Simultaneous visible and near-infrared time resolvedobservations of the outer solar system object

(29981) 1999 TD10

Beatrice E. A. MuellerNational Optical Astronomy Observatory1, 950 N Cherry Ave, Tucson AZ 85719

E-mail: muller@noao.edu

Carl W. HergenrotherLunar and Planetary Laboratory, University of Arizona, Tucson AZ 85721

Nalin H. SamarasinhaNational Optical Astronomy Observatory1, 950 N Cherry Ave, Tucson AZ 85719

Humberto CampinsUniversity of Central Florida, Orlando FL 32186 and Lunar and Planetary

Laboratory, University of Arizona, Tucson AZ 85721

Donald W. McCarthy, Jr.Steward Observatory, University of Arizona, Tucson AZ 85721

Accepted by Icarus , June 17, 2004.

1The National Optical Astronomy Observatory is operated by the Association of Universities forResearch in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foun-dation.

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ABSTRACT

The outer solar system object (29981) 1999 TD10 was observed simultaneouslyin the R, and J and H bands in September 2001, and in B, V, R, and I in Octo-ber 2002. We derive B-V=0.80±0.05mag, V-R=0.48±0.05mag, R-I=0.44±0.05mag,R-J=1.24±0.05mag, and J-H=0.61±0.07mag. Combining our data with the data fromRousselot et al. (Astron. Astrophys. 407, 1139, 2003) we derive a synodic period of15.382±0.001 hr in agreement with the period from Rousselot et al. Our observationsat the same time, with better S/N and seeing, show no evidence of a coma, contraryto the claim by Choi et al. (Icarus 165, 101, 2003).

Key Words: Asteroids, rotation; Centaurs; Photometry; Infrared Observations;(29981) 1999 TD10

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I. INTRODUCTION

The outer solar system object (29981) 1999 TD10, referred to hereafter as TD10, wasdiscovered by the Spacewatch survey (Scotti et al. 1999). It has a perihelion distance of12AU, similar to Centaurs and an aphelion distance of 190AU characteristic of manyscattered disk objects. Consolmagno et al. (2000), assuming an albedo of 0.04, estimatethe object’s major and minor axes as 130 and 70 km respectively (i.e. an effective radiusof 50 km). This indicates a large outer solar system object with an irregular shape.They find a periodicity in the lightcurve of 5.8 hr (which corresponds to a single peakedlightcurve) and a gray color. Rousselot et al. (2003) derive a solar phase dependencein the H-G scattering formalism and a period of 15.382±0.002 hr. Choi et al. (2003)report a period of 15.448±0.012 hr and derive a solar phase dependence and an effectiveradius of 58 km. They also claim to have detected a coma in TD10.

We compare our results with the literature above and investigate the claim of comaactivity. In section II we describe our visible and near-infrared observations and reduc-tions, in section III, we discuss the photometry, the lightcurve and the colors of TD10.In section IV we consider and reject the claim of coma activity, and we summarize ourresults in section V.

II. OBSERVATIONS AND REDUCTIONS

TD10 was observed simultaneously in the visible and near-infrared from September21–22UT, 2001, with two additional nights in the visible on September 23 and 25, 2001.It was also observed about one year later in the visible from October 30 – November 1,2002. The summary of the observational and the geometrical circumstances are givenin Table I.

Table IObservational and Geometric Circumstances for TD10

Date Filters ra ∆b αc sky condition[UT] [AU] [AU] [deg]

09/21/01 R,J 12.69 11.76 1.7 photometric09/22/01 R,J,H 12.70 11.75 1.6 photometric09/23/01 R 12.70 11.75 1.5 photometric09/25/01 R 12.70 11.74 1.4 photometric10/30/02 B,V,R,I 13.27 12.28 0.6 photometric10/31/02 R 13.27 12.29 0.7 photometric11/01/02 R 13.27 12.29 0.7 not photometrica heliocentric distanceb geocentric distancec solar phase angle

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A. Visible data

The visible observations in both runs were taken with the KPNO-2.1m telescopeon Kitt Peak equipped with a 2048×2048 Tektronix CCD chip with 24µm pixels. At afocal ratio of f/7.5, the plate scale is 0.′′305 pix−1 and the field of view is 10.′4. StandardHarris broad-band filters which are optimized for the Kron-Cousins system were used.The integration time was limited as not to smear out the rotational signature. Thetrailing rates corresponded to less than half the seeing disk. Standard IRAF procedureswere used to reduce the data. The images were bias subtracted and then flat fieldedwith combined dome flats to remove the pixel to pixel variations. Dithered objectframes were combined with a rejection criteria to remove the objects and smoothed tocorrect for the difference in slope between the dome flats and the dark sky. In addition,fringe correction frames were constructed from combined object frames in the I-bandto remove the fringing. The resulting reduced images were flat to better than 1%.

Relative photometry was carried out with at least 6 comparison stars in the sameframe to extract the magnitude of TD10. An aperture diameter of 14 pix (4.′′3) whichwas always at least twice as large as the maximum seeing was used for the photometryof the object and an aperture diameter of 30 pix (9.′′2) was used for the comparisonand standard stars. The aperture correction to tie the object to the comparison starswas done for every frame separately. There were common stars in frames of subsequentnights, so that the nights could be tied to a reference night. There were at least twophotometric nights per observing run (Table I) and the absolute calibration was doneusing Landolt Standard Star fields (Landolt 1992).

B. Near-infrared data

The near-infrared observations were obtained with the University of Arizona’s Stew-ard Observatory 2.3m Bok telescope on Kitt Peak with the PISCES infrared camera.PISCES consists of a 1024x1024 Rockwell Hawaii detector with 18.5µm pixels (Mc-Carthy et al. 2001). The PISCES camera has a final focal ratio of f/3.3 and a platescale of 0.′′50 pix−1 at the 2.3m Bok telescope. Standard IRAF procedures were em-ployed to reduce the data. Images were dark subtracted and flat fielded with mediansky flats produced from the individual dithered J and H-band images. A routine wasused to remove the effect of cross-talk between CCD readout quadrants (McCarthy etal. 2001).

Absolute photometry in J (λc=1.25µm) and H (λc=1.65µm) was determined fromobservations of standards P309-U, P533-D and S677-D (Persson et al. 1998) at mul-tiple airmasses. Variations in seeing did not allow for the use of a single aperture forphotometry. The standards were measured with an aperture diameter of 6 FWHMwhich ranged from 8 to 16 pixels (4′′ to 8′′). TD10 was measured with an aperturediameter of 2 FWHM. An aperture correction was determined and used to normalizethe photometry. Both nights were photometric allowing absolute calibrations. Thesolar colors in the Arizona infrared photometric system (Campins et al. 1985) were

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used for conversion from fluxes to reflectances (section III.D).

III. Photometry

A. Lightcurve Analysis

The magnitudes R(1,1,α), i.e. the observed R magnitude corrected to heliocentricand geocentric distances of 1AU each, but not corrected for solar phase angle effectsare given in Table II for September 2001 and in Table III for October 2002. The timesin both Tables are light time corrected and refer to the midpoint of the integrations.Figure 1 shows the R-band data from September 23, 2001 and from October 30, 2002.In September 2001 we observed a maximum but no minima and in October 2002 weobserved a minimum and a maximum. The peak to peak variation in the lightcurveis ≈0.50mag. This variation is consistent with the peak to peak variation of 0.68maggiven by Consolmagno et al. (2000). As the geometry was different for our observationscompared to that of Consolmagno et al. (2000) and their lightcurve amplitude islarger than ours, our observations must have been taken when the rotational angularmomentum vector was closer to the line of sight.

The J data for September 21 and 22, 2001 are given in Table IV and the H datafor September 22, 2001 are given in Table V. The times in both Tables are again lighttime corrected and refer to the midpoints of the integrations. The lightcurve for the Jdata from September 21, 2001 is shown in Figure 2.

B. Solar phase dependence

Before doing a periodicity analysis, our data will have to be corrected first forchanges in heliocentric and geocentric distances and phase angle. The solar phasedependence of TD10 is not known. We did not adopt the phase dependence of Choiet al. (2003) because our derived mean absolute magnitudes using their solar phasecoefficient for the September 2001 and October 2002 data do not agree with eachother. Their derivation is also widely different from the range of linear solar phasecoefficients for 7 TNOs by Sheppard and Jewitt (2002). The derived phase coefficient ofβ=0.121mag deg−1 by Rousselot et al. (2003) is in agreement with the mean solar phasecoefficient by Sheppard and Jewitt (2002). However, our mean absolute magnitudes inSeptember 2001 and October 2002 using this coefficient are still inconsistent with eachother. This could be due to the different nuclear orientations caused by the observinggeometries, but the difference in geocentric (or heliocentric) ecliptic longitude of TD10

between these two dates is only 10◦ and in ecliptic latitude is 1◦.We used the maxima in the R-band lightcurve from September 23, 2001 and

from November 1, 2002 to derive the solar phase angle dependence. We get forthe absolute mean magnitude R0=8.43±0.01mag, and for the solar phase coefficientβ=0.181±0.015mag deg−1. As the two maxima in a double peaked lightcurve do nothave to be equal, we derived the solar phase coefficient by comparing the mean magni-tudes for the September 2001 and October 2002 run. We get the same coefficient as that

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Table IIR-band photometry for TD10 in September 2001

JDa-2450000 R(1,1,α) error JDa-2450000 R(1,1,α) error2173.8181 8.633 0.014 2174.8306 8.798 0.0182173.8230 8.635 0.015 2174.8347 8.813 0.0182173.8292 8.671 0.015 2174.8392 8.838 0.0182173.8353 8.719 0.015 2174.8433 8.843 0.0182173.8402 8.710 0.016 2174.8533 8.858 0.0182173.8443 8.792 0.018 2175.6512 8.447 0.0172173.8485 8.799 0.017 2175.6558 8.456 0.0162173.8544 8.802 0.018 2175.6600 8.469 0.0172173.8590 8.820 0.019 2175.6645 8.439 0.0162173.8640 8.860 0.020 2175.6686 8.438 0.0162173.8685 8.876 0.019 2175.6850 8.432 0.0152174.6474 8.590 0.020 2175.6895 8.453 0.0152174.6525 8.577 0.020 2175.6947 8.476 0.0152174.6587 8.539 0.018 2175.7046 8.480 0.0152174.6657 8.524 0.017 2175.7091 8.489 0.0142174.6715 8.551 0.016 2175.7136 8.495 0.0152174.6765 8.520 0.016 2175.7635 8.737 0.0172174.6905 8.480 0.016 2175.7683 8.755 0.0172174.6956 8.473 0.015 2175.7726 8.814 0.0182174.7005 8.500 0.016 2175.7777 8.812 0.0192174.7049 8.467 0.015 2175.7888 8.852 0.0202174.7158 8.465 0.014 2175.7933 8.850 0.0182174.7277 8.431 0.014 2177.7864 8.856 0.0192174.7840 8.583 0.016 2177.7912 8.838 0.0192174.7901 8.645 0.014 2177.7974 8.781 0.0182174.7951 8.628 0.014 2177.8037 8.802 0.0182174.7992 8.667 0.015 2177.8079 8.762 0.0172174.8040 8.688 0.016 2177.8220 8.644 0.0152174.8081 8.685 0.015 2177.8262 8.584 0.0142174.8210 8.747 0.017 2177.8305 8.609 0.0152174.8256 8.814 0.017

a light time corrected, at midpoint of integration

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Table IIIR-band photometry for TD10 in October 2002

JDa-2450000 R(1,1,α) error JDa-2450000 R(1,1,α) error2577.5761 8.410 0.017 2578.8030 8.274 0.0142577.5839 8.387 0.017 2578.8087 8.291 0.0152577.5900 8.423 0.017 2578.8145 8.293 0.0162577.6129 8.527 0.018 2578.8193 8.280 0.0162577.6240 8.566 0.017 2578.8242 8.320 0.0182577.6360 8.612 0.018 2578.8544 8.347 0.0212577.6635 8.703 0.018 2578.8606 8.380 0.0212577.6693 8.671 0.018 2578.8658 8.398 0.0212577.6880 8.722 0.019 2578.8709 8.360 0.0212577.6938 8.749 0.020 2579.5627 8.579 0.0372577.7044 8.729 0.018 2579.6350 8.636 0.0382577.7100 8.802 0.020 2579.6422 8.679 0.0282577.7326 8.657 0.018 2579.6495 8.570 0.0392577.7383 8.650 0.019 2579.7169 8.317 0.0332577.7561 8.576 0.017 2579.7480 8.265 0.0362577.7610 8.560 0.017 2579.7592 8.280 0.0132577.7725 8.500 0.017 2579.7662 8.251 0.0112577.7782 8.462 0.018 2579.7730 8.254 0.0212577.7982 8.433 0.017 2579.7798 8.270 0.0312577.8158 8.385 0.018 2579.7867 8.279 0.0392577.8217 8.370 0.018 2579.8006 8.252 0.0302577.8323 8.348 0.018 2579.8086 8.318 0.0222577.8762 8.360 0.022 2579.8561 8.510 0.0332577.8819 8.332 0.022 2579.8640 8.504 0.0442578.7968 8.315 0.017 2579.8713 8.539 0.044

a light time corrected, at midpoint of integration

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Figure 1: Visible lightcurve in R from September 23, 2001 (top) and from October 30,2002 (bottom).

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Table IVJ-band photometry for TD10 in September 2001

JDa-2450000 J(1,1,α) error JDa-2450000 J(1,1,α) error2173.68555 7.15 0.09 2173.75518 7.23 0.102173.68721 7.10 0.09 2173.79145 7.20 0.092173.68888 7.10 0.09 2173.79601 7.32 0.092173.69055 7.06 0.09 2173.79837 7.28 0.092173.69221 7.22 0.09 2173.80073 7.35 0.092173.69388 7.22 0.09 2173.80309 7.30 0.092173.69553 7.18 0.09 2173.80546 7.32 0.092173.69720 7.11 0.09 2173.80782 7.41 0.092173.69887 7.27 0.10 2173.81018 7.28 0.092173.70052 7.09 0.09 2173.81254 7.25 0.092173.70219 7.26 0.10 2173.81490 7.46 0.092173.70386 7.19 0.09 2173.82396 7.41 0.092173.70552 7.20 0.09 2173.82632 7.40 0.092173.70719 7.03 0.09 2173.82869 7.43 0.092173.70886 7.15 0.09 2173.83105 7.44 0.092173.71051 7.31 0.09 2173.83341 7.56 0.092173.71218 7.13 0.09 2173.83577 7.49 0.092173.72436 7.11 0.09 2173.83813 7.59 0.102173.72688 7.29 0.10 2173.84049 7.53 0.092173.72855 7.29 0.09 2173.84284 7.54 0.092173.73021 7.31 0.09 2173.84599 7.49 0.092173.73187 7.17 0.09 2173.84834 7.63 0.112173.73353 7.26 0.09 2173.85071 7.50 0.102173.73520 7.16 0.09 2173.85307 7.50 0.092173.73684 7.36 0.10 2173.85542 7.62 0.102173.73850 7.02 0.09 2173.85778 7.47 0.092173.74017 7.11 0.09 2173.86016 7.66 0.102173.74183 7.11 0.09 2173.86252 7.66 0.102173.74351 7.26 0.09 2173.86487 7.67 0.102173.74518 7.23 0.09 2173.86722 7.76 0.112173.74684 7.00 0.09 2173.86958 7.71 0.122173.74851 7.23 0.09 2173.87194 7.66 0.102173.75018 7.21 0.09 2173.87430 7.66 0.112173.75183 7.09 0.09 2173.87666 7.61 0.102173.75350 7.27 0.09

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Table IVcontinued

JDa-2450000 J(1,1,α) error JDa-2450000 J(1,1,α) error2174.67352 7.20 0.09 2174.79308 7.56 0.092174.67638 7.21 0.09 2174.79474 7.46 0.102174.67805 7.21 0.09 2174.79641 7.76 0.112174.67972 7.21 0.10 2174.79808 7.52 0.092174.68137 7.28 0.10 2174.79973 7.56 0.092174.68304 7.13 0.09 2174.80140 7.36 0.092174.68470 7.20 0.09 2174.80306 7.52 0.102174.68637 7.24 0.10 2174.80484 7.38 0.092174.68804 7.28 0.10 2174.80650 7.56 0.092174.68969 7.14 0.09 2174.80818 7.40 0.092174.69894 7.14 0.08 2174.80985 7.57 0.102174.70061 7.23 0.09 2174.81151 7.50 0.102174.70227 7.25 0.09 2174.81317 7.49 0.102174.70394 7.16 0.09 2174.81484 7.48 0.102174.70561 7.16 0.09 2174.81650 7.63 0.102174.70752 7.17 0.09 2174.81816 7.54 0.102174.70918 7.16 0.08 2174.82007 7.61 0.102174.71084 7.22 0.08 2174.82172 7.53 0.102174.71251 7.27 0.09 2174.82339 7.52 0.102174.71416 7.13 0.08 2174.82504 7.60 0.102174.71582 7.11 0.08 2174.82671 7.54 0.102174.71748 7.26 0.09 2174.82838 7.59 0.092174.71914 7.27 0.09 2174.83004 7.53 0.102174.72082 7.24 0.09 2174.83170 7.56 0.092174.78974 7.44 0.09 2174.83337 7.68 0.102174.79141 7.53 0.09

a light time corrected, at midpoint of integration

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Table VH-band photometry for TD10 in September 2001

JDa-2450000 H(1,1,α) error JDa-2450000 J(1,1,α) error2174.64825 7.22 0.30 2174.65844 6.76 0.202174.65054 6.76 0.14 2174.65895 6.50 0.172174.65105 6.74 0.18 2174.65946 6.78 0.172174.65156 6.95 0.20 2174.66018 6.31 0.152174.65207 6.80 0.19 2174.66069 7.18 0.332174.65257 6.87 0.18 2174.66121 6.67 0.212174.65307 6.50 0.21 2174.66539 6.40 0.172174.65358 6.60 0.17 2174.66590 7.02 0.242174.65409 6.49 0.21 2174.66641 6.67 0.192174.65460 6.74 0.15 2174.66692 6.64 0.232174.65539 6.49 0.13 2174.66742 6.76 0.242174.65590 7.02 0.19 2174.66792 6.68 0.252174.65641 6.84 0.16 2174.66843 6.50 0.152174.65692 7.01 0.22 2174.66894 6.51 0.152174.65742 6.57 0.16 2174.66945 6.56 0.192174.65793 6.32 0.17

a light time corrected, at midpoint of integration

derived from comparing the maxima. This is not surprising since, as the phase plot inFigure 7 shows, the two maxima in the lightcurve are indeed equal in magnitude. Thissolar phase coefficient is inside the range of coefficients quoted by Sheppard and Jewitt(2002). The derivation of the solar phase coefficient is dependent on the accuracy ofthe absolute calibration of the data. We had two photometric nights each in bothobserving runs and the difference in absolute magnitude within a run is 0.007mag inSeptember 2001 and 0.005mag in October 2002. If we assume a maximum differencein absolute magnitudes of 0.012mag between the two runs, then the correspondingchange in the derived solar phase coefficient β is 0.015mag deg−1 which is of the sameorder as the error of β quoted above. R0 is consistent with Ropp derived by Choi et al.(2003) and HR by Rousselot et al. (2003).

C. Period Analysis

In order to identify as well as to minimize artifacts, we used two different techniquesfor the period search analysis of the R-band lightcurves: fitting harmonics and usinga Fourier Transform (FT) with a subsequent WindowCLEAN algorithm (Belton andGandhi 1988, Mueller et al. 2002). The clean spectrum is a deconvolution of the FTof the observed data (dirty spectrum) with the spectral window. The spectral window(or dirty beam) is the FT of the sampling function which is 0 when no observation was

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Figure 2: Near-infrared lightcurve in J from September 21, 2001

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taken and 1 when an observation was taken.The χ2 plot of fitting the harmonics and the dirty and clean power spectra for the

WindowCLEAN for the individual data sets from September 2001 and October 2002are shown in Figure 3.

The minimum for September 2001 in the χ2 plot is at a frequency f of 3.13 dy−1.The maximum in the clean power spectrum is at 3.11 dy−1. Taking the standarddeviations of the power spectrum signatures and the χ2 signatures as the maximumof the error, we obtain 0.12 dy−1 for the error. The two frequencies are in excellentagreement with each other. Assuming that the lightcurve is due to an aspherical shape,the corresponding double peaked periods (P= 2

f) are 15.34±0.3 hr and 15.43±0.3 hr.

For October 2002, the frequencies derived are 3.12 dy−1 from the χ2 plot and3.14 dy−1 from the clean spectrum, again in good agreement with each other. Thecorresponding periods are 15.38±0.3 hr from fitting harmonics and 15.29±0.3 hr fromWindowCLEAN. The minor adjacent peaks to the maximum peak in the dirty spectrafor the 2001 and 2002 data are likely daily aliases. This is confirmed as the phase plotswith their corresponding periods are unacceptable.

The periods derived from our 2001 and 2002 data sets seem to be in conflict with thesingle peak period of 5.8 hr quoted by Consolmagno et al. (2000). The correspondingfrequency of the double peaked equivalent is 4.14 dy−1, which is the same as one of ourminor peaks in the dirty spectrum and is a daily alias. They did not do a detailedperiod analysis and their data fit well with any of our periods quoted above (priv.comm. by S. C. Tegler, 2003). The simultaneous near-infrared data agree well withthe period derived from the R-band data.

We can derive a very accurate period by combining the September 2001 and October2002 data because of the large time baseline of over one year. We applied the FT andWindowCLEAN as well as fitting harmonics to the combined data set from September2001 and October 2002 (Figure 4). Two frequencies at 3.0758±0.0005 dy−1 and at3.1379±0.0005 dy−1 are left by the WindowCLEAN. Both of these frequencies are alsopresent in the χ2 plot. The double peaked period corresponding to the first frequencydoes not give a satisfactory phase plot and is rejected. The second frequency gives adouble peaked period of 15.297±0.001 hr.

As the gap between the two data sets is about a year, the results of the Window-CLEAN algorithm need to be checked. The maximum peaks in the dirty spectrumarea at frequencies 3.0784 dy−1 and 3.0759 dy−1. However, phase plots with the cor-responding periods are not acceptable. We then checked all the peaks from the dirtyspectrum between frequencies of 3.11 dy−1 and 3.14 dy−1. This is the frequency rangederived from the September 2001 and October 2002 data separately. Although, some ofthe corresponding periods give unacceptable results, some give phase plots that are asgood as the one with the period of 15.297 hr. We also applied WindowCLEAN to thecombined data sets with slightly different solar phase corrections. The dirty spectrawere the same within the errors. The WindowCLEAN picked the adjacent peaks to theone corresponding to the 15.297 hr period. We do not have enough temporal coverage

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Figure 3: χ2 plot (top), and dirty (middle) and clean (bottom) power spectra for theR-band data from September 2001 on the left and for October 2002 on the right.

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Figure 4: χ2 plot (top), and dirty (middle) and clean (bottom) power spectra for theR-band data from September 2001 and October 2002 combined. The resolution of thex-axis increases from left to right.

15

in this case for WindowCLEAN to break all the aliases.The error in the period due to synodic effects has to be estimated. The time

difference between the maxima in October 2002 and September 2001 corresponds to634 cycles. The difference in geometry is roughly 10◦ between September 2001 andOctober 2002 (see above). This gives a change in period of the order of P[sec]×10

(634×360)

resulting in a synodic effect of less than 0.001 hr.A phase plot of our data with the period from Rousselot et al. (2003) is acceptable.

A phase plot with the period from Choi et al. (2003) does not fit our data at all. Wechecked the dirty spectra from our combined data sets and the data set of Rousselotet al. (2003) for frequency peaks close to the frequency corresponding to the periodderived by Choi et al. (2003). The frequencies found were different but inside the errorbars from the frequency from Choi et al. (2003). None of the corresponding periodsresulted in phase plots that fit our data or the Rousselot et al. (2003) data.

We applied the same WindowCLEAN algorithm to the data of Rousselot et al.(2003) and get two peaks, one at 3.120±0.004 dy−1 (P=15.385±0.009 hr) and the otherat 3.138±0.004 dy−1 (P=15.296±0.009 hr) in order of falling power. The first peakis the same that Rousselot et al. (2003) derive from their data and the second oneis one of the acceptable periods from our combined 2001 and 2002 data. We thencombined our data set with the data set from Rousselot et al. (2003) and obtain apeak at 3.1205±0.0005 dy−1 (P=15.382±0.001 hr) from the clean spectrum. The samefrequency is present in the χ2 plot as well. The corresponding χ2-plot and the dirtyand clean spectra are shown in Figure 5.

Because of the large gap in our combined data sets, caution has to be exercised inaccepting the result from the WindowCLEAN algorithm without further checks. Weused the dirty spectra from the Rousselot et al. (2003) data alone, our September2001 and October 2002 data combined, as well as our data sets combined with theRousselot et al. (2003) data, to compare the dirty spectra in the relevant frequencyrange. There were three common peaks. The peaks from the data of Rousselot etal. (2003) are: 3.1025 dy−1, 3.1205 dy−1, and 3.1390 dy−1. The standard deviation forall three peaks is 0.004 dy−1. From our combined 2001 and 2002 data sets they are:3.1030 dy−1, 3.1205 dy−1, and 3.1379 dy−1. The standard deviation for all three peaksis 0.0005 dy−1. From our data set combined with the Rousselot et al. (2003) datathey are: 3.1027 dy−1, 3.1205 dy−1, and 3.1383 dy−1. The standard deviation is again0.0005 dy−1. The comparison of the dirty spectra is shown in Figure 6. Although thecorresponding peaks are inside the error bars, phase plots of the data from Rousselotet al. (2003) with periods from our peaks do not fit except for the peak at 3.1205 dy−1.The same is true for using the periods from the peaks from the Rousselot et al. (2003)data for our data set. We conclude that the peak at 3.1205 dy−1 is the only oneconsistent with all three dirty spectra. A phase plot with all the data for a period of15.382 hr is given in Figure 7. The spread and small systematic differences in the phaseplot between our data and the data from Rousselot et al. (2003) can be attributed tothe uncertain solar phase dependence corrections.

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Figure 5: χ2 plot (top), and dirty (middle) and clean (bottom) power spectra for ourR-band data combined with the Rousselot et al. (2003) data. The resolution of thex-axis increases from left to right.

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Figure 6: Comparison of the dirty spectra in the frequency range from 3.09 dy−1 to3.15 dy−1. The top panel shows the dirty spectrum from the Rousselot et al. (2003)data alone, the middle panel from our September 2001 and October 2002 data com-bined, and the bottom panel shows the dirty spectrum from our combined data setscombined with the Rousselot et al. (2003) data. The marks on the peaks denote the 3common peaks discussed in the text.

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Figure 7: Phase plot of our R-band data from September 2001 (blue), October 2002(green), and the data from Rousselot et al. (2003) (red) for a period of 15.283 hr.

19

As Choi et al. (2003) did not publish their data in tabulated form, we cannotphase their data with the period of 15.382 hr. However, in their figure 2, they give thelocation of their minimum and we can therfore deduce the locations of their maximaand compare those with the loaction of our maximum from September 2003 (see ourfigure 1). The locations of their maxima is consistent with our period of 15.382 hrwithin the errors.

D. Near-infrared and visible colors

In addition to the lightcurve observations in R, we took observations in B, V, andI on October 30, 2002. Because of the good rotational phase coverage we can deriveaccurate colors unaffected by rotational effects. The average of four color observa-tions in each of the above filters gives B-R=1.28±0.02mag, V-R=0.48±0.05mag, andR-I=0.44±0.05mag. This gives B-V=0.80±0.05mag. These colors are in excellentagreement with colors from the literature. A comparison of our colors and those fromthe literature are given in Table VI.

The near-infrared data are noisier but have a higher time resolution than the visibledata. We binned the J and H data in order to derive colors. We obtained simultaneousobservations of J and R and of H and R. Comparing the R lightcurve with the J dataand H data and comparing the rotationally phased data gives consistent results. Wederive R-J=1.24±0.05mag and R-H=1.85±0.05mag, resulting in J-H=0.61±0.07mag.With the above colors this results in V-J=1.72±0.08mag. Our near-infrared colors andcolors from the literature are again compared in Table VI.

Table VIComparison of visible and near-infrared colors for TD10

Object B-V V-R R-I V-J J-H Referencea

TD10 0.80±0.05 0.48±0.05 0.44±0.05 1.72±0.06 0.61±0.07 MuTD10 0.75±0.06 0.47±0.02 RTD10 0.77±0.05 0.50±0.04 0.47±0.03 HDTD10 0.77±0.02 0.47±0.01 TRTD10 1.81±0.06 McSun 0.665 HSun 0.36 0.33 HTMCSun 1.116 0.310 CRLa Mu = this work, R= Rousselot et al. (2003), HD= Hainaut and Delsanti (2002),

TR= Tegler and Romanishin (2003), Mc= McBride et al. (2003), H= Hardorp(1980), HTMC= Hartmann et al. (1990), CRL = Campins et al. (1985)

The solar colors from Table VI were subtracted from the colors of TD10. We thenderived the relative reflectances of TD10 which are normalized with respect to theR-band. They are shown in Figure 8.

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Figure 8: Relative reflectance plot. The plot has been normalized with respect to theR-band.

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We also converted our colors to the so called slope parameter or spectral index S[% (0.1µm)−1] (Hainaut and Delsanti 2002) using S = 100 R(λ2)−R(λ1)

(λ2−λ1)/0.1with R the rela-

tive reflectivity and λ the central wavelength in µm. Following the approach of Hainautand Delsanti (2002), we derived S for V, R, and I through linear regression. The resultis S=7.5±3.1% (0.1µm)−1 compared with S=11.9±1.9% (0.1µm)−1 from Hainaut andDelsanti (2002). These values are within each other’s error bars. Jewitt (2002) usesthe V and R filters to derive the normalized reflectivity gradient S ′ [% (0.1µm)−1] fromV-R=(V-R)¯ + 2.5 log(2+S′ ∆λ

2−S′ ∆λ), with (V-R)¯ as the solar color. For TD10 we derive

S ′=10.8±0.9% (0.1µm)−1. This value is close to the median S ′=10% (0.1µm)−1 for 9Centaurs (Jewitt 2002).

IV. ACTIVITY

Choi et al. (2003) claim to have detected cometary activity in TD10 on November 2and 5, 2000 and on September 22, 2001. Our September 21–23 data are bracketing theirdata from September 22. To investigate this claim in our data, we spatially shifted andcombined all the images from September 21–23, 2001 to enhance the signal-to-noise forTD10. The same procedure was applied to the stars in the field. The resulting effectiveintegration time is 3.6 hr, a factor of 1.5 longer than that of Choi et al. (2003). Weused a 2.1m telescope which gives a collection area 4.4 times larger than that from their1m telescope. Additionally the seeing in our combined image is 1.′′3 compared to theiraverage seeing of 2.′′3. No evidence of a resolved coma could be detected in our data, inagreement with the non-detection of a coma in the November 2002 data by Rousselotet al. (2003). As our September data bracket the data from Choi et al. (2003) onSeptember 22, an intermittent coma can also be excluded. In Figure 9a the combinedimage centered on TD10 is shown. No indication of a coma extension in any directionis seen. Figure 9b shows a comparison of the spatial profiles of TD10 and a star in thesame field with a similar flux, normalized to the same peak surface brightness. Theerror bars have been omitted for the star for clarity.

There seem to be inconsistencies in the paper by Choi et al. (2003). The positionangle of the antisolar direction is 252◦ for the September 2001 data and 66◦ for theNovember 2000 data, according to the JPL Horizons ephemeris. This leads to theconclusion that their coma extension is not in the antisolar direction contrary to theirclaim. In addition, the shape of this claimed coma extension is very unusual. Theradial brightness profile for TD10 dips below the profile of the comparison star andthen rises back up to the previous level (see their Figure 6b). Also, the deviation of theprofile of TD10 from that of the comparison star starts after a distance from the centerof the object at 5′′. At that distance the noise is dominant in our profile of TD10 (seeour Figure 9b), as well as in the profile from Rousselot et al. (2003) (see their Figure2), both of which have better S/N than that from Choi et al. (2003).

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Figure 9: a) Combined image of TD10 from September 21–23, 2001. The figure is 30′′

(99 pixels) on the side. North is to the top and East is to the left. The antisolardirection is to the Southwest (PA=252◦).

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Figure 9: b) Comparison of the spatial profiles of TD10 and a comparison star fromthe combined images with a total integration time of 3.6 hr.

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V. DISCUSSION AND SUMMARY

TD10 is a large outer solar system body with an irregular shape. Its visible colorsare not unusual. Tegler et al. (2003) find a bimodal distribution for Centaurs in(B-R) for a sample of 22 Centaurs. Peixinho et al. (2003) also find a bimodal colordistribution in (V-R) versus (B-V) for a sample of 20 Centaurs and they group theCentaurs into either “blue” (like 2060 Chiron) or “red” (like 5145 Pholus). The visiblecolors (Table VI) place TD10 in the “blue” group (at the red end). There are only7 Centaurs in common between the paper of Tegler et al. (2003) and Peixinho et al.(2003) and only one is outside each others error bars. On the other hand, Bauer etal. (2003) find a continuous color distribution for 24 Centaurs in their (V-R) versus(R-I) color-color plot. A direct comparison between the plots of Tegler and Romanishin(2003) and Peixinho et al. (2003) with those of Bauer et al. (2003) is not possible.Tegler et al. (2003) have 10 objects in common with Bauer et al. (2003) and only one isoutside each others error bars. Peixinho et al. (2003) and Bauer et al. (2003) have 18objects in common with 11 objects agreeing well, 2 objects having colors outside theirerror bars and 5 objects within each others combined error bars. If we take the V-Rcolors from Bauer et al. (2003) and the B-V colors from Peixinho et al. (2003), thegap in the (V-R) versus (B-V) plot vanishes. However, the error bars from the colorsof Bauer et al. (2003) are generally larger than those from Peixinho et al. (2003). Thisemphasizes the need for careful extraction of colors. This includes taking into accounteffects of large amplitude variations in the lightcurves due to rotation. On the otherhand, the difference in colors for some objects could be real due to large scale colorvariegations on the surface or due to temporal changes of the surface colors.

As other authors noted, there seems to be a prevalence of large aspherical objectsamong the TNOs and Centaurs (e.g. Ortiz et al. 2003, Sheppard and Jewitt 2002).Approximately 8% of main belt asteroids with diameters larger than 90 km have axialratios larger than 1.5. With the caveat of small number statistics (total number ofobjects is 27), the percentage of large (i.e. HV ≤7.5) TNOs and Centaurs with axialratios larger than 1.5 is≈22%. TD10 was excluded from the statistic as its value of HV islarger than 7.5. What processes make main belt asteroids of this size more spherical onaverage than TNOs and Centaurs. Is this a selection effect or small statistics effect? Isit an inherent property of TNOs and Centaurs or an evolutionary effect? Sheppard andJewitt (2002) note that the distribution of axial ratios of TNOs is not consistent withthe distribution of impact fragment shapes. The knowledge of shapes and rotationalperiods of TNOs and Centaurs is clearly a very powerful tool in understanding theintrinsic and evolutionary properties of these objects.

The summary of our results is listed below.• The synodic period of TD10 is 15.382±0.001 hr assuming the lightcurve variation

is due to an aspherical nucleus.• No coma activity is detected. An intermittent coma can be excluded as well.

• We derive visible and near-infrared colors for TD10 including its first J-H color.

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• The normalized reflectivity gradient is S ′=10.8±0.9% (0.1µm)−1, and is close tothe median for Centaurs.

ACKNOWLEDGMENTS

It is a pleasure to thank Drs Michael J. S. Belton and Tod R. Lauer for help withacquiring the data. We are very grateful to Drs Stephen Tegler and Philippe Rousselotfor providing their data in electronic form. This work was supported by PlanetaryAstronomy Grants from NASA for BEAM, NHS and HC, and by an NSF Grant forHC.

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