A first look at the damocloids

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 AE 0:01, V ÀR ¼ 0:48 AE 0:01, and RÀI ¼ 0:48 AE 0:01. The normalized reflectivity spectra are generally linear, with a mean gradient S 0 = 11.9% AE 1.0% per 1000 8. The latter is consistent with the mean S 0 = 11.6% AE 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 (S 0 = 21.1% AE 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 SCIENTIFIC MOTIVATION AND THE DAMOCLOID SAMPLE The nuclei of comets are scientifically interesting as likely carriers of volatile and other material captured $4.6 Gyr ago in the protoplanetary disk of the Sun. Unfortunately, because of their small size and the presence of near-nucleus coma when near perihelion (and thus at their brightest), cometary nuclei have also proved to be among the most difficult objects to study using astronomical techniques. Despite substantial observational efforts over the past two decades, the number of nuclei that are well characterized in terms of reliable measurements of their shapes, sizes, colors, albedos and spin states is only of order 10 (Campins & Fernández 2003; The formation locations and dynamical histories of the long-period comets are likely different from those of the better studied JFCs. Most long-period comets probably formed not in the Kuiper belt but in the vicinity of the giant planets at 5 to 30 AU, where equilibrium temperatures were in the $50 to $150 K range. They were scattered out of the planetary region during near-miss interactions with the growing planets, and the perihelia of some (maybe $10%) were lifted by the extragalactic tide and by random perturbations from passing stars, producing the Oort cloud In this paper, we begin to address this question using a set of objects known informally as Damocloids. Damocloids are solar system objects with orbits like those of Halley-family and longperiod comets but without visible signs of outgassing. Like the archetypal object (5335) Damocles, their high eccentricities and inclinations suggest that the Damocloids are the dead or dormant nuclei of long-period comets The Tisserand parameter, T J , which is a constant of the motion in the restricted circular three-body problem, provides a useful metric with which to identify Damocloids. The parameter relative to Jupiter is defined by 1 Guest observer at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. 1 (V129/204320) 11/2/04 The Astronomical Journal, 129:000 -000, 2005 January where a, e, and i are the semimajor axis, eccentricity, and inclination of the orbit while a J ¼ 5:2 AU is the semimajor axis of the orbit of Jupiter. For reference, the Halley-family comets (HFCs) have T J 2 and the JFCs have 2 < T J 3, while most asteroids have T J > 3. The Tisserand parameter is an imperfect metric in the sense that Jupiter's orbit is not a circle and the gravitational influences of other planets cannot in general be neglected in the calculation of long-term motions. Nevertheless, T J provides a simple and revealing way to dynamically classify interplanetary objects We define Damocloids as point-source objects having T J 2. With this definition, a search of the orbital elements database for asteroids maintained at the Lowell Observatory yields 20 Damocloids, which we list in order of T J in As discussed below, some objects identified as Damocloids on the basis of their Tisserand parameters and asteroidal appearance have subsequently been found to display weak comae. This provides clear evidence that our simple, dynamical definition of the Damocloids picks out objects that are volatilerich and, therefore, of a cometary nature. The coma in these objects is generally weak enough that scattered light from the nucleus dominates the observations. Weissman & Levison (1997) have argued that Damocloid 1996 PW could have an origin in the main asteroid belt. The stochastic nature of solar system dynamics makes it impossible to determine the origin of any particular object with absolute confidence, but the cometlike inclination distribution OBSERVATIONS Observations were taken at the University of Hawaii (UH ) 2.2 m diameter telescope and the 10 m Keck I Telescope, both located on Mauna Kea in Hawaii. We worked at the f=10 Cassegrain focus of the UH 2.2 m using a Tektronix 2048 ; 2048 pixel CCD camera and the ''OPTIC'' CCD camera, which contains two CCDs each 2048 ; 4104 pixels in size. The Tektronix is a standard back-illuminated device antireflectioncoated to provide $90% quantum efficiency at wavelengths JEWITT 2 (V129/204320) 11/2/04 Vol. 129 k $0:7 m. OPTIC is an orthogonal transfer CCD, in which charge packets can be moved on the detector during the integration in order to provide tip-tilt corrections to the wave front Photometric calibration of the data was obtained on the Johnson-Kron-Cousins system from observations of faint stars in the catalog by Landolt (1992). We observed stars close in air mass to the target fields and measured the extinction coefficients from the nightly data. These extinction coefficients varied slightly from run to run but were generally close to k B ¼ 0:18 mag per air mass, k V ¼ 0:10 mag per air mass, k R ¼ 0:07 mag per air mass, and k I ¼ 0:04 mag per air mass at both telescopes (both are located at $4200 m altitude). The extinction corrections from the nearest available Landolt stars were typically only a few times 0.01 mag and therefore not a major source of error in our photometry. Larger errors were encountered as a result of variable backgrounds to the Damocloids, in particular with the removal of background stars and galaxies from the target signals. In fact, reliable measurements of 2000 DG 8 proved impossible because of the particularly dense backgrounds against which this object was observed, and we have expunged it from this paper. Other objects [1999 LE 31 , 2002 2002 CE 10 ] were observed on more than one night to provide protection from errors due to background sources (see Photometry was performed using aperture correction to negate the effects of temporal variability in the point-source function (the latter principally caused by variable seeing and guiding-error differences between images) Q1 . In this, the standard stars are measured using a large aperture to assess the total light. Typically, we used an 11 00 diameter aperture for this purpose. The Damocloids were measured using a smaller aperture (typically 2 00 in diameter) in order to minimize the effects of uncertainty in the background sky. Ideally, we would like to use even smaller apertures, but we determined (by trial and error) that differential motion between the targets and the stars introduced photometric errors when smaller apertures were used. The relation between large-aperture and small-aperture photometry was established using measurements with both apertures of five or more field stars in each image. Three objects in RESULTS The results of the photometry are listed in where r and Á are the heliocentric and geocentric distances, is the phase (Sun-object-Earth) angle, and is the phase coefficient. We used a nominal value ¼ 0:04 mag deg À1 that is typical for the nuclei of JFCs. This value is uncertain perhaps by AE0.02 mag deg À1 . The implied uncertainty on R(1, 1, 0) due to uncertainties in could be as large as $0.4 mag for 2000 HE 46 and (65407) 2002 RP 120 (both observed near ¼ 20 ; see In DISCUSSION Albedos The albedos of four Damocloids have been determined from simultaneous optical and thermal infrared photometry by different investigators. Using slow-rotator models (which tend to give higher albedos than are obtained from fast-rotator or isothermal models), all four objects are found to be extremely dark. Specifically, 1999 LE 3