Spectral characteristics and modeling of the trans-neptunian object (55565) 2002 AW197 and the Centaurs (55576) 2002 GB10 and (83982) 2002 GO9: ESO Large Program on TNOs and Centaurs

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<ul><li><p>Planetary and Space Science 53</p><p>lienP</p><p>.N</p><p>219</p><p>ri, LcInstitut dAstrophysique Spatiale, Universite Paris-Sud, 91405 Orsay Cedex, France</p><p>intimate mixtures of organics compounds (Triton tholins, amorphous carbon) and contaminated water ice, although other possibilities</p><p>exist.</p><p>evolutionary scenarios to explain the origin of the differentcategories of objects (for instance, the dynamically hot andcold CDO populations) (Levison and Morbidelli, 2003).</p><p>ARTICLE IN PRESS</p><p>Corresponding author. Fax: +331 45077719.E-mail address: alain.doressoundiram@obspm.fr</p><p>(A. Doressoundiram).</p><p>In the meanwhile observational studies of TNOs and</p><p>Centaurs using visible and near-IR imaging and spectro-scopy at large ground-based telescopes and from space</p><p>0032-0633/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.</p><p>doi:10.1016/j.pss.2004.11.007</p><p>1Based on observations obtained at the VLT Observatory Cerro Paranal</p><p>of European Southern Observatory, ESO in Chile within the framework of</p><p>program 167.C-0340.r 2005 Elsevier Ltd. All rights reserved.</p><p>Keywords: Kuiper Belt objects; Trans-neptunian objects; Centaurs; Spectroscopy; Radiative transfer; Solar system</p><p>1. Introduction</p><p>The Kuiper Belt beyond the orbit of Neptune is areservoir of primordial objects from the formation periodof the planetary system around the Sun. Therefore thestudy of these objects can provide information about theprocesses that governed the evolution of our young solarsystem as well as of other planetary systems around youngstars. These bodies frequently called Kuiper Belt Objects(KBOs) or Trans-Neptunian Objects (TNOs), are thus</p><p>believed to contain pristine material from the early periodof the solar system. However, some objects could haveevolved over the past 4.5 billion years due to collisions and/or physical alteration processes at the surface and under-neath (Barucci et al., 2004). The rst decade after thediscovery of 15760 (1992 QB1), the rst TNO after Pluto/Charon, saw the classication of about 1000 TNOs in thedynamical types like classical disk objects (CDOs),scattered disk objects (SDOs), Plutinos and Centaurs.Peculiarities in the orbit dynamics called for innovativeeCentro de Astronomia e Astrofisica da Universidade de Lisboa, PT-1349-018 Lisboa, Portugal</p><p>Received 6 August 2004; received in revised form 30 September 2004; accepted 26 November 2004</p><p>Available online 10 October 2005</p><p>Abstract</p><p>We present in this paper rst results on broadband photometry (JHK lters) and low-dispersion infrared spectroscopy performed at</p><p>ESO Very Large Telescope (VLT) for the trans-neptunian object (55565) 2002 AW197 and Centaurs (55576) 2002 GB10 and (83982) 2002</p><p>GO9. These observations were obtained in the framework of ESOs Large Program on Physical Studies of TNOs and Centaurs. All the</p><p>spectra are characterized by a strong red visiblenear infrared slope. There is no clear detection of water ice, except for the Centaur</p><p>(83982) 2002 GO9.</p><p>Analysis of these visiblenear infrared reectance spectra with radiative transfer models are compatible with a surface composed ofdMax-Planck-Institute for Astronomy, Koenigstuhl 17, D-69117 Heidelberg, GermanySpectral characteristics and mode(55565) 2002 AW197 and the C(83982) 2002 GO9</p><p>1: ESO Large</p><p>A. Doressoundirama,, M.A. Baruccia, GC. de Bergha,</p><p>aLESIA, Observatoire de Paris, F-9bINAF, Osservatorio astrofisico di Arcet(2005) 15011509</p><p>ng of the trans-neptunian objecttaurs (55576) 2002 GB10 androgram on TNOs and Centaurs</p><p>P. Tozzib, F. Pouletc, H. Boehnhardtd,. Peixinhoa,e</p><p>5 Meudon Principal Cedex, France</p><p>argo E. Fermi 5, I-50125 Firenze, Italy</p><p>www.elsevier.com/locate/pss</p></li><li><p>ARTICLE IN PRESSandhave become instrumental challenge for the rst census ofthe physical surface properties of these bodies. Even if theTNOs science has rapidly evolved from the dynamical andtheoretical point of view in the last few years, ourknowledge of the physical and compositional propertiesremains still very limited.Broadband photometry in the visible wavelength range</p><p>has been obtained for more than 150 objects to establishrm links between color and dynamical properties for thetwo CDO populations as well as for Centaurs. Despite allefforts, similar success has not (yet) been achieved forPlutinos and SDOs (see, for example, Hainaut andDelsanti, 2002; Tegler et al., 2003; Doressoundiram et al.,2002; Peixinho et al., 2004). The wide bandpasses of thecolor lters limit diagnostics for composition, whereasspectroscopy covering the wavelength 0.42.5 mm range isthe most effective tool to investigate the surface composi-tion of these objects. Only the use of 810m telescopes(VLT, Keck, Gemini and Subaru) allows us to obtainresults for, at least, the brightest objects.Visible spectra are mostly found to be featureless and</p><p>conrm the overall spectral slope characteristics of theobjects as measured in photometry. Thus, it came as asurprise that a few objects (Lazzarin et al., 2003; Fornasieret al., 2004) display visible spectra with weak and wideabsorption dips in the red and blue wavelength ranges thatare analogous to the features of hydrated silicates (deBergh et al., 2004). These materials, associated with theaction of the aqueous alteration process (Vilas et al., 1994),are commonly revealed on primitive main belt asteroids butthey are not expected on Centaurs and TNOs.Infrared results on TNOs are much sparser. Infrared</p><p>photometric colors of about 30 objects (Boehnhardt et al.,2001; McBride et al., 2003 and references therein) arepublished showing a general attening of the spectral slopetowards longer wavelength. Spectroscopy in the near-IR isstill more difcult since even more telescope time isrequired (typically one night per bright TNO at 810mtelescopes). Only about 15 objects have been observed inboth the visible and near-infrared regions and they show ahuge variety of spectral behaviour and composition.Conclusions from this small sample of objects are still ofsomewhat speculative character (Barucci et al., 2004).The visible and the infrared regions encompass diag-</p><p>nostic spectral features of minerals (like pyroxene, olivine,carbonaceous assemblages, organics, etc.) and ices (water,methanol, hydrocarbon, etc.). The visible slope is also veryimportant and can give constraints on the compositionparticularly for the red objects, diagnostic for the presenceof organic compounds like tholins or kerogen.In this paper we present new observational results of 3</p><p>objects observed photometrically and spectroscopically inthe visible and near-infrared range. The data have beenobtained as part of ESOs Large Program on PhysicalStudies of TNOs and Centaurs and results to date are</p><p>A. Doressoundiram et al. / Planetary1502published in Boehnhardt et al. (2002), Barucci et al. (2002),Lazzarin et al. (2003), Dotto et al. (2003), Doressoundiramet al. (2003), Peixinho et al. (2004) and Fornasier et al.(2004).</p><p>2. Observations and data reduction</p><p>We observed the TNO (55565) 2002 AW197 and twoCentaurs (55576) 2002 GB10 and (83982) 2002 GO9. Wepresent in this paper results from broadband photometry(JHK lters) and low-dispersion infrared spectroscopyperformed at ESO Very Large Telescope (VLT) in Chile.Visible data (BVRI photometry and visible spectroscopy)have also been obtained during the same observing run buthave been published in another paper (Fornasier et al.,2004). However, these data are briey reported here as theywill be discussed and modeled together with the infrareddata. All the observations and aspect data are reported inTable 1.</p><p>2.1. Near-infrared photometry</p><p>The near-infrared photometric measurements reportedhere were recorded with the ISAAC instrument on the rstUnit (UT1, Antu) 8m telescope of the VLT on nights ofMarch 911, 2003. The ISAAC IR camera was equippedwith a Rockwell Hawaii 1024 1024 pixel array. The pixelscale is 0.1484 arcsec/pixel and the eld of view 2.5 arc-min 2.5 arcmin.We recorded series of images in the J, H and Ks lters,</p><p>centered at 1.25, 1.65 and 2.16 mm, respectively, before eachspectroscopic observation. Observations were obtainedthrough the photometric sequence JHKJ aimed at mini-mizing systematic errors due to objects rotation whenderiving JH and JK colors. For each target a set offrames moved in dither pattern was recorded as is commonin infrared photometric acquisition.Exposures were performed under clear to photometric</p><p>conditions with dark skies and a seeing between 0.6 and1.2 arcsec. We observed several infrared photometricstandard stars (Persson et al., 1998) for ux calibration.The photometric reduction was performed using the jitterroutine of the ECLIPSE package and the IRAF (ImageReduction and Analysis Facility) software package ofNOAO, following the data processing steps described inRomon et al. (2001) for image combination and skysubtraction. The magnitudes of stars and objects werenally measured using aperture correction photometry foreach lter. The results of our photometric measurementsare listed in Table 2.</p><p>2.2. Visible photometry</p><p>The visible photometry was obtained using the FORS1instrument also on the rst Unit (UT1) 8m telescope of theVLT during the night of 8 March 2003 under photometricconditions, dark skies and a seeing between 0.5 and</p><p>Space Science 53 (2005) 150115090.8 arcsec. The observations covered the B, V, R and Ibands, with images obtained through a photometric</p></li><li><p>2.4. Visible spectroscopic data</p><p>ARTICLE IN PRESS</p><p>r</p><p>4</p><p>4</p><p>4</p><p>4</p><p>1</p><p>1</p><p>1</p><p>1</p><p>1</p><p>1</p><p>c di</p><p>.html).</p><p>andsequence RVBIV aimed at minimizing systematic errorsdue to objects rotation (see Doressoundiram and Boehn-hardt (2003) for details on the specic observationalstrategy of TNOs). These photometric results have beenpublished in Fornasier et al. (2004) and are also reported inTable 2.</p><p>2.3. Near-infrared spectroscopy</p><p>Table 1</p><p>Aspect data during the observations</p><p>Object Group UT date</p><p>(55565) 2002 AW197 Classical 2003 Mar. 09</p><p>2003 Mar. 10</p><p>2003 Mar. 11</p><p>2003 Mar. 12</p><p>(55576) 2002 GB10 Centaur 2003 Mar. 09</p><p>2003 Mar. 11</p><p>2003 Mar. 12</p><p>(83982) 2002 GO9 Centaur 2003 Mar. 09</p><p>2003 Mar. 10</p><p>2003 Mar. 12</p><p>Group: dynamical class of objects. r, D, a are, respectively, the heliocentriPlanet Ephemeris Service http://cfa-www.harvard.edu/iau/MPEph/MPEph</p><p>A. Doressoundiram et al. / PlanetaryNear infrared spectroscopic measurements were per-formed using ISAAC in its Low Resolution mode. We useda slit width of 1 arcsec oriented along the EW direction.The resulting spectral resolution was about 500. Theobservations were done by nodding the object along theslit in two different positions (A and B) separated by10 arcsec. Each night, at least 3 solar analog stars werealso observed at similar air masses. Since the spectra of thestars, once normalized, were very similar (difference lessthan 1%/100 nm) we used their average to compute thereectivity of each object. The stars were Landolt 98-978,Landolt 102-1081 and Landolt 107-998. All the J spectra of2002 AW197 were completely blank for an unknownreason, probably technical problems with the ISAACinstrument.The spectroscopic data were reduced using the ECLIPSE</p><p>package and the MIDAS-ESO software. The basis of thedata reduction process is to combine pairs of imagedifferences (e.g. AB and BA) in order to properlyremove sky contribution and get an improved S/N ratio.For details of the method, see Barucci et al. (2000). Allindividual spectra were checked and those with very lowS/N ratio were discarded from the image combinationprocess. Table 3 gives the details of the spectroscopicobservations with the effective exposure time. Due toVisible spectra of the same 3 objects were obtainedduring the same run with the FORS1 instrument (togetherwith the visible photometry) and have been published in aobject visibility and exposure time needed to achieve goodsignal-to-noise ratio (SNR), we could not obtain all the J,H and K spectra for each object on a single night.(AU) D (AU) a (deg) Observations</p><p>7.26 46.45 0.7 BVRI photometry</p><p>7.26 46.46 0.7 JHK photometry</p><p>JH spectroscopy</p><p>7.26 46.47 0.7 K spectroscopy</p><p>7.26 46.48 0.7 JHK photometry</p><p>K spectroscopy</p><p>5.19 14.33 1.9 BVRI photometry</p><p>5.19 14.31 1.8 JHK photometry</p><p>JHK spectroscopy</p><p>5.19 14.30 1.8 JHK photometry</p><p>K spectroscopy</p><p>4.06 13.44 3.2 BVRI photometry</p><p>4.06 13.43 3.2 JHK photometry</p><p>JHK spectroscopy</p><p>4.06 13.40 3.1 JHK photometry</p><p>K spectroscopy</p><p>stance, the topocentric distance and the phase angle of the object (Minor</p><p>Space Science 53 (2005) 15011509 1503separate paper (Fornasier et al., 2004).</p><p>3. Results</p><p>3.1. Photometry</p><p>Table 4 gives a summary of the derived averaged colorsand Table 5 gives other useful physical parameters derivedfrom our observations: the absolute magnitude in the Vband, HV and the size (estimated). Table 5 also gives thealbedo and rotational period (when available from theliterature) and the orbital elements.Observations of TNOs are made at small phase angles</p><p>where the opposition-brightening effect is likely to occur.Recent studies (Belskaya et al., 2003) have shown that thiseffect could be signicant for TNOs and Centaurs.Absolute magnitudes (HV) are computed using the linearphase function fa 10ab:HV V 1; 1; 0 V 5 logrD ab,where V is the V-band magnitude, r is the objectsheliocentric distance (AU), D is the objects geocentricdistance (AU), a is the phase angle (deg) and b is the phasecurve slope (mag/deg).</p></li><li><p>ARTICLE IN PRESS</p><p>Table 3</p><p>Spectroscopic observations</p><p>Object Date UT-start (hh:mm) Air mass start Air mass end Spectral band Texp (min)</p><p>(55565) 2002 AW197 2003 Mar. 10 01:53 1.22 1.23 H 120</p><p>2003 Mar. 11 00:23 1.38 1.22 K 120</p><p>2003 Mar. 12 01:00 1.28 1.22 K 90</p><p>(55576) 2002 GB10 2003 Mar. 11 03:57 1.14 1.05 J 42</p><p>2003 Mar. 11 04:54 1.05 1.05 H 120</p><p>2003 Mar. 11 07:14 1.05 1.47 K 120</p><p>2003 Mar. 12 03:29 1.22 1.05 K 72</p><p>(83982) 2002 GO9 2003 Mar. 10 05:54 1.29 1.16 J 42</p><p>2003 Mar. 10 06:50 1.16 1.11 H 120</p><p>2003 Mar. 10 09:11 1.11 1.17 K 30</p><p>2003 Mar. 12 06:29 1.17 1.16 K 156</p><p>Table 2</p><p>Photometric observations and results</p><p>Object Date UT-start</p><p>(hh:mm)</p><p>Filter Texp (s) Mag. Colors</p><p>(55565) 2002 AW197 2003 Mar. 09 02:17 R 60 19.8270.03 VR 0.6270.0302:19 V 60 20.4470.0202:21 B 180 21.3470.04 BV 0.9070.0302:26 I 120 19.2670.04 VI 1.1870.0302:29 V 60 20.4470.02</p><p>2003 Mar. 10 00:22 J 120 18.6870.0400:25 H 300 18.3370.05 JH 0.3570.0600:34 K 600 18.0670.04 JK 0.6270.06</p><p>2003 Mar. 12 00:23 J 120 18.6270.0400:27 H 360 18.3270.05 JH 0.3070.0600:37 K 600 18.0570.04 JK 0.5070.0600:57 J 120 18.5570.04</p><p>(55576) 2002 GB10 2003 Mar. 09 04:22 R 60 19.2870.03 VR 0.7270.0304:24 V 60 20.0070.0204:26 B 180 21.1270.04 BV 1.1270.0304:30 I 120 18.6770.04 VI 1.3270.0304:34 V 60 19.9970.02</p><p>2003 Mar. 11 03:23 J 120 17.9970.0703:27 H 360 17.6470.05 JH 0.3570.0903:37 K 600 17.6370.05 JK 0.2770.0903:54 J 120 17.9070.07</p><p>2003 Mar. 12 02:51 J 120 17.8770.0402:55 H 384 17.6170.04 JH 0.2670.0603:06 K 640 17.6170.04 JK 0.1970.0603:25 J 120 17.8070.04</p><p>(83982) 2002 GO9 2003 Mar. 09 07:24 R 60 20.0770.03 VR 0.7670.0307:27 V 60 20.8370.0307:29 B 180 21.9670.04 BV 1.1370.0307:33 I 120 19.4070.04 VI 1.4470.0307:37 V 60 20.8470.03</p><p>2003 Mar. 10 05:19 J 120 18.4270.0405:33 K 600 18.0170.04 JK 0.3...</p></li></ul>