Thermal Infrared and Optical Photometry of Asteroidal Comet C/2002 CE 10
Tomohiko Sekiguchi, Seidai Miyasaka, Budi Dermawan, Thomas Mueller, Naruhisa Takato, Junichi Watanabe, Hermann Boehnhardt
aa r X i v : . [ a s t r o - ph . E P ] D ec Icarus 00 (2018) 1–9
ICARUS
Thermal Infrared and Optical Photometry of Asteroidal CometC / Tomohiko Sekiguchi a , Seidai Miyasaka b , Budi Dermawan c , Thomas Mueller d , Naruhisa Takato e f , JunichiWatanabe e , Hermann Boehnhardt g a Hokkaido University of Education, 9 Hokumon, Asahikawa, Japan b Tokyo Metropolitan Government, 2-8-1, Nishishinjuku, Shinjuku, Tokyo, Japan c Department of Astronomy, Bandung Institute of Technology, Bandung 40132, Indonesia d Max-Planck-Institute for Extraterrestrial Physics, Giessenbachstrasse, 85748 Garching, Germany e National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, Japan f Subaru Telescope, 650 North A’ohoku Pl., Hilo, HI, USA g Max-Planck-Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 G¨ottingen, Germany
Abstract C / is an object in a retrograde elliptical orbit with Tisserand parameter − .
853 indicating a likely origin in the OortCloud. It appears to be a rather inactive comet since no coma and only a very weak tail was detected during the past perihelionpassage. We present multi-color optical photometry, lightcurve and thermal mid-IR observations of the asteroidal comet. Withthe photometric analysis in
BVRI , the surface color is found to be redder than asteroids, corresponding to cometary nuclei andTNOs / Centaurs. The time-resolved di ff erential photometry supports a rotation period of 8.19 ± ff ective diameter andthe geometric albedo are 17.9 ± ± / may be attribute to devolatilized material by surface aging su ff ered from the irradiation bycosmic rays or from impact by dust particles in the Oort Cloud. Alternatively, C / was formed of very dark refractorymaterial originally like a rocky planetesimal. In both cases, this object lacks ices (on the surface at least). The dynamical andknown physical characteristics of C / are best compatible with those of the Damocloids population in the Solar System,that appear to be exhaust cometary nucleus in Halley-type orbits. The study of physical properties of rocky Oort cloud objects maygive us a key for the formation of the Oort cloud and the solar system.c (cid:13) Keywords:
Asteroids, Asteroids surface, Comets, Comets nucleus, Infrared observations, Photometry,
1. Introduction
On February 2, 2002, the LINEAR project of the MIT Lincoln Laboratory in Socorro, USA, discovered an objectapproaching the Sun at about Jupiter’s distance. Although of asteroidal appearance without sign of activity, it wasclassified as cometary object, C / , based upon its retrograde comet-like orbit (see Table. 1). Deep imagingof the object, obtained with the Subaru telescope around the period of closest approach to Earth, revealed a short faint1 Icarus 00 (2018) 1–9 / (orbital elements taken from M.P.E.C. 2003-R41 a : semimajor axis 9.815 (au) q : perihelion distance 2.047 (au) Q : aphelion distance 17.585 (au) e : eccentricity 0.7915 i : inclination 145.46 ( ◦ ) ω : argument of perihelion 126.19 ( ◦ ) Ω : longitude of the ascending node 147.44 ( ◦ ) M : mean anomaly 0.0609 ( ◦ ) n : mean motion 0.0320 ( ◦ / d)Perihelion Passage 2003, June 22.10 TTEarth Approach 2003, Sept 04.90 TT ( ∆ = P : orbital period 30.75 (years) T J : Jupiter Tisserand parameter -0.853tail of C / (Takato et al. 2003). The faint tail may either be caused by very weak or by temporal cometaryactivity (sublimation of gas and release of embedded dust), or it may be due to a recent impact event of low, but non-zero occurrence probability. Since cometary coma activity in C / has not been reported so far despite deepimaging attempts using the Subaru telescope’s Prime Focus Camera: Suprime-Cam, and despite the object passedthrough perihelion well within the water sublimation limit, C / may represent a transitional object betweenthe population classes of comets and asteroids. This paper primarily presents an analysis of the nucleus properties ofthis object and the properties of dust tail with the Finson-Probstein analysis and no coma activity will be analyzed ina future paper.The criteria for classification a minor body as asteroid or as comet are appearance (coma and tail versus point-like) and orbit parameters, namely the Tisserand parameter; T J . The parameter T J characterizing the dynamical link ofminor bodies to the gravitational disturbance by planet Jupiter (Carusi et al. 1995) is used to di ff erentiate the Halley-type comets ( T J <
2) from the Jupiter-family comets (3 > T J >
2) and objects with T J > T J = a J a + n a J a (1 − e ) o cos( i ) , where a J and a are the semi-major axisof Jupiter and the object (asteroid, comet or others) respectively, i and e are the object’s inclination and eccentricity,respectively. Recently, this classification approach is challenged by the discovery of objects of cometary appearancein asteroid-like orbits, the so called ”Main Belt Comets” (MBCs), and of objects of point-like appearance in cometaryorbits. Jewitt (2005) assigned Halley-type orbit asteroids and inactive comets into a new group, the “Damocloids”,named after asteroid (5335) Damocles. Meech et al. (2016) reported that Oort cloud comet C / µ mwavelength, indicating that the comet may be physically similar to an inner main belt rocky S-type asteroid.The T J -value of C / is − .
853 (Tab. 1), indicating in object in an Halley-type orbit. This result togetherwith no coma appearance (Takato et al. 2003) suggests that C / might be either an extinct comet or aquasi-inert object that has been eject from the Oort Cloud, giving us a good opportunity to investigate basic physicalcharacteristics of an Oort Cloud object. Furthermore, the results may provide insights in the surface aging of minorbodies and the link between asteroids and comets. After an outline of the observations and data reduction performed,we present results on physical properties of C / : rotation period, axis ratio, dimension, albedo and colortaxonomy. In the final section of the paper we discuss, based upon our findings, the relation of C / withother minor body populations in the solar system.
2. Observations and Data Reduction C / was observed in September and October 2003 in the visible and thermal infrared wavelength ranges.2 Icarus 00 (2018) 1–9 / Date R ha ∆ b Phase Angle Observation Type (& band) Telescope sky condition(UT) (au) (au) (deg.)2003-Sep-06 2.20 1.23 9.8 ◦ thermal IR ( N ) ESO 3.6 m photometric2003-Oct-02 2.31 1.57 20.1 ◦ color ( BVRI ) & lightcurve ( I ) Kiso 1.05 m photometric2003-Oct-03 2.32 1.59 20.5 ◦ lightcurve ( I ) Kiso 1.05 m thin clouds2003-Oct-04 2.32 1.61 20.9 ◦ lightcurve ( I ) Kiso 1.05 m thin clouds2003-Oct-06 2.33 1.66 21.6 ◦ lightcurve ( I ) Kiso 1.05 m thin clouds2003-Oct-08 2.34 1.71 22.2 ◦ lightcurve ( I ) Kiso 1.05 m thin clouds a R h is the heliocentric distance. b ∆ is the geocentric distance. The
BVRI observations of C / were carried out on a photometric night, 2003-Oct-3, using the 1.05 mSchmidt telescope at the Kiso observatory, Japan. The CCD camera used has 2048 × µ m and covers a field of view (fov) of 50 ′ × ′ . It is well-suited for time-resolved observations of Solar Systemobjects using an adequate number of reference stars in the fov for di ff erential photometry of moving targets. B and V exposures were taken through Johnson-type filters, R and I exposures through Cron-Cousins-type filters. The multi-color photometric observations were embedded in the series of I -band exposures for lightcurve sampling (i.e. – I – I – B – I – V – I – R – I – I – sequence) in order to follow and compensate for brightness variations due to non-spherical shape oralbedo in combination with the rotation motion of the object. The photometric parameters of the telescope-instrumentcombination and of the atmosphere were determined by measuring various standard star fields at di ff erent airmassesLightcurve observations were performed between 2003-Oct-2 and 2003-Oct-8, occasionally with thin clouds. In orderto minimize the fluctuation of sky conditions and scattering of lunar light by thin cloud, I -band filter was used. Thestandard calibration frames (CCD bias and flatfield exposures) were also obtained as needed. Table 2 summarizes theobserving geometry, exposures types and sky conditions for the C / observations.Di ff erential photometry between C / and comparison stars in the fov is applied. In order to reducethe influence of possible variable stars on the photometric results, to gain a high signal-to-noise ratio, and to ensurethe confidence of the measurements and a good coverage for the lightcurve analysis, we selected comparison starsaccording to the following criteria: (1) as many as possible, (2) as bright as possible, (3) with maximum exposurelevel below 40,000 ADU to stay well inside the linearity range of the CCD detector, (4) visible and measurable in thewhole set of images of a single night. Daily extinction and zero-point parameters were derived from the exposures ofthe Landolt standard star fields and by measuring the comparison stars in the object fov. Magnitudes of comparisonstars are derived per night series using photometric data of the standard stars at the same airmasses. Thermal observations of C / were carried out on 2003-Sep-6. N -band images were taken in service modewith the 3.6 m telescope and the TIMMI2 instrument at the La Silla site of the European Southern Observatory ESOin Chile. TIMMI2, the Thermal Infrared Multi-Mode Instrument 2 (K¨aufl et al., 2003) has a 240 ×
320 pixel SiAsdetector, and it is operated at 6.5-7.5 K. The image scale used for our observations was 0 . ′′
202 pixel − which o ff ersa field of view of 64 ′′ × ′′ on the sky. The N ff ective central wavelengthof 8.6 µ m) was chosen because of an expected advantageous sensitivity for the observations of C / . Theindividual TIMMI2 detector integration time (DIT) was set to 20.8 milliseconds. The observations were performed asa series of 4 exposures using secondary mirror chopping and telescope nodding as follows: On target position 3 DITread-outs were taken at two chopping positions o ff set in North-South direction by 10 ′′ . This chopping-integrationcycle was repeated 60 times. Thereafter, the telescope was moved 10 ′′ in East-West direction and 80 chopping-integrations were repeated as before. An exposure series of C / were made with a total integration time of3 Icarus 00 (2018) 1–9 (solid line). Di ff erential photometric observations were performed from 2003-Oct-2 to 2003-Oct-8 withthe 1.05 m Schmidt telescope at the Kiso observatory in Japan. / the sky conditions were photometric with an average seeing of1 ′′ . The basic reduction for C / data makes use the TIMMI2 reduction pipeline (Relke et al. 2000, Sieben-morgen et al., 2004). The pipeline procedure automatically subtracts the pairs of “chopped” images and co-adds allthe frames of the whole chopping / nodding sequence (equivalent to one exposure series). Hence, the resulting imageshows 2 positive and 2 negative sub-images of C / . The two negative ones are multiplied by -1 and all sub-images of C / are shifted such that the pixel positions of the brightness center in the sub-images overlap. Atthe end the shifted subimages are coadded to the result frame of the respective exposure series. The TIMMI2 data areflux-calibrated using observations of standard star HD156277 which were obtained during the same night applying thesame filter setup and observing mode as for the observations of C / . An airmass correction factor of 7 % isapplied to compensate for the di ff erent airmasses of the standard star (airmass = = // / sci / facilities / lasilla / instruments / timmi / Reports / oschuetz / Projects / T2 Extinc / TIMMI2 extinc.html)and Schuetz & Sterzik (2004)). Because of the di ff erent spectral types of the standard star (K2-III) and the target(solar-type), the color correction factor is estimated to be 1–3 % in the N / at 8.7 µ m is 0.50 ±
3. Results
Two independent methods were used for the periodogram analysis of the I -filter lightcurve of C / , e.g.Lomb’s spectral analysis (Lomb 1976) which is widely used as a standard method to calculate the power spectraof unevenly spaced time-series data, and the Window-CLEAN method (Roberts et al. 1987) which was originallydeveloped in radio astronomy for UV-plane image synthesis. More specifically, we applied the implementation ofthe time series analysis by Yoshida et al. (2016) used for asteroid lightcurve data. This analysis approach wasused previously to determine the rotational period of asteroid 25143 Itokawa, which was later confirmed by theHayabusa space mission (Dermawan et al., 2002). The two highest peaks of Window-CLEAN periodograms of theC / lightcurve data coincide with those of Lomb-Scargle approach applied in parallel to the same dataset.The Lomb-Scargle scheme shows both peaks at significance level of 98 % (Fig. 2). A rotation period of 8.19 ± / . However, other solutions for the rotation period of the objectcannot completely ruled out based upon the dispersion inherent in the photometric data of C / (Fig. 1).4 Icarus 00 (2018) 1–9 / . The solid line shows the result for Window-CLEAN method, and thedashed-line is for Lomb-Scargle spectral analysis. The uncertainty of the rotation period was calculated considering the frequency interval (Fouriertransform) of Window-CLEAN around the highest peak of the obtained spectral power, i.e. 0.04656 h. The second highest peak is located at20.58 h. Note that the two highest peaks of Window-CLEAN coincide with those of Lomb-Scargle, and the Lomb-Scargle scheme shows bothpeaks with 98 % significance level. Table 3. Colors of C / in magnitude B − V V − R V − I V (1 , , a ± ± ± ± a V (1 , , = H is the absolute magnitude in V -bandAssuming that the lightcurve of C / are caused by the non-spherical shape of its nucleus alone (e.g.Sekiguchi et al., 2002), a lower limit for the shape elongation, i.e. the a / b axis ratio, can be estimated as a / b ≥ . ∆ m = . ± .
1, where ∆ m is the peak-to-valley amplitude of the fitted lightcurve (in magnitude). For the determination of the absolute magnitude of C / the analysis approach based upon the HG system(Bowell et al., 1989) of the International Astronomical Union IAU is chosen: V (1 , , α ) = V (1 , , − log (cid:20) (1 − G ) Φ ( α ) + G Φ ( α ) (cid:21) , (1)where V (1 , , α ) is the V -band magnitude reduced to unity Sun and Earth distances R h = ∆ = α ,and V (1 , ,
0) is the absolute magnitude H . G represents the slope parameter in V -band. Φ and Φ are describingthe phase function and are considered filter-independent. Since for C / the phase angle coverage of ourobservations is not su ffi cient, we applied a predetermined values of 0.15 as a default slope parameter G (e.g. Bowellet al., 1989). The estimated absolute magnitude of C / in V -band is H = . ± .
07 mag. This result isused together with the thermal flux of the object in order to constrain the size and albedo of C / - see nextsection.The photometric results of C / are illustrated in terms of colors B − V , V − R , V − I (see Tab. 3) thatare representing coarse measures of the global surface taxonomy of the object. In Fig. 3 the three broadband colorsof C / (marked by circle with error bars) are plotted together with the colors of asteroids (colored squares),comets, Transneptunian Objects (TNOs) and Centaurs (colored triangles). The surface taxonomy of C / is5 Icarus 00 (2018) 1–9 ff erent minor body groups including C / . The comet and Damocloids data were taken from Abellet al. (2003), Campins et al. (2006), Doressoundiram et al. (2007), Hicks and Bauer (2007), Jewitt et al. (2009), Lamy and Toth (2009), Jewitt(2015). The square symbols in Fig. 3 denote the taxonomy ranges of S-type, C-type, M-type and D-type asteroids, respectively, from the eight-colors asteroid survey data by Zellner et al. (1985) and from NASA’s Planetary Data System: Small Bodies Node (https: // pds.nasa.gov / ). The colorsfor TNOs are taken from Boehnhardt et al. (2003), Sheppard and Jewitt (2002), Mueller et al. (2004), Peixinho et al. (2004), Doressoundiram etal. (2007), Sheppard (2010), Perna et al. (2013) while the Centaurs data are taken from the papers of Gutierrez et al. (2001), Bauer et al. (2003),Doressoundiram et al. (2007), Perna et al. (2013), Jewitt (2015). clearly redder than and much beyond the colors of the di ff erent asteroid types. It falls well in the middle of the rangefor TNOs, Centaurs, and cometary nuclei. The thermal IR count rates of C / and of the standard star were measured from the respective N -bandimages using the aperture photometry method. Using the determined thermal flux of the standard star, the N -bandflux of C / was determined to be 0.50 ± S ν = π ε (cid:18) r N ∆ (cid:19) B ν ( T ) , (2)where r N is the radius of C / (in m), ε = ∆ is thegeocentric distance (in m) and B ν ( T ) is the Planck function for the e ff ective surface temperature T . The e ff ectivesurface temperature of minor bodies is determined from the energy balance at the surface (e.g. Lebofsky & Spencer,1989). However, it also implies knowledge or an estimation of the surface temperature of the object. Since bymeasurements of a single thermal filter band, the surface temperature distribution of a minor body cannot be estimated,a thermal model approximation is used instead. Usually, this approach has to adopt additional values for in principleunconstrained parameters like the chemical composition, material distribution (albedo map), emissivity, density (andporosity), heat conductivity and heat capacity of the surface materials, of the rotation period and the orientation of therotation axis with respect to the Sun.For our analysis of the TIMMI2 data of C / , we applied the so called NEATM model (Near EarthAsteroid Thermal Model; Harrris, 1998) that is adopted to a specific object group. The NEATM is modified from the6 Icarus 00 (2018) 1–9 / . The figure shows a graphical representation of the radiometric method: The solid(with error) curve represents the ”optical constraint”, based on the H -magnitude. The almost horizontal curve (with error) represents the ”thermalconstraint” coming from the TIMMI2 measurements. The thermal flux is tightly connected to the size of the object. Standard Thermal Model (STM) of Lebofsky et al. (1986) which is usually used for minor body applications (e.g.Sekiguchi et al., 2003).Fern´andez et al. (2013) and Licandro et al. (2016) show that comets as well as asteroids in cometary orbits(including Damocloids) present beaming parameters of η = . ± .
2. The bond albedo A and geometric albedo arerelated by A = p q = p V q , where p and p V are the bolometric geometric albedo and the geometric albedo in V -band,respectively. q is the bolometric phase integral which in the H - G system is derived from the slope parameter G , via q = . + . G (Bowell et al., 1989).The absolute magnitude H , geometric albedo p V and diameter D (km) for asteroids are related by (e.g., Fowler &Chillemi, 1986) log D = . − . H − . ( p V ) , (3)Fig. 4 shows the diameter versus the geometric albedo relationship for C / using the measurement resultsin the visible and thermal wavelength range (including uncertainties). In the figure the optical constraint, representingthe H magnitude of the object in Eq. (3), is shown as decreasing curves. The thermal constraint representing the IR fluxof the object is an almost horizontal curve (plus parallel lines indicating the uncertainty ranges). The simultaneoussolution of thermal and optical constraint in Eq. 2 and Eq. 3, respectively, is the the intersection of the respectivecurves. Thus, the best estimate for the radius r and albedo p V of C / is found to be: r = . ± . p V = . ± .
4. Discussion
The dynamical and physical characteristics of C / suggest several similarities with minor bodies fromthe outer solar system: very low Tisserand parameter, red color, low albedo. The presence of a tail in 2002 indicatesa link to the comets population, although it is not known whether it is driven by repetitive and extended periods ofcometary activity due to ice sublimation, or by a rare and singular activity event, caused for instance by an impact.7 Icarus 00 (2018) 1–9 C / is in an orbit that crosses the orbital distances of the giant planet. For this reason it is likely ina transitional orbit state, since it is exposed to encounters with the gas giants in the planetary system and thus togravitational scattering in the future and it was so in the past. Various reservoirs of minor bodies can be consideredas the original home region of C / : The Kuiper Belt and the Oort Cloud as main reservoirs for the short-periodic and long-periodic comets, respectively. While the known physical characteristics (size, color, albedo, axisratio) are no very conclusive (whether the object has to be seen as short-periodic comet coming from the Kuiper Beltor as long-periodic comet from the Oort Cloud), the Tisserand parameter and the high inclination orbit of C / clearly favor an origin as for the long-periodic comets.The Oort cloud objects (a ∼ / passed through in 2002 to 2003 with solar distances well within the water sublimation limit, may indicate a ratherexhausted state of the nucleus activity. So, C / can be seen as a member of the group of Damocloids. Itssurface color is in good agreement with the average color of Damocloids: B − V = . ± . V − R = . ± . R − I = . ± . B − R = . ± .
02 (Jewitt, 2015). Jewitt (2005) reported the lack of ultrared matter in Damocloidsas for instance seen in the population of the dynamically cold Classical Disk Objects in the Kuiper Belt (Hainaut etal., 2012). The estimated size of C / of 17 . ± . . ± .
01 (or0.02 if the an η range 0.8–1.2 is considered) agrees with the albedo range of 0.02–0.06 for cometary nuclei (see forinstance Campins and Fern´andez 2002, Lamy et al. 2004). It is close to the mean visible geometric albedo of 0.04 forJupiter-family comets obtained by Fern´andez et al. (2013), and also close to the mean geometric albedo of 0 . ± . / rep-resents the lower limit of the albedos found for Kuiper Belt objects (Lacerda et al. 2014) and is clearly below that ofseveral dynamical sub-populations therein. However, its surface colors would comply with these, while its retrogradeorbit may not be easily accomplished through gravitational scattering from that region by a giant planet.Comparing C / with short / long-periodic comets requires an explanation of its low level or absence ofactivity during perihelion: Exhaustion of the sublimating ices or coverage of the surface by thick layers of regolithfrom previous activity cycles that may prevent the solar heat wave to reach the still present ice reservoirs underneath.The fraction of active regions on cometary surfaces with volatile ices is generally small and most of the activityoriginates from sub-surface layers (e.g. Keller et al. 1986). Recently, the ROSETTA mission has obtained a significantnumber of detailed images of 67P / Churyumov-Gerasimenko that reveal a lack of distinct active region with exposedfresh ice chunks (Thomas et al., 2015). The mission has also demonstrated the existence of meter-thick regolith onthe nucleus surface that represents ballistic fall-back material from cometary activity.The ”Grand Tack” model showed that the giant planets scattered inner solar system material outward during theirinward migration, and vice versa, they scattered icy planetesimals into the inner solar system during their outwardmigration (Walsh et al, 2011). The surface color of C / is much redder than that of asteroids and perhaps itsextremely low Tisserand parameter and retrograde orbit which make on origin of the object in the main asteroid beltunlikely. However, a scenario of an inward scattered and stranded object from the outer solar system may be a validexplanation for C / .Recently inactive minor body with hyperbolic eccentricity, 1I / ′ Oumuamua) was discovered and its phys-ical properties are studied (Meech et al, 2017). It can be either from interstellar originally or from other planetarysystems, or from our Oort cloud as a result of multiple scattering due to stellar encounters. The investigation of theevolution of the Oort cloud may give a hint to understand the relationship among such outer rocky minor bodies.
Acknowledgments
We are grateful for the recommendations and suggestions to this manuscript made by Olivier Hainaut, HenryHsieh and one anonymous reviewer. This work is based on observations collected at the European Organization forAstronomical Research in the Southern Hemisphere ESO under programme 60.A-9126(F).8
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