THE ASTRONOMICAL JOURNAL, 120 :496È500, 2000 July2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.(

COMPOSITIONAL SURFACE DIVERSITY IN THE TRANS-NEPTUNIAN OBJECTS1M. A. BARUCCI, J. ROMON AND A. DORESSOUNDIRAM

Observatoire de Paris, Section de Meudon, 5, Place Jules Janssen, Meudon Cedex, F- 92195, France ; antonella.barucci=obspm.fr

AND

D. J. THOLEN

Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822Received 2000 January 4 ; accepted 2000 March 15

ABSTRACTThe knowledge of the physical and chemical properties of trans-Neptunian objects (TNOs) is still

incomplete and confused. To investigate their physical properties, we are continuing the TNO obser-vational program started in 1997 at ESO in La Silla, Chile with the 3.5 m New Technology Telescope.In February 1999, broadband optical colors were obtained for eight new objects : 1993 FW, 1995 HM5,1997 1997 1997 1997 1998 and 1998 Particular attention hasCQ29, CS29, CT29, CU29, FS144, WH24.been paid to the observations and data reduction of these faint objects. These new data increase theavailable statistical sample and will help us to understand the surface properties and the mechanisms ofthe surface modiÐcation of the TNOs. The observed objects present a wide spread of colors. The colordistribution does not show any bimodality. Knowledge of the colors of a large number of these objects isimportant to understand this population, which represents an important reservoir of primordial material.Key words : Kuiper belt, Oort cloud È techniques : photometric

1. INTRODUCTION

Since the discovery in 1992 (Luu & Jewitt 1993), theknown population of trans-Neptunian objects (TNOs) hasgrown rapidly. More than 2300 TNOs have already beendetected,2 and on the basis of the sky surveys done, Jewitt(2000) hypothesized that there are more than 100,000 TNOswith diameter larger than 100 km in the radial zone extend-ing outward from Neptune between 30 and 50 AU. It is veryimportant to understand their physical characteristics aswell as their dynamical behavior. They are believed to bethe source of short-period comets (Levison & Duncan1997), but, more important still, the knowledge of this newpopulation is related to the study of the solar system forma-tion and evolution. Located so far from the Sun comparedwith the planets and some other small bodies, the TNOshave probably not been thermally processed since their for-mation. They are expected to be well-preserved fossils of theprotoplanetary disk, as they were stored at very low tem-perature and they are believed to be the remnants of solarsystem formation. Exploration of the TNOs is still at thenascent stage, but the knowledge of their physical proper-ties could constrain the formation and evolution models ofour own solar system and other planetary systems.

Very little is known about the physical and chemicalcharacteristics of the TNOs, and the study of their composi-tion is very difficult. Because of their faintness, spectro-scopic techniques can be applied to only a very few objects.Near-infrared spectra with low signal-to-noise ratios (S/Ns)have been obtained at the Keck observatory for only threeTNOs. The spectrum of 1993 SC was obtained by Brown etal. (1997) for the range 1.4È2.4 km. Although this spectrumhas been heavily smoothed, it shows absorption features atwavelengths near those of Pluto or Triton, suggesting the

ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ1 Based on observations carried out at the European Southern Obser-

vatory, La Silla, Chile (ESO program 62.S-0305).2 See http ://cfa-www.harvard.edu/cfa/ps/lists/TNOs.html.

presence of hydrocarbon ices. Luu and Jewitt (1998)obtained a spectrum of 1996 for the range 1.0È2.5 km.TL66It is almost Ñat and very similar to that of Centaur Chiron.In 1999, Brown, Cruikshank, & Pendleton (1999) observed1996 and the spectrum shows strong absorption nearTO66,1.5 and 2.0 km, characteristic of water ice.

Though a few objects are brighter than 22nd magnitude,the typical apparent magnitude of TNOs is more than 23,so they cannot be observed by spectroscopy. The photo-metric technique is the only way now available to study alarge sample of these objects. Visual photometric obser-vations of TNOs have been carried out by Luu & Jewitt(1996), Green et al. (1997), Tegler & Romanishin (1998), andBarucci et al. (1999). In some cases the reported results seemdiscrepant and the color di†erences exceed the quotederrors by a considerable margin. To enlarge the availablesample and to obtain homogeneous data of high quality, weare continuing spectrophotometric observations of TNOs.In this paper we present the observations for eight newobjects, and we carefully explain the method used for thereduction.

2. OBSERVATIONS

Observations of eight new TNOs have been carried outon 1999 February 14È17 at ESO (European SouthernObservatory, La Silla, Chile). The observational character-istics are reported in Table 1. The SUSI2 CCD camera

at the f/11 Nasmyth focus of the 3.5 m New(5@.5] 5@.5)Technology Telescope (NTT) was used to obtain directimages. The EEV camera (ESO No. 46) has been selectedfor our observations with the Bessel B, V , R, and I Ðlters.The observations were carried out in 2 ] 2 binning mode,yielding a pixel scale of The seeing throughout the run0A.16.was in the range Because of the faintness of these0A.5È1A.9.objects, careful observations and reductions are necessaryto minimize potential systematic errors due to faintbackground-source contamination. We sequence the obser-vations by repeating measurements through the V Ðlter

496

TRANS-NEPTUNIAN OBJECTS 497

TABLE 1

OBSERVATIONAL CHARACTERISTICS OF THE OBSERVED TNOS

Date R.A. Decl. r * PhaseObject (UT) (2000.0) (2000.0) (AU) (AU) (deg)

1993 FW . . . . . . . . 1999 Feb 16 13 01 35 [05 28 05 41.97 41.32 1.031993 FW . . . . . . . . 1999 Feb 17 13 01 32 [05 27 48 41.97 41.31 1.011995 HM5 . . . . . . . 1999 Feb 14 12 58 34 [05 32 56 32.09 31.45 1.321995 HM5 . . . . . . . 1999 Feb 17 12 58 24 [05 31 48 32.09 31.41 1.311997 CQ29 . . . . . . 1999 Feb 16 10 41 30 ]09 39 33 41.30 40.33 0.281997 CQ29 . . . . . . 1999 Feb 17 10 41 26 ]09 40 01 41.30 40.33 0.251997 CS29 . . . . . . . 1999 Feb 14 07 52 11 ]21 15 09 43.61 42.74 0.621997 CS29 . . . . . . . 1999 Feb 15 07 52 07 ]21 15 20 43.61 42.75 0.641997 CT29 . . . . . . . 1999 Feb 15 09 13 52 ]16 59 32 44.85 43.88 0.221997 CT29 . . . . . . . 1999 Feb 16 09 13 47 ]16 59 51 44.85 43.88 0.241997 CU29 . . . . . . 1999 Feb 16 07 45 20 ]22 28 24 44.75 43.92 0.681997 CU29 . . . . . . 1999 Feb 17 07 45 17 ]22 28 33 44.75 43.93 0.701998 FS144 . . . . . . 1999 Feb 14 11 06 21 [03 47 12 41.86 40.96 0.591998 FS144 . . . . . . 1999 Feb 15 11 06 17 [03 46 50 41.86 40.96 0.571998 WH24 . . . . . . 1999 Feb 15 03 31 25 ]20 34 54 42.79 42.78 1.321998 WH24 . . . . . . 1999 Feb 17 03 31 27 ]20 35 01 42.79 42.82 1.32

NOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declination aredegrees, arcminutes, and arcseconds.

(e.g., V , B, V , R, V , I, V ). Making multiple observationsthrough the same Ðlter allows us to interpolate or toaverage through this Ðlter to minimize the errors in thecolor index caused by the variation in brightness with rota-tion.

The exposure time ranged between 300 and 900 s for theV , R, and I Ðlters and 600 and 1350 for the B Ðlter. Weavoid longer exposure times, not only because of potentialbrightness variation, but also because of the increasedpossibility of data loss due to cosmic-ray hits on the imageof the object. We repeated the observations by making asecond independent measurement for all the objects. Stan-dard stars in multiple Ðelds (Landolt 1992) have beenobserved each night at di†erent air masses to determine theextinction coefficients and to perform a complete photo-metric transformation. The nights were photometric exceptpart of the second night, during which isolated cirri werepresent near the horizon.

3. DATA REDUCTION

Photometric reduction was performed using speciÐc rou-tines developed with MIDAS. First, bias and Ñat-Ðeld cor-rections were performed. Because of the faintness of theseobjects, a very accurate data reduction method wasrequired. Automatic methods cannot be used in this case.Several observations performed on the same objects by dif-ferent authors give di†erent results (Green & McBride2000). To determine the best way to reduce TNO photo-metric data, we studied the error sources in magnitude. Wetook into account the main error sources : (a) the sky back-ground photon noise, (b) the error in the sky backgroundestimator, (c) the object photon noise, and (d) the readoutnoise.

The classical data reduction method consists of using abig aperture, one large enough to include the majority of theÑux of the object, as well as a signiÐcant contribution fromthe sky background. For objects as faint as TNOs (i.e.,

the error from the sky background dominates if weV Z 22),use the classical method. For this reason a small aperture isneeded to reduce the sky background contribution and the

probability of contamination by a background source. If wechoose a small aperture to perform photometric datareduction, a signiÐcant loss of object Ñux accompanies thereduction in sky background. To calibrate this e†ect, weneed to perform an aperture correction. This method con-sists of measuring a bright star3 in the same frame(V [ 19)as the object, both with the small aperture and with a bigone. The comparison of both measurements allows us todetermine the correction to apply (Howell 1989). To esti-mate the error on the correcting value, several bright starsin the frame have been used. For each one, we construct agraph showing the sky-subtracted Ñux of the source versusaperture size (growth curves). To subtract the sky back-ground, we select six subframes (50] 50 pixels) around theobject. For each one, we compute the mode of the intensitydistribution. The mode corresponds to the most probablevalue of a distribution :

mode\ 3(median)[ 2(mean)

(Stetson 1987 ; Da Costa 1992). Using these six sky back-ground estimators, we compute an average value and therms standard deviation. If the chosen sources are brightenough, the sky background estimate is not critical. Thegrowth curves converge on a value normalized to 1 for largeaperture sizes. We determine growth curves for severalbright stars, then we calculate an average growth curve,which will be used to determine the correcting value foreach aperture size. The shape of the curve depends only onthe seeing proÐle in the frame. As this proÐle changes fromone frame to another, such a curve must be produced foreach frame. The average growth curve also provides theerror on the correcting value, which has to be included inthe total error computation.

The next step consists in choosing the aperture size. Thesignal-to-noise ratio depends on the chosen aperture sizeand the seeing-proÐle FWHM (through the truncated Ñux

ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ3 This upper limit depends on many parameters, such as exposure time

and sky brightness.

19 20 21 22 23 24 25Magnitude of the source

0

0.05

0.1

0.15

0.2

0.25

0.3E

rror

(in

mag

nitu

de)

Classical method (D=4")Aperture correction (D=1")

498 BARUCCI ET AL. Vol. 120

FIG. 1.ÈTheoretical error in magnitude vs. the magnitude of thesource. The parameters used to calculate these analytical curves are thefollowing : exposure time, 300 s ; collecting area, 10 m2 ; spectral bandwidth, 0.09 k ; sky magnitude, 21.8 mag arcsec~2 ; and error on the skybackground estimator, 10 photons pixel~2. It is evident that the aperturecorrection method is better for faint objects. For bright objects, a smallererror is obtained with the classical method.

contained in an aperture of diameter D). It can be demon-strated that the signal-to-noise ratio is maximized when theaperture size is equal to the seeing-proÐle FWHM (Romon1999). In general, we use an aperture a few pixels biggerthan the seeing to avoid edge e†ect.

If the aperture correction method is used, the total errorin the magnitude includes (a) the error on the correctingvalue, (b) the errors relative to the sky background, (c) theobject photon noise, and (4) the readout noise. If the clas-sical method is used, only the three last error sourcesremain. To compare the error obtained using the classicalmethod and that obtained using the aperture-correctionmethod, the total error is computed analytically. In Figure 1the error (in magnitude) versus the magnitude of the sourcefor each method is shown. The best method, giving thesmallest error, depends on the magnitude of the source : forbright objects, the classical method should be used, whilefaint ones require aperture correction. The analytical com-putation does not allow us to give an accurate limit. Tochoose between one method or the other, growth curves areused. If the average curve converges, the classical methodcan be applied ; otherwise, aperture correction must be used.This method has been applied and validated using synthetic

objects to test di†erent reduction techniques on faintTNOs.4

The aperture correction method has been used for all theobjects studied here. The aperture size depends on theseeing. Therefore, it could vary from one frame to another.The aperture size used varies from to Because this0A.6 1A.9.method cannot be applied to trailed objects, we checkedthat the proper motion of each object is smaller than theseeing.

Photometric calibration was performed using the usualcalibration method. We checked the stability of severalcomparison stars for each night, and we used six standardstars, observed at di†erent air masses, to compute the atmo-spheric extinction coefficients. We computed the extinctioncoefficient, the color term, and the zero point by using alinear regression method. The errors on these coefficientshave been included in the total error in the magnitude.

4. RESULTS

The optical photometry is presented in Table 2. Thereported magnitudes represent the weighted average of dif-ferent measurements, and the error listed is its standarddeviation. In a few cases we were not able to measure themagnitudes, especially in the B and I Ðlters. The B magni-tude was particularly difficult to measure as it is very faintfor most of the objects and the SUSI2 CCD camera sensi-tivity is poor in the B Ðlter. Through the I Ðlter, strongfringes were observed in the images. They are due to theinterference of night-sky lines in the thin CCD. ESO NTTsta† provided an efficient method to remove these fringes,5but the high noise level that remained after correction madeit difficult to measure the I magnitude of the faintest objects.We also found that the seeing is a critical parameter : verygood seeing is needed to measure the faintest objects. Themissing measurements in B and I Ðlters for some objects aredue to their faintness as well as poor seeing.

The resulting color indexes are reported in Table 3 andthe relative spectra, normalized to 1 at the V Ðlter, areshown in Figure 2. The reÑectivity was computed using thesolar colors (Hardorp 1980 ; Hartmann, Cruikshank, &Degewij 1982). The basic result is that a continuous spreadexists among the obtained large-band spectra : from 1995

which shows a quite neutral spectrum, to 1997HM5, CQ29or 1997 which have very red spectra. This wide di†er-CU29,

ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ4 See http ://scorpio.tn.cornell.edu/ACM/web–abs.html.5 See

http ://www.ls.eso.org/lasilla/Telescopes/NEWNTT/NTT-MAIN.html.

TABLE 2

MAGNITUDES OF THE OBSERVED TNOS

Object B V R I

1993 FW . . . . . . . . 24.48^ 0.09 23.47 ^ 0.02 22.81^ 0.04 22.42 ^ 0.081995 HM5 . . . . . . . 24.07^ 0.06 23.40 ^ 0.02 22.85^ 0.04 22.48 ^ 0.101997 CQ29 . . . . . . 24.52^ 0.12 23.53 ^ 0.03 22.85^ 0.05 22.23 ^ 0.081997 CS29 . . . . . . . 23.02^ 0.06 21.97 ^ 0.01 21.31^ 0.02 20.78 ^ 0.031997 CT29 . . . . . . . . . . 23.42^ 0.03 22.68^ 0.06 . . .1997 CU29 . . . . . . 24.65^ 0.12 23.33 ^ 0.02 22.72^ 0.03 21.98 ^ 0.051998 FS144 . . . . . . 24.30^ 0.07 23.39 ^ 0.03 22.83^ 0.06 . . .1998 WH24 . . . . . . 22.29^ 0.06 21.36 ^ 0.02 20.80^ 0.04 20.10 ^ 0.04

400 500 600 700 800Wavelength (nm)

0.4

0.9

1.4

1.9

Nor

mal

ized

ref

lect

ivity

1993 FW1995 HM51997 CQ291997 CS291997 CT291997 CU291998 FS1441998 WH24

0.4 0.6 0.8 1 1.2 1.4B−V

0.25

0.45

0.65

0.85

V−

R

Our dataTR 98

No. 1, 2000 TRANS-NEPTUNIAN OBJECTS 499

TABLE 3

ABSOLUTE MAGNITUDES AND COLORS OF THE OBSERVED TNOS

Object HV B[V V [R R[I

1993 FW . . . . . . . . 7.15^ 0.02 1.01 ^ 0.09 0.66^ 0.04 0.39 ^ 0.091995 HM5 . . . . . . . 8.23^ 0.02 0.67 ^ 0.06 0.55^ 0.04 0.37 ^ 0.111997 CQ29 . . . . . . 7.38^ 0.03 0.99 ^ 0.12 0.68^ 0.06 0.62 ^ 0.091997 CS29 . . . . . . . 5.54^ 0.01 1.05 ^ 0.06 0.66^ 0.02 0.53 ^ 0.041997 CT29 . . . . . . . 6.92^ 0.03 . . . 0.74 ^ 0.07 . . .1997 CU29 . . . . . . 6.77^ 0.02 1.32 ^ 0.12 0.61^ 0.04 0.74 ^ 0.061998 FS144 . . . . . . 7.14^ 0.03 0.91 ^ 0.08 0.56^ 0.07 . . .1998 WH24 . . . . . . 4.93^ 0.02 0.93 ^ 0.06 0.56^ 0.04 0.70 ^ 0.06

ence is also seen in infrared spectra available for four TNOs(Jewitt & Luu 1998). No evidence for bimodality, as claimedby Tegler & Romanishin (1998, hereafter TR98) has beenfound in these new high quality data. On the basis of theirsurvey of B, V , and R colors, TR98 reported the existence oftwo populations : one with objects having neutral or slightlyred colors, and the other one with the reddest objectsknown in the solar system. Barucci et al. (1999) found, byanalyzing all the available data in literature on B, V , R, andI, no conÐrmation of any cluster, indicating a more complexpopulation. If we compare our high quality data (this paperand those in Barucci et al. 1999) with those published byTR98, as shown in Figure 3, we can conclude that our datapractically Ðll the gap hypothesized by TR98. Davies et al.(2000) on the basis of their visibleÈinfrared (V ÈJ) colors alsoconÐrm the lack of bimodality in their sample. The di†er-ence in colors increases in the infrared ; moreover, diversityin visibleÈIR colors is even more striking than in opticalcolors. For this reason, it is a better diagnostic to observeTNOs in the infrared.

FIG. 2.ÈRelative reÑectivity for the eight observed TNOs, normalizedat the V Ðlter (550 nm).

5. CONCLUSION

We presented optical broadband colors for eight newtrans-Neptunian objects. The observed objects present awide spread of colors, ranging from nearly neutral to veryred. Our data do not conÐrm the bimodality previouslyfound by TR98 and are consistent with our previous results(Barucci et al. 1999). The color diversity may originate fromcomposition variation within the TNO population or fromtime-dependent irradiation processes. Of course, it is verydifficult to explain di†erent compositions for objects thatprobably have been formed in the same zone at about thesame low temperature. The observed compositional di†er-ences would involve a more complex formation mechanismthan that currently believed. The composition of the mantleor crust has to be di†erent from the interior, which retainspristine ice, and this color variety may imply at least com-positional diversity of the surface layers. A plausible expla-nation is that these objects have experienced collisions. Infact Farinella & Davis (1996) and Davis & Farinella (1997),on the basis of their numerical simulation, showed that col-lisions played a substantial role in the inner part of thetrans-Neptunian belt and that this e†ect decreases with theincrease of heliocentric distance. Collisions could reworkthe surface of the objects, and the fresh, excavated, unex-

FIG. 3.ÈColor plot of B[V vs. V [R for TNOs, showing our data(squares ; Barucci et al. 1999 and present observations) and data from TR98(circles). The six objects (1994 TB, 1995 1996 1996 1996HM5, TP66, TO66,and 1997 are reported twice, as they have been observed byTL66, CS29)both teams. Our new data show no evidence for bimodality.

500 BARUCCI ET AL.

posed material should be of di†erent composition than theirradiated mantle. Luu & Jewitt (1996) demonstrated with amodel that the impact resurfacing may explain the TNOcolor diversity. Older surfaces would be redder. Di†erentdegrees of space weathering alteration could have modiÐedthe surfaces of di†erent ages. Moroz et al. (1998) demon-strated, on the basis of laboratory spectroscopic study, thatthe diversity of complex organics depends signiÐcantly onparticle size and texture. In some cases the colors seem tobecome progressively more neutral with the increase of par-ticle size. However they concluded that the variation insurface texture is a less important factor than spaceweathering.

Another possible explanation of the color diversity maybe cometary activity, as suggested in the case of 1996 TO66(Hainaut et al. 2000) and 1994 TB (Fletcher et al. 2000).

The di†erence in colors could be not genetic, but it ismore probably due to the di†erent history of the objects.More broadband colors will help to classify the populationof TNOs and to have a general view of the evolution ofthese bodies, but only infrared spectroscopy with a newlarge-aperture telescope will allow researchers to under-stand the real composition of these objects.

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