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Spectral characteristics and modeling of the trans-neptunian object(55565) 2002 AW197 and the Centaurs (55576) 2002 GB10 and

(83982) 2002 GO91: ESO Large Program on TNOs and Centaurs

A. Doressoundirama,�, M.A. Baruccia, G.P. Tozzib, F. Pouletc, H. Boehnhardtd,C. de Bergha, N. Peixinhoa,e

aLESIA, Observatoire de Paris, F-92195 Meudon Principal Cedex, FrancebINAF, Osservatorio astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, ItalycInstitut d’Astrophysique Spatiale, Universite Paris-Sud, 91405 Orsay Cedex, FrancedMax-Planck-Institute for Astronomy, Koenigstuhl 17, D-69117 Heidelberg, Germany

eCentro de Astronomia e Astrofisica da Universidade de Lisboa, PT-1349-018 Lisboa, Portugal

Received 6 August 2004; received in revised form 30 September 2004; accepted 26 November 2004

Available online 10 October 2005

Abstract

We present in this paper first results on broadband photometry (JHK filters) and low-dispersion infrared spectroscopy performed at

ESO Very Large Telescope (VLT) for the trans-neptunian object (55565) 2002 AW197 and Centaurs (55576) 2002 GB10 and (83982) 2002

GO9. These observations were obtained in the framework of ESO’s Large Program on ‘Physical Studies of TNOs and Centaurs’. All the

spectra are characterized by a strong red visible–near infrared slope. There is no clear detection of water ice, except for the Centaur

(83982) 2002 GO9.

Analysis of these visible–near infrared reflectance spectra with radiative transfer models are compatible with a surface composed of

intimate mixtures of organics compounds (Triton tholins, amorphous carbon) and contaminated water ice, although other possibilities

exist.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Kuiper Belt objects; Trans-neptunian objects; Centaurs; Spectroscopy; Radiative transfer; Solar system

1. Introduction

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

e front matter r 2005 Elsevier Ltd. All rights reserved.

s.2004.11.007

ing author. Fax: +331 45077719.

ess: [email protected]

iram).

servations obtained at the VLT Observatory Cerro Paranal

uthern Observatory, ESO in Chile within the framework of

-0340.

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 first decade after thediscovery of 15760 (1992 QB1), the first TNO after Pluto/Charon, saw the classification 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 ‘innovative’evolutionary scenarios to explain the origin of the differentcategories of objects (for instance, the dynamically hot andcold CDO populations) (Levison and Morbidelli, 2003).In the meanwhile observational studies of TNOs and

Centaurs using visible and near-IR imaging and spectro-scopy at large ground-based telescopes and from space

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ARTICLE IN PRESSA. Doressoundiram et al. / Planetary and Space Science 53 (2005) 1501–15091502

have become instrumental challenge for the first 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 rangehas been obtained for more than 150 objects to establishfirm 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 filters limit diagnostics for composition, whereasspectroscopy covering the wavelength 0.4–2.5 mm range isthe most effective tool to investigate the surface composi-tion of these objects. Only the use of 8–10m telescopes(VLT, Keck, Gemini and Subaru) allows us to obtainresults for, at least, the brightest objects.

Visible spectra are mostly found to be featureless andconfirm 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. Infraredphotometric colors of about 30 objects (Boehnhardt et al.,2001; McBride et al., 2003 and references therein) arepublished showing a general flattening of the spectral slopetowards longer wavelength. Spectroscopy in the near-IR isstill more difficult since even more telescope time isrequired (typically one night per bright TNO at 8–10mtelescopes). 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-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 3objects observed photometrically and spectroscopically inthe visible and near-infrared range. The data have beenobtained as part of ESO’s Large Program on ‘PhysicalStudies of TNOs and Centaurs’ and results to date arepublished in Boehnhardt et al. (2002), Barucci et al. (2002),Lazzarin et al. (2003), Dotto et al. (2003), Doressoundiram

et al. (2003), Peixinho et al. (2004) and Fornasier et al.(2004).

2. Observations and data reduction

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 filters) 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 briefly reported here as theywill be discussed and modeled together with the infrareddata. All the observations and aspect data are reported inTable 1.

2.1. Near-infrared photometry

The near-infrared photometric measurements reportedhere were recorded with the ISAAC instrument on the firstUnit (UT1, Antu) 8m telescope of the VLT on nights ofMarch 9–11, 2003. The ISAAC IR camera was equippedwith a Rockwell Hawaii 1024� 1024 pixel array. The pixelscale is 0.1484 arcsec/pixel and the field of view 2.5 arc-min� 2.5 arcmin.We recorded series of images in the J, H and K’s filters,

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 object’s rotation whenderiving J–H and J–K 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

conditions with dark skies and a seeing between 0.6 and1.2 arcsec. We observed several infrared photometricstandard stars (Persson et al., 1998) for flux 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 werefinally measured using aperture correction photometry foreach filter. The results of our photometric measurementsare listed in Table 2.

2.2. Visible photometry

The visible photometry was obtained using the FORS1instrument also on the first Unit (UT1) 8m telescope of theVLT during the night of 8 March 2003 under photometricconditions, dark skies and a seeing between 0.5 and0.8 arcsec. The observations covered the B, V, R and Ibands, with images obtained through a photometric

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Table 1

Aspect data during the observations

Object Group UT date r (AU) D (AU) a (deg) Observations

(55565) 2002 AW197 Classical 2003 Mar. 09 47.26 46.45 0.7 BVRI photometry

2003 Mar. 10 47.26 46.46 0.7 JHK photometry

JH spectroscopy

2003 Mar. 11 47.26 46.47 0.7 K spectroscopy


K spectroscopy

(55576) 2002 GB10 Centaur 2003 Mar. 09 15.19 14.33 1.9 BVRI photometry


JHK spectroscopy


K spectroscopy

(83982) 2002 GO9 Centaur 2003 Mar. 09 14.06 13.44 3.2 BVRI photometry


JHK spectroscopy


K spectroscopy

Group: dynamical class of objects. r, D, a are, respectively, the heliocentric distance, the topocentric distance and the phase angle of the object (Minor

Planet Ephemeris Service http://cfa-www.harvard.edu/iau/MPEph/MPEph.html).

A. Doressoundiram et al. / Planetary and Space Science 53 (2005) 1501–1509 1503

sequence RVBIV aimed at minimizing systematic errorsdue to object’s rotation (see Doressoundiram and Boehn-hardt (2003) for details on the specific observationalstrategy of TNOs). These photometric results have beenpublished in Fornasier et al. (2004) and are also reported inTable 2.

2.3. Near-infrared spectroscopy

Near infrared spectroscopic measurements were per-formed using ISAAC in its Low Resolution mode. We useda slit width of 1 arcsec oriented along the E–W 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 by�10 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 thereflectivity 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 ECLIPSEpackage and the MIDAS-ESO software. The basis of thedata reduction process is to combine pairs of imagedifferences (e.g. A–B and B–A) 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 to

object 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.

2.4. Visible spectroscopic data

Visible spectra of the same 3 objects were obtainedduring the same run with the FORS1 instrument (togetherwith the visible photometry) and have been published in aseparate paper (Fornasier et al., 2004).

3. Results

3.1. Photometry

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

where the opposition-brightening effect is likely to occur.Recent studies (Belskaya et al., 2003) have shown that thiseffect could be significant for TNOs and Centaurs.Absolute magnitudes (HV) are computed using the linearphase function fðaÞ ¼ 10�ab:

HV ¼ V ð1; 1; 0Þ ¼ V � 5 logðrDÞ � ab,

where V is the V-band magnitude, r is the object’sheliocentric distance (AU), D is the object’s geocentricdistance (AU), a is the phase angle (deg) and b is the phasecurve slope (mag/deg).

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Table 3

Spectroscopic observations

Object Date UT-start (hh:mm) Air mass start Air mass end Spectral band Texp (min)

(55565) 2002 AW197 2003 Mar. 10 01:53 1.22 1.23 H 120

2003 Mar. 11 00:23 1.38 1.22 K 120

2003 Mar. 12 01:00 1.28 1.22 K 90

(55576) 2002 GB10 2003 Mar. 11 03:57 1.14 1.05 J 42

2003 Mar. 11 04:54 1.05 1.05 H 120

2003 Mar. 11 07:14 1.05 1.47 K 120

2003 Mar. 12 03:29 1.22 1.05 K 72

(83982) 2002 GO9 2003 Mar. 10 05:54 1.29 1.16 J 42

2003 Mar. 10 06:50 1.16 1.11 H 120

2003 Mar. 10 09:11 1.11 1.17 K 30

2003 Mar. 12 06:29 1.17 1.16 K 156

Table 2

Photometric observations and results

Object Date UT-start

(hh:mm)

Filter Texp (s) Mag. Colors

(55565) 2002 AW197 2003 Mar. 09 02:17 R 60 19.8270.03 V–R ¼ 0.6270.03

02:19 V 60 20.4470.02

02:21 B 180 21.3470.04 B–V ¼ 0.9070.03

02:26 I 120 19.2670.04 V–I ¼ 1.1870.03

02:29 V 60 20.4470.02

2003 Mar. 10 00:22 J 120 18.6870.04

00:25 H 300 18.3370.05 J–H ¼ 0.3570.06

00:34 K 600 18.0670.04 J–K ¼ 0.6270.06

2003 Mar. 12 00:23 J 120 18.6270.04

00:27 H 360 18.3270.05 J–H ¼ 0.3070.06

00:37 K 600 18.0570.04 J–K ¼ 0.5070.06

00:57 J 120 18.5570.04

(55576) 2002 GB10 2003 Mar. 09 04:22 R 60 19.2870.03 V–R ¼ 0.7270.03

04:24 V 60 20.0070.02

04:26 B 180 21.1270.04 B–V ¼ 1.1270.03

04:30 I 120 18.6770.04 V–I ¼ 1.3270.03

04:34 V 60 19.9970.02

2003 Mar. 11 03:23 J 120 17.9970.07

03:27 H 360 17.6470.05 J–H ¼ 0.3570.09

03:37 K 600 17.6370.05 J–K ¼ 0.2770.09

03:54 J 120 17.9070.07

2003 Mar. 12 02:51 J 120 17.8770.04

02:55 H 384 17.6170.04 J–H ¼ 0.2670.06

03:06 K 640 17.6170.04 J–K ¼ 0.1970.06

03:25 J 120 17.8070.04

(83982) 2002 GO9 2003 Mar. 09 07:24 R 60 20.0770.03 V–R ¼ 0.7670.03

07:27 V 60 20.8370.03

07:29 B 180 21.9670.04 B–V ¼ 1.1370.03

07:33 I 120 19.4070.04 V–I ¼ 1.4470.03

07:37 V 60 20.8470.03

2003 Mar. 10 05:19 J 120 18.4270.04

05:33 K 600 18.0170.04 J–K ¼ 0.3770.06

05:50 J 120 18.3870.04

2003 Mar. 12 05:54 J 120 18.3670.04

05:58 H 360 17.9970.04 J–H ¼ 0.3770.06

06:08 K 600 18.1370.05 J–K ¼ 0.3170.06

06:25 J 120 18.4470.04

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Table 4

Mean colors

Object Va B–V V–R V–I V–Jb J–Hc J–Kc

Solar colors 0.67 0.36 0.69 1.08 0.29 0.35

(55565) 2002 AW197 20.4470.04 0.9070.03 0.6270.03 1.1870.03 1.8270.06 0.3370.04 0.5670.08

(55576) 2002 GB10 20.0070.04 1.1270.03 0.7270.03 1.3270.03 2.1170.08 0.2970.06 0.2170.06

(83982) 2002 GO9 20.8470.03 1.1370.03 0.7670.03 1.4470.03 2.4470.06 0.3770.06 0.3470.04

aV-band magnitude.bComputed from mean V and mean J (2–4 measurements).cWhen multiple colors were available, a weighted mean has been computed. Solar colors from Hardorp (1980) and Hartmann et al. (1982).

Table 5

Physical properties

Object Group HVa Sizeb (km) Albedo Rot period (h) Amplitude ae (AU) ee (AU) ie (deg) qe (AU)

(55565) 2002 AW197 Clas 3.6370.02 886c 0.101c 9.345 or 15.17 47.50 0.13 24.3 41.34

�0.09 magf

(55576) 2002 GB10 Cent 8.1070.02 143 25.24 0.40 13.3 15.19

(83982) 2002 GO9 Cent 9.1070.03 90 6.97 or 9.67 19.50 0.28 12.8 14.04

0.14 magd

aHV ¼ absolute V-band magnitude from this work.bThe equivalent diameter has been derived from the absolute magnitude and assuming an albedo of 0.05.cSize and red geometric albedo measured by Margot et al. (2002).dData from Ortiz et al. (2003).eData from the Minor Planet Center (http://cfa-www.harvard.edu/iau/lists/MPLists.html).fData from Ortiz, pers. comm.


For TNOs we took the modal value of the measurementspublished by Sheppard and Jewitt (2002): b ¼ 0:14� 0:03.For Centaurs, we used a value of b ¼ 0:11� 0:01 obtainedfrom a least-squares fit of the linear approximation fðaÞ tothe published data. These rather steep slopes show that thephase correction is important when calculating absolutemagnitude of TNOs. However, these numbers have to beconsidered as rough estimates due to low statistics.

3.2. Spectroscopy

The final spectra (visible and near-infrared) are plottedin Figs. 1–3. The spectra have been divided by the averagespectrum of the solar analogs and normalized to 1 at0.55 mm. We combined the individual K spectra obtainedon two nights because the individual spectrum of the night10 March 2003 (30mn exposure, see Table 3) had too lowS/N to be useful alone. Note that the spectrum of night 12March 2003 actually mostly contributes to the final Kspectrum (156mn exposure). Due to the specific opera-tional mode of ISAAC, the different J, H and K spectra areobtained separately, so that the three spectra have to beadjusted with the accompanying photometric measure-ments in the J, H and Ks filters.

To improve the SNR, the spectra were convolved with aGaussian. This convolution improves the SNR at the costof the spectral resolution. The full-width at half-maximumof the Gaussian filtering was 5 pixels resulting in a finalspectral resolution of about 300.

All the spectra are characterized by a strong red slopefrom the V to the J wavelengths. (83982) 2002 GO9 inparticular is a very red object with a V–J ¼ 2.44. Thespectra of (55565) 2002 AW197 and (55576) 2002 GB10

appear featureless, with no clear evidence for water icesignatures. On the other hand, the spectrum of (83982)2002 GO9 shows a strong absorption feature at about 2 mm,diagnostic of water ice. However, no water ice absorptionband at 1.5 mm is clearly detected, possibly because of lowSNR. Also, one should bear in mind that the wholespectrum, from the visible to infrared wavelengths has notbeen obtained simultaneously, and thus each part of thespectrum (visible, J, H, K) may monitor different parts ofthe surface due to the object’s rotation.

4. Spectral modeling

The extraction of quantitative information on thecomposition and physical state from the spectra of theseicy outer solar system objects requires accurate models ofradiative transfer within their regolith. In this work, we usethe approach adopted by Shkuratov et al. (1999). Thisapproach, based on the geometrical optics approximationas is the more familiar Hapke model (1981), is used totransform optical constants into a reflectance spectrum andprovide the spectral albedo of powdered surfaces. Inaddition to common intimate mixtures, an interesting typeof mixture can be formulated: the intramixture. Someindividual large particles (likely made of water ice) can

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Fig. 1. Spectra of (55565) 2002 AW197 in the V, H and K ranges. The spectra have been adjusted using broadband photometry (BVRI and JHK filters)

and have been normalized to 1 at 0.55mm. The visible spectrum is from Fornasier et al. (2004). Two compositional models are proposed. One (solid line) is

composed of 66% of amorphous carbon (10 mm grain size), 23% of Triton tholin (8 mm grain size) and 11% of contaminated water ice (intramixture of

water ice and 7% of Titan tholin, 10mm grain size). The mean optical albedo at V-band wavelength is 0.05. The second suggested model (dashed line) is

composed of 48% of amorphous carbon (24 mm grain size), 40% of Triton tholin (5 mm grain size) and 12% of contaminated water ice (intramixture of

water ice and 0.07% of ice tholin, 5 mm grain size). The mean optical albedo at V-band wavelength is 0.08.

Fig. 2. Spectra of (55576) 2002 GB10 in the V, J, H and K ranges. The spectra have been adjusted using broadband photometry (BVRI and JHK filters)

and have been normalized to 1 at 0.55mm. The compositional model shown (solid line) is composed of 56% of amorphous carbon (9mm grain size), 11%

of Triton tholin (5mm grain size) and 33% of contaminated water ice (intramixture of water ice and 14% of Titan tholin, 6mm grain size). The mean

optical albedo at V-band wavelength is 0.05. The visible spectrum is from Fornasier et al. (2004).

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Fig. 3. Spectra of (83982) 2002 GO9 in the V, J, H and K ranges. The spectra have been adjusted using broadband photometry (BVRI and JHK filters)

and have been normalized to 1 at 0.55mm. The compositional model shown (solid line) is composed of 38% of amorphous carbon (5mm grain size), 40%

of Triton tholin (5mm grain size) and 22% of contaminated water ice (intramixture of water ice and 10% of Titan tholin, 39mm grain size). The mean

optical albedo at V-band wavelength is 0.05. The visible spectrum is from Fornasier et al. (2004).


contain a mixture of different materials visualized as smallinclusions in the bulk, as might result from initially purewater ice evolving in composition under the effects of spaceweathering and meteoritic bombardment. The non-icycontaminants can also be seen as an intrinsic material.More details on the physical realism of this type of mixturecan be found in Poulet et al. (2002).

For each object, the compositional model has to satisfy 2major constraints: the strong red slope from UV to NIRand the assumed dark albedo. The tholins have been usedas coloring agents in spectral compositional models ofseveral surfaces in the outer Solar System. Therefore, 3kinds of tholins are used here, Triton tholin, Titan tholinand Ice tholin. Spectrally neutral low albedo amorphouscarbon is included to lower the average spectral albedowithout altering the spectral shape overall. Although nowater ice absorption band is clearly detected possiblybecause of the low SNR, grains of crystalline water ice arenevertheless considered as end-member in order to evaluatethe limits on the water ice abundance.

The data are fitted using a simplex minimizationalgorithm. We first tried to model the spectra with atypical intimated mixture of tholins and amorphouscarbon, but to achieve the red slope at the correct albedolevel, we need tholin grain sizes of the order of 1 mm whichviolates Shkuratov’s theory. We finally find, for eachobject, a best fit consisting of an intimate mixture of grainsof amorphous carbon, tholins plus water ice contaminatedby Titan (Ice Tholin in one case) tholin inclusions with an

upper limit of polluted water ice concentration of 33% inthe case of 2002 GB10.The numeric best-fit values of the fractional carbon,

Triton tholin and water ice is only meant to be taken as anindication of the slope of the spectra; the models are notunique and many different combinations of material grainsizes and mixing spatial scales can be used to fit the redslopes and weak water–ice absorption features. The best-fitmodels are shown in Figs. 1–3, as solid lines superimposedon the observed spectra.

4.1. (55565) 2002 AW197

Margot et al. (2002) have found a red geometric albedoof pR ¼ 0:101þ0:038�0:022 for this object. Given the V–R ¼ 0.62(see Table 4), we then compute the albedo in the V-band,pV ¼ 0:079. This value is a prime constraint for our model.The solid line in Fig. 1 is our proposed best solution for thesurface composition of (55565) 2002 AW197. The model isan intimate mixture composed of 66% of amorphouscarbon (10 mm grain size), 23% of Triton tholin (8 mm grainsize) and 11% of contaminated water ice (intramixture ofwater ice and Titan tholin, 10 mm grain size). The meanoptical albedo at V-band wavelength is 0.05. The albedo is,however, a little low compared to the measured value. Analternative solution is given by the dashed line (Fig. 1) withan albedo of 0.08. The model is an intimate mixturecomposed of 48% of amorphous carbon (24 mm grain size),40% of Triton tholin (5 mm grain size) and 12% of

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contaminated water ice (intramixture of water ice and Icetholin, 5 mm grain size). However, with the later model, thefit is poor in the visible part of the spectrum. These twoproposed models show that the solution is not unique.

4.2. (55576) 2002 GB10

We propose for the surface composition of (55576) 2002GB10 a model (Fig. 2) composed of an intimate mixture of56% of amorphous carbon (9 mm grain size), 11% ofTriton tholin (5 mm grain size) and 33% of contaminatedwater ice (intramixture of water ice and Titan tholin, 6 mmgrain size). The computed mean optical albedo at V-bandwavelength is 0.05.

This model has a poor fit for the J–H (1.1–1.8 mm) partof the spectrum. Even if no detectable signature werepresent, we still used mixtures including various minerals inour model, but without any success. The compositionalmodel proposed here is our best fit including tholins,carbon and water ice as end members.

4.3. (83982) 2002 GO9

We found an intimate mixture (solid line in Fig. 3)composed of 38% of amorphous carbon (5 mm grain size),40% of Triton tholin (5 mm grain size) and 22% ofcontaminated water ice (intramixture of water ice andTitan tholin, 39 mm grain size), with a mean optical albedoat V-band wavelength of 0.05. Our model reproduces quitenicely the high SNR visible spectrum. However, it fails toreproduce the H part of spectrum where the 1.5 mm waterice band seems absent while the 2 mm band is clearlydetected. This discrepancy in the spectral behavior of(83982) 2002 GO9 may be explained if the Centaur has aheterogeneous surface composition. Indeed, due to thecharacteristics of the ISAAC instrument, each of the J, Hand K spectra ars recorded separately, and the longexposures times coupled with the rotation (see Table 5) ofthe object may lead to observations of different parts of theCentaur.

The infrared spectrum of (83982) 2002 GO9 showspossibly a feature around 2.3 mm. Such a band has beenobserved on the Centaur Pholus, and has been tentativelyattributed to solid methanol (Cruikshank et al., 1998). Inorder to reproduce this 2.3 mm feature, we included in ourcompositional model solid methanol. However, the addi-tion of this material yields a poor model fit to the data,especially around 1.6 mm where solid methanol has aprominent band.

5. Discussion and conclusion

Spectral reflectance for the trans-neptunian object(55565) 2002 AW197 and the Centaurs (55576) 2002 GB10

and (83982) 2002 GO9 in the 0.4–2.4 mm wavelength rangewere obtained from observations at ESO-VLT, as part ofthe ESO’s Large Program on ‘‘Physical Studies’’. All the

spectra have a red slope from visible to near-infraredwavelengths, with no detectable features, except for (83982)2002 GO9. The spectrum of this Centaur shows a strong2 mm absorption band possibly diagnostic of water ice.Analysis of these visible–near infrared reflectance spectra

with radiative transfer models suggests surface compositioncomposed of intimate mixtures of organics compounds(Triton tholins, amorphous carbon) and contaminatedwater ice (intramixture). Due to the lack of constraints, ourmodels give only an indication of possible material presenton these surfaces.Our modeling suggests that the surface of TNOs is

contaminated by complex organic molecules and elementalcarbon which may mask any water ice absorptions, ifpresent. By investigating the presence of water ice grainscontaminated by small amounts of tholins which has theadvantage both to reproduce the reddening and to reducethe contrast in the water ice absorptions, we obtain arelative abundance of water ice which can reach a few tensof percents.The non-icy absorber present in an intramixture can be

defined as a part of the initial intrinsic composition. Theintrinsic composition of the TNOs (water ice and redorganic material) that we infer here may be representativeof the surface composition of the primitive Kuiper Beltregion. Alternately, the large quantity of neutral extrinsicmaterial (represented in this work as amorphous carbon)could be the result of the processing of organics by avariety of sources (UV, charged particles, etc.), whichcauses the spectral changes from gray to red to gray again(Moroz et al., 2004). Additionally, the impact processcould play a selective role in preserving preferentially someconstituents and/or in redistributing material at the surfaceof the objects. In any case, weathering processes need to beunderstood and modeled in more details before stronglyconstraining the conditions of formation of the trans-neptunian objects.

Acknowledgements

We are grateful to D. Strobell for a careful reading of themanuscript and helpful comments. We also thank S.Fornasier for quick refereeing and valuable suggestions.

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