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Damocloid Paper The Astronomical Journal, 129:000 –000, 2005 January # 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A. A FIRST LOOK AT THE DAMOCLOIDS David Jewitt1 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822; [email protected] Receivedv 2004 August 3; accepted 2004 September 21 ABSTRACT The Damocloids are objects thought, on dynamical grounds, to be inactive Halley-family and long-period comets. We present optical measurements of 12 such objects, finding that their mean Kron-Cousins colors are BÀV ¼ 0:79 Æ 0:01, V ÀR ¼ 0:48 Æ 0:01, and RÀI ¼ 0:48 Æ 0:01. The normalized reflectivity spectra are generally linear, with a mean gradient S0 = 11.9% Æ 1.0% per 1000 8. The latter is consistent with the mean S0 =11.6%Æ 2.3% per 1000 8 measured for the nuclei of (short-period) Jupiter-family comets, a surprising result given the expected very different formation locations and dynamical histories of these two types of body. The Damocloids are devoid of the ultrared matter (with S 0 25% per 1000 8) that is present on many Kuiper belt objects and Centaurs, and the mean colors of the Damocloids are inconsistent with those of the Kuiper belt objects (S0 = 21.1% Æ 1.4% per 1000 8). The data suggest that the ultrared matter, widely thought to consist of a complex organic compound processed by prolonged exposure to cosmic rays, cannot survive long in the inner solar system. Timescales for ejection or burial of ultrared matter on the nuclei of both Jupiter-family comets and Damocloids are short. Such material may also be chemically unstable to the higher temperatures experienced in the inner planetary region. Key words: comets: general — Kuiper belt — minor planets, asteroids 1. SCIENTIFIC MOTIVATION producing the Oort cloud (Duncan et al. 1987). The suggestion AND THE DAMOCLOID SAMPLE by Ferna´ndez et al. (2003) that about 1% of the Oort cloud comets might be former members of the Kuiper belt’s scat- The nuclei of comets are scientifically interesting as likely tered-disk population does not materially change this scenario. carriers of volatile and other material captured 4.6 Gyr ago in the protoplanetary disk of the Sun. Unfortunately, because of Once in the Oort cloud, comets fall to equilibrium temperatures of 10 K or less, suffer no collisions, and are subject to the their small size and the presence of near-nucleus coma when full flux of ionizing cosmic radiation. In these respects they near perihelion (and thus at their brightest), cometary nuclei have also proved to be among the most difficult objects to study differ from comets originating in the Kuiper belt, which have radiation equilibrium temperatures near 40 K, suffer colli- using astronomical techniques. Despite substantial observa- sions (indeed, the observed JFC nuclei may be produced by tional efforts over the past two decades, the number of nuclei fragmentation of larger Kuiper belt objects; Farinella & Davis that are well characterized in terms of reliable measurements 1996), and are partially protected from cosmic rays by the of their shapes, sizes, colors, albedos and spin states is only of heliosphere. It is interesting to ask whether different formation order 10 (Campins & Ferna´ndez 2003; Lamy et al. 2004), al- locations and processing histories might have led to measur- though partial information is available for a larger number. able differences in the properties of cometary nuclei from Almost all of the well-observed nuclei are members of the short- Kuiper belt and Oort cloud sources. period Jupiter-family comet (JFC) class. The JFCs are thought In this paper, we begin to address this question using a set of to have a source beyond Neptune within the Kuiper belt (from roughly 30 to 50 AU), although the precise location of the objects known informally as Damocloids. Damocloids are solar system objects with orbits like those of Halley-family and long- source remains unclear. The nuclei of the so-called long-period period comets but without visible signs of outgassing. Like or ‘‘nearly isotropic’’ comets are observationally undersampled, in part because their strong mass-loss rates and bright comae the archetypal object (5335) Damocles, their high eccentricities and inclinations suggest that the Damocloids are the dead or typically swamp the light scattered from the nucleus. dormant nuclei of long-period comets (Asher et al. 1994). With The formation locations and dynamical histories of the no likely source in the Kuiper belt, the Damocloids constitute long-period comets are likely different from those of the better a sample from which the physical properties of long-period studied JFCs. Most long-period comets probably formed not comet nuclei might be determined. Owing to the small number in the Kuiper belt but in the vicinity of the giant planets at 5 of known Damocloids and to their general faintness, no sys- to 30 AU, where equilibrium temperatures were in the 50 to tematic study of these objects has yet been published, although 150 K range. They were scattered out of the planetary region a few detailed studies of particular objects exist (Davies et al. during near-miss interactions with the growing planets, and the 1998, 2001; Harris et al. 2001; Ferna´ndez et al. 2001). perihelia of some (maybe 10%) were lifted by the extraga- lactic tide and by random perturbations from passing stars, The Tisserand parameter, TJ, which is a constant of the motion in the restricted circular three-body problem, provides a useful metric with which to identify Damocloids. The pa- 1 Guest observer at the W. M. Keck Observatory, which is operated as a rameter relative to Jupiter is defined by scientific partnership among the California Institute of Technology, the Uni- versity of California, and the National Aeronautics and Space Administration. 1=2 aJ 2 a The Observatory was made possible by the generous financial support of the TJ ¼ þ 2 (1 À e ) cos i; ð1Þ W. M. Keck Foundation. a aJ 1 (V129/204320) 11/2/04 2 (V129/204320) 11/2/04 JEWITT Vol. 129 TABLE 1 Currently Known Damocloids aa i qd b c e Name (AU) e (deg) (AU) TJ 20461 Dioretsa (1999 LD31) ....... 23.777 0.900 160.4 2.390 À1.542 2000 HE46 .................................... 23.985 0.902 158.38 2.355 À1.508 1999 LE 31 .................................... 8.163 0.472 151.88 4.310 À1.310 f C/LINEAR (2002 CE10) ............. 9.816 0.791 145.46 2.047 À0.853 (65407) 2002 RP120..................... 55.940 0.956 119.11 2.473 À0.845 2000 DG8 ..................................... 10.786 0.793 129.43 2.231 À0.631 f C/LONEOS (2001 OG108) ......... 13.30 0.925 80.26 0.994 0.597 2000 AB 229.................................. 52.497 0.956 68.72 2.292 0.773 1997 MD10................................... 26.740 0.942 59.04 1.543 0.975 f C/LINEAR (2002 VQ 94) ........... 218.161 0.969 70.50 6.800 1.095 5335 Damocles (1991 DA) ......... 11.834 0.867 62.10 1.573 1.143 2002 XU93.................................... 67.426 0.689 77.88 20.983 1.173 1998 WU24................................... 15.221 0.907 42.56 1.419 1.404 1999 XS35 .................................... 18.079 0.948 19.47 0.948 1.411 2000 KP65 .................................... 88.323 0.963 45.62 3.274 1.613 1996 PW ...................................... 287.127 0.991 29.76 2.547 1.732 2003 WG166 ................................. 5.160 0.644 55.41 1.838 1.873 2003 WN188 ................................. 14.566 0.849 26.94 2.200 1.933 (15504) 1999 RG33...................... 9.634 0.775 35.13 2.164 1.946 2004 DA62 ................................... 7.709 0.467 52.23 4.107 1.993 Note.—As of 2004 July 1. Objects are ordered by Tisserand parameter, a Orbital semimajor axis. b Orbital eccentricity. c Orbital inclination. d Perihelion distance. e Tisserand parameter. f These objects satisfied the working definition of Damocloids at discovery but were later observed to possess faint comae. where a, e,andi are the semimajor axis, eccentricity, and in- Damocloids in Table 1 satisfy this condition. Therefore, we clination of the orbit while aJ ¼ 5:2 AU is the semimajor axis conclude that while it is possible that the sample may be of the orbit of Jupiter. For reference, the Halley-family comets weakly contaminated by highly dynamically evolved, inactive (HFCs) have TJ 2andtheJFCshave2< TJ 3, while most nuclei of JFCs (particularly for objects with TJ 2), most asteroids have TJ > 3. The Tisserand parameter is an imperfect Damocloids are unrelated to the JFCs. metric in the sense that Jupiter’s orbit is not a circle and the As discussed below, some objects identified as Damocloids gravitational influences of other planets cannot in general be on the basis of their Tisserand parameters and asteroidal ap- neglected in the calculation of long-term motions. Neverthe- pearance have subsequently been found to display weak co- less, TJ provides a simple and revealing way to dynamically mae. This provides clear evidence that our simple, dynamical classify interplanetary objects (Carusi et al. 1995). definition of the Damocloids picks out objects that are volatile- We define Damocloids as point-source objects having TJ rich and, therefore, of a cometary nature. The coma in these 2. With this definition, a search of the orbital elements database objects is generally weak enough that scattered light from the for asteroids maintained at the Lowell Observatory yields 20 nucleus dominates the observations. Weissman & Levison Damocloids, which we list in order of TJ in Table 1. So defined, (1997) have argued that Damocloid 1996 PW could have an the Damocloids exhibit a wide range of orbital properties but origin in the main asteroid belt.
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