planets, asteroids planets and satellites: general celestialmechanics, stellar dynamics Kuiper belt

    1 INTRODUCTION

    The Centaurs are a transition population of minor bodiesbetween the trans-Neptunian objects and the Jupiter-familycomets (see, for example, Horner et al. 2003 and the refer-ences therein). Centaurs typically cross the orbits of one ormore of the giant planets and have relatively short dynam-ical lifetimes (106 yr). Their properties are exemplified bythe first known Centaur, Chiron, which was found in 1977 onPalomar plates (Kowal et al. 1979). Chiron is a large minorbody with perihelion close to or within the orbit of Saturnand aphelion close to the orbit of Uranus. The Centaurs haveso far largely eluded the attention of numerical integrators.The only ones that have hitherto been the subject of detaileddynamical investigations are Chiron itself (Hahn & Bailey1990, Nakamura & Yoshikawa 1993) and Pholus (Asher &Steel 1993). Dones et al. (1996) also looked briefly at fourCentaurs, including Chiron and Nessus. All these investi-gations were for durations of less than 1 Myr and involvedmodest numbers of clones.

    Horner et al. (2004, hereafter Paper I) integrated theorbits of 23 328 clones of 32 selected Centaurs and used the

    dataset to evaluate statistical properties of the Centaurs ina model Solar system containing the Sun and the four gi-ant planets. Hence, these longer numerical integrations withlarge numbers of clones provide better statistics and high-light some unusual past histories and future fates for Cen-taurs. In this companion paper, the behaviour of clones offive of these Centaurs namely, 1996 AR20, Chiron, 1995SN55, 2000 FZ53 and 2002 FY36 are studied in more de-tail. The objects are chosen to span a wide range of prop-erties. 1996 AR20 has the shortest half-life in our sample,while 2000 FZ53 has the longest half-life. 1995 SN55 is theCentaur with the brightest absolute magnitude (hence po-tentially the largest Centaur known), while Chiron is theonly one confirmed to display cometary out-gassing.

    Horner et al. (2003) introduced a new classification sys-tem for cometary-like bodies according to the planets underwhose control the perihelion and aphelion lie. For exam-ple, we classify Chiron as an SU object, by which we meanthat the position of its perihelion lies within Saturns zoneof control, and that the position of its aphelion lies withinUranus zone of control. It is apparent that perturbations atperihelion, by Saturn, will act primarily to move the position

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  • 2 J. Horner, N.W. Evans & M.E. Bailey

    of the aphelion, and vica-versa. In other words, the motionnear Saturn determines whether or not the body gets toUranus, or is captured to a more tightly bound orbit, or ex-pelled. Conversely, perturbations by Uranus, near aphelion,largely determine the future perihelion distance. So, in awider sense, Saturn also controls the aphelion (and Uranusthe perihelion), as it determines its numerical value. How-ever, in this paper, whenever we talk of a planet controllinga minor body at perihelion (or aphelion), we mean that themotion at perihelion (or aphelion) lies in the zone of controlof that planet.

    For our selected 5 Centaurs, there is one object withperihelion under the control of Jupiter (1996 AR20), two un-der the control of Saturn (Chiron and 1995 SN55), and oneeach under the control of Uranus (2000 FZ53) and Neptune(2002 FY36). Clones of the objects were created by incre-mentally increasing (and decreasing) the semi-major axis aof the object by 0.005 au, the eccentricity e by 0.005, and theinclination i by 0.01. Nine values were used for each of theseelements, with the central (fifth) value of the nine having theoriginal orbital elements for the Centaur, as taken from TheMinor Planet Center. The other orbital elements aside froma, e and i are unchanged (see Paper I for more details). Thisprocedure yielded 729 clones of each Centaur, all of whichwere numerically integrated for up to 3 Myr. In this paper,we restrict ourselves to just 2 particularly interesting clonesfor each Centaur.

    Although all 5 of our selected Centaurs have reason-ably reliable ephemerides, only Chiron has been the subjectof sustained interest from observers. For Chiron, there arelong-term photometric studies of the behaviour of the ob-ject (Dufford et al., 2002), detailed analyses of its reflectancespectrum (Foster et al. 1999), as well as the use of archivalpre-discovery images of the object (Bus et al., 2001). Thereare little observational data on the remaining four objects.

    The detailed studies of individual clones of these objectsare important to illustrate some of the dynamical pathwaysin the Solar system. Objects in very stable regimes in theSolar system (such as some resonances) are long-lived andcould be potential targets for new surveys. A good exam-ple is the possible long-lived belt of objects between Uranusand Neptune claimed by Holman (1997). Objects in unstableregimes must evolve, and correlations between observablesand orbital properties are then expected. For example, bluercolours might indicate a younger, fresher surface and so beindicative of recent cometary activity. So, a Centaur withblue colours (such as Chiron) could be a candidate for apassage through a cometary phase in the recent past. Indi-vidual examples allow us to match an orbital history to sucha presumed pathway.

    The paper is organized according to object, with 1996AR20 studied in 2, Chiron in 3, 1995 SN55 in 4, 2000FZ53 in 5 and 2000 FY36 in 6.

    2 EVOLUTION OF A JN OBJECT: 1996 AR20

    1996 AR20 is a JN object with its perihelion under the con-trol of Jupiter and its aphelion under the control of Neptune.Among the Centaurs, 1996 AR20 has the shortest knownhalf-lives, namely 540 kyr in the forward and 594 kyr in thebackward direction. Its orbit is interesting as its initial po-

    Figure 1. The evolution of the population of clones of 1996 AR20

    subdivided according to the planet controlling the perihelion (ob-jects controlled by Jupiter, Saturn, Uranus and Neptune are red,green, yellow and cyan respectively). Also shown are the evolutionof the number of short-period comets (black), trans-Neptunianobjects and ejected objects (blue). The left panel shows the re-sults from the forward integration, the right the backward inte-gration. [This colour convention is employed in all following plotsof this nature.]

    sition lies close to two prominent mean-motion resonances.The initial value of semi-major axis in the integrations was15.2 au, which is within 0.02 au of the 1:5 mean-motionresonance with Jupiter and within 0.06 au of the 1:2 mean-motion resonance with Saturn. In addition, 1996 AR20 hasan eccentricity of 0.627 so that it can approach all the majorouter planets close enough to be perturbed. These factors allcontribute towards making 1996 AR20 one of the least stableCentaurs. Of the 729 clones, 62 become Earth-crossers, 154become Mars-crossers and 340 become short-period cometsin the forward integration. These numbers are all slightlylarger in the backward integration, namely 89, 194, and 406respectively.

    Figure 1 shows how the population of clones of 1996AR20 changes over time. Initially, all 729 clones have peri-helion under the control by Jupiter, but by the end of thesimulation, in both the forward and backward directions,over 650 of the clones have been ejected. The number of ob-jects under the control of Jupiter rapidly decays, with mosteither being ejected, or moving to the control of Saturn, ortransferred to cometary orbits. The numbers in each of theseclasses peaks early within the simulation and then decays asmore and more objects are ejected. Only a small number ofclones of 1996 AR20 evolve so that the perihelion is underthe control of Uranus and Neptune. The great majority ofobjects are ejected by either Jupiter or Saturn, giving veryfew the opportunity to evolve all the way out to Neptune.

    2.1 A Source for Jovian Trojan Asteroids

    Figure 2 shows the evolution of the 12th clone of 1996AR20, integrated in the forward direction. The initial semi-major axis, eccentricity and inclination of this clone area = 15.177 au, e = 0.617 and i = 6.17. The clone is rapidlycaptured into a 1:1 mean-motion resonance with Jupiter,which it then occupies for over 0.5 Myr before ejection fromthe Solar system. The clone displays quite large variations in

    The clone label is useful for our internal data management butcarries no other physical meaning.

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    Figure 2. The evolution of the 12th clone of 1996 AR20 in theforward direction. Sub-panels show the evolution of semimajoraxis, perihelion and aphelion distance (all in au), inclination (indegrees) and eccentricity. In the plot of Tisserand parameter, thevalue of TJ is plotted in blue and TS in yellow. This convention isfollowed in all similar plots. Note that the clone is rapidly trappedinto a 1:1 mean motion resonance with Jupiter until ejection after 0.5 Myr.

    a, e and i whilst in the resonance. By plotting the positionsover time, it is clear that the clone follows a tadpole orbitlibrating about the Lagrange point. This is significant as itshows that Centaurs can be captured into the 1:1 resonancewith Jupiter. Hence, there may well be Jovian Trojans thatwere originally Centaurs and vice versa. It would be inter-esting to see whether any Jovian Trojans display cometaryout-gassing, since recently captured Centaurs may still con-tain volatiles, whilst any Trojans captured from an originalMain Belt asteroidal orbit are unlikely to display such ac-tivity.

    In our Centaur orbital integrations, we find that clonesare quite frequently trapped into 1:1 mean-motion reso-nances with all the giant planets.

    2.2 A Collision with Saturn

    Figure 3 shows the behaviour of the 66th clone of 1996 AR20,whose initial orbital elements were a = 15.177 au, e = 0.617,i = 6.23 (almost the same as the 12th clone!). The 66thclone impacts upon Saturn at the end of its lifetime, 18 kyrafter the start of the integration. In Paper I, we calculatedthat Centaurs impact onto the surface of Saturn at a rateof 1 every 28 kyr. The perihelion of the clone starts thesimulation under the control of Jupiter, and perturbationsby this planet cause a number of changes in the semi-major

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    Figure 3. The evolution of the 66th clone of 1996 AR20 in theforward direction. The clone hits the surface of Saturn after 18kyr.

    Figure 4. The numbers of clones of Chiron controlled by Jupiter(red), Saturn (green), Uranus (yellow) and Neptune (cyan), to-gether with the numbers of short-period comets (black), trans-Neptunian and ejected objects (blue), plotted against time. Theleft (right) panel shows the results from the forward (backward)integration.

    axis of the clone. Finally, a series of close encounters reducethe perihelion and aphelion distances for the object untilit twice becomes a cometary body (at around 12 kyr, verybriefly, and then for a more prolonged period from 13 kyrto 15 kyr). After this, the clones perihelion and apheliondistances increase until the perihelion lies just beyond theorbit of Jupiter and the aphelion lies under Saturns control.The object finally collides with Saturn at aphelion, roughly18 kyr from the present.

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    Figure 5. The evolution of the 206th clone of Chiron in theforward direction. and eccentricity. In the plot of Tisserand pa-rameter, the value of TJ is plotted in blue and TS in yellow. Notethe prolonged spell ( 1 Myr) as a short-period comet.

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    Figure 6. The evolution of the 78th clone of Chiron in the for-ward direction. Note the stable, nearly constant behaviour of theorbital elements as the clone is transferred to a long-lived orbit.

    3 EVOLUTION OF A SU OBJECT: CHIRON

    Chiron was the first Centaur to be discovered in 1977. Pre-discovery images allow the orbit to be traced all the way backto the perihelion passage of 1895 (see Kowal et al. 1979).Chiron has a coma which undergoes variations in bright-ness (Meech & Belton 1989, Luu & Jewitt 1990). Chironsphotometric activity is sporadic and apparently unrelatedto heliocentric distance (Duffard et al. 2002). For example,the increase in brightness during 1988-1991 (e.g., Tholen etal. 1988) was followed by a period of minimal activity asthe object passed through perihelion in 1996. Also unusualis the size of Chiron with an absolute magnitude H of 6.5,it is one of the largest Cent...