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The Rings of Chariklo Under Close Encounters with the Giant Planets R The Astrophysical Journal, 824:80 (7pp), 2016 June 20 doi:10.3847/0004-637X/824/2/80 © 2016. The American Astronomical Society. All rights reserved. THE RINGS OF CHARIKLO UNDER CLOSE ENCOUNTERS WITH THE GIANT PLANETS R. A. N. Araujo, R. Sfair, and O. C. Winter UNESP - Univ. Estadual Paulista, Grupo de Dinâmica Orbital e Planetologia, CEP 12516-410, Guaratingueta, SP, Brazil; [email protected], [email protected], [email protected] Received 2015 December 11; accepted 2016 April 5; published 2016 June 15 ABSTRACT The Centaur population is composed of minor bodies wandering between the giant planets that frequently perform close gravitational encounters with these planets, leading to a chaotic orbital evolution. Recently, the discovery of two well-defined narrow rings was announced around the Centaur 10199 Chariklo. The rings are assumed to be in the equatorial plane of Chariklo and to have circular orbits. The existence of a well-defined system of rings around a body in such a perturbed orbital region poses an interesting new problem. Are the rings of Chariklo stable when perturbed by close gravitational encounters with the giant planets? Our approach to address this question consisted of forward and backward numerical simulations of 729 clones of Chariklo, with similar initial orbits, for a period of 100 Myr. We found, on average, that each clone experiences during its lifetime more than 150 close encounters with the giant planets within one Hill radius of the planet in question. We identified some extreme close encounters that were able to significantly disrupt or disturb the rings of Chariklo. About 3% of the clones lose their rings and about 4% of the clones have their rings significantly disturbed. Therefore, our results show that in most cases (more than 90%), the close encounters with the giant planets do not affect the stability of the rings in Chariklo-like systems. Thus, if there is an efficient mechanism that creates the rings, then these structures may be common among these kinds of Centaurs. Key words: minor planets, asteroids: individual (10199 Chariklo) – planets and satellites: dynamical evolution and stability – planets and satellites: rings 1. INTRODUCTION to be in the equatorial plane of Chariklo, with circular orbits. From the orbital positions of the rings, they also estimated the Among the orbits of the giant planets there is a population of − density of the central body to be 1 g cm 3. Apart from the small objects called Centaurs. There is no consensus on the equivalent radius of 124 km, derived from the same stellar definition of the Centaur population. According to the Minor Planet Center (MPC) of the IAU, Centaurs are celestial bodies occultation, little information about the physical properties of with a perihelion beyond the orbit of Jupiter and with Chariklo is available. Table 1 summarizes some of the physical semimajor axes smaller than the semimajor axis of Neptune.1 and orbital parameters of Chariklo and its rings. ( )/ fi A detailed study of the orbital evolution of the Centaurs was Similarly, the Jet Propulsion Laboratory JPL NASA de nes ( ) the Centaur population as objects with semimajor axes between presented by Horner et al. 2004a , where they analyzed the 2 ( ) orbital evolution of 32 cataloged objects through numerical 5.5 and 30.1 AU. Duffard et al. 2014 classify Centaurs as fl celestial bodies with orbits mostly in the region between Jupiter simulations of an ensemble of particles under the in uence of and Neptune that typically cross the orbits of the giant planets. the Jovian planets. They followed the particles both forward A similar definition is also found in Horner et al. (2004b). and backward in time and registered the dynamical evolution 2060 Chiron was the first observed body of this population and fate of the particles. The orbital radius shows that Chariklo U (Kowal et al. 1979). Since then, the number of known Centaurs orbits between Saturn and Uranus, corresponding to a typical has grown. Currently, more than 400 objects are cataloged3, class object, i.e., those whose evolution is controlled by Uranus ( ) and from the flux of short period comets Horner et al. (2004a) Horner et al. 2003 . For Chariklo, they found the half-life to be ( ) estimated the total number of objects with a diameter >1kmto 10.3 9.68 Myr. be approximately 44,000. The Centaur objects are transient. Therefore, a source is The Centaur 10199 Chariklo was discovered in 1997 by the required to maintain a steady-state population. The idea of Spacewatch program.4 In 2013, a stellar occultation revealed bodies coming from regions of the solar system beyond the existence of symmetric features encircling Chariklo, the Neptune and populating the region between the planets is well- second largest known Centaur. Braga-Ribas et al. (2014) accepted. ( ) showed that these structures are a system of two narrow and Levison & Duncan 1997 estimated, through numerical well-defined rings. This discovery was the first example of integrations, the number of comets transiting between the inner and outer solar system originating from the Kuiper Belt as minor bodies within the select group of ringed objects. 7 In Braga-Ribas et al. (2014), the authors estimated that the ≈1.2 × 10 . Horner et al. (2004a) estimated a flux of one body rings have orbital radii of approximately 391 km and 405 km, coming from the Kuiper Belt and joining the Centaur and widths of 7 km and 3 km, respectively. They are assumed population every 125 years. Sisto & Brunini (2007), present the scattered disk objects (SDO; bodies with a distance to the 1 MPC, www.minorplanetcenter.org/iau/lists/Unusual.html. perihelion of q < 30 AU and semimajor axes of a > 50 AU) as 2 JPL, http://ssd.jpl.nasa.gov/sbdb_help.cgi?class=CEN. the most probable source of the Centaurs. Emel’Yanenko et al. 3 MPC, www.minorplanetcenter.net/iau/lists/Centaurs.html. (2007), analyzed the role of the Oort cloud in determining the 4 Spacewatch Program, http://spacewatch.lpl.arizona.edu/discovery.html. flux of cometary bodies through the planetary system. They 1 The Astrophysical Journal, 824:80 (7pp), 2016 June 20 Araujo, Sfair, & Winter Table 1 Orbital and Physical Parameters of Chariklo and its Rings Chariklo Rings R1 R2 aa 15.74 AU ea 0.171 Orbital radius (km)b 390.6 ± 3.3 404.8 ± 3.3 ia ° ( )b ± +1.1 23.4 Width km 7.17 0.14 3.4-1.4 Equivalent radius (km)b 124 Radial separation (km)b 14.2 ± 0.2 Mass (kg)c 7.986 × 1018 Gap between rings (km)b 8.7 ± 0.4 Notes. a Orbital elements obtained from JPL’s Horizons system for the epoch MJD 56541. According to JPL the uncertainties in a, e, and i are of the order 10−5,10−6, and 10−5, respectively. b Braga-Ribas et al. (2014). c Calculated considering a density of 1 g cm−3, the equivalent radius of Chariklo and a spherical body. concluded that a substantial fraction of all known cometary extreme close encounters, i.e., those encounters that could be bodies may have a source in the Oort cloud, including the capable of disrupting the rings of Chariklo. In Section 4,we Centaur population, which they defined as the population of describe how the rings were simulated during a close small bodies with a perihelion 5 < q < 28 AU and encounter. In Sections 5–7 we present the results, and Section 8 a < 1000 AU. Following the same definition for Centaurs, provides our final comments and the major conclusions of Emel’Yanenko et al. (2013) suggest that more than 90% of all the work. Centaurs with a > 60 AU and ≈50% with a < 60 AU come from the Oort cloud. In Brasser et al. (2012), the Oort cloud is also indicated out as the source of Centaurs, in particular those 2. CLOSE ENCOUNTERS WITH THE GIANT PLANETS with high inclination. In that work the Centaur population is The first step consisted on selecting a representative sample defined as small bodies with a perihelion between 15 and of close encounters of Chariklo with the giant planets. We 30 AU and a semimajor axis shorter than 100 AU. They considered a system composed of the Sun, the giant planets of showed that these objects probably originated from the Oort the solar system (Jupiter, Saturn, Uranus, and Neptune), and a cloud rather than the Kuiper Belt or the scattered disc. sample of clones, i.e., objects with the same mass and radius as Throughout its orbital evolution a Centaur is strongly Chariklo, but with small deviations in their orbits. perturbed by the giant planets. Horner et al. (2004b), illustrate The clones were created following the procedure presented in detail the effects of these perturbations on the orbit of five in Horner et al. (2004a), where 729 clones were created from selected Centaurs. The close encounters with the giant planets the original orbit assuming a variation of semimajor axis of are quite frequent. As consequence, the Centaurs present a 0.005 AU, a variation of eccentricity of 0.005, and a variation characteristic chaotic orbital evolution. of inclination of 0°.01. The existence of a small body with a well-defined system of The orbital elements of Chariklo and of the planets were circular rings within such a perturbed population poses an obtained through JPL’s Horizons system for the epoch MJD interesting new problem. This scenario has motivated the 56541. For the orbit of Chariklo at this epoch we have a = development of the present work. We investigated the stability 15.74 AU, e = 0.171, and i = 23°.4.
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