Icarus152, 246–250 (2001)

doi:10.1006/icar.2001.6627, available online at http://www.idealibrary.com on

VR Photometry of Sixteen Kuiper Belt Objects

R. Gil-Hutton1

Felix Aguilar Observatory, Av. Benavidez 8175 oeste, 5407 San Juan, Argentina

and

Javier Licandro

Centro Galileo Galilei, S/C de la Palma, Tenerife, Spain

Received September 12, 2000; revised February 8, 2001; Posted online June 27, 2001

We present V and R photometry of 16 Kuiper belt objects (KBOs)from the 3.6-m Telescopio Nazionale Galileo and the Complejo As-tronomico El Leoncito 2.1-m telescope. We find a wide dispersionin the (V−R) colors of the objects, indicating nonuniform surfaceproperties. If we assume near constant albedos, there does not ap-pear to be a general trend of redness with size, but the color range forclassical KBOs in our sample appears to be wider than for Plutinos.Unless the albedo value is variable for different objects, 1998 SN165becomes the largest Plutino so far identified, apart from Pluto(diameter= 2400 km) and Charon (1200 km). c© 2001 Academic Press

1. INTRODUCTION

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The existence of objects in the region beyond Neptunefirst proposed by Edgeworth (1943) and Kuiper (1951) in whthey suggested that the solar system beyond Neptune conof progressively smaller icy bodies confined near the planthe ecliptic. The so-called Kuiper belt may also supply the shperiod comets (Fern´andez 1980, Duncanet al. 1988) and maybe a source of interplanetary dust (Stern 1996). More thanKuiper belt objects (KBOs) have been found since thecovery in 1992 of the first member of this class, 1992 Q1

(Jewitt and Luu 1992). It is estimated (Jewitt 1999) thatleast 100,000 KBOs with diameters larger than 100 km movnearly circular orbits at heliocentric distances betweenr ≈ 30and 50 AU. They comprise three dynamical classes: objecthe 3 : 2 mean motion resonance with Neptune having beenscribed as “Plutinos” (Jewitt and Luu 1996a), those beyond a41 AU as “classical Kuiper belt objects,” and those with a mularger semi-major axis and higher eccentricity than the prevclasses are known as “scattered disk objects” (Luuet al.1997).

1 Visiting astronomer, Complejo Astron´omico El Leoncito, operated un-der agreement between the Consejo Nacional de Investigaciones Cient´ıficasy Tecnicas de la Rep´ublica Argentina and the National Universities of La PlataCordoba and San Juan.

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based on photometry and spectroscopy in the visible and ninfrared regions, but there are very few observational stuavailable due to the faintness of the objects. However, the larCentaurs, which presumably originated in the Kuiper belt, hlow albedos and similar sizes, but very different specChiron has neutral colors similar to C-type asteroids and socometary nuclei (Hartmannet al. 1990). On the other handPholus was found to have an astonishing spectrum (Finket al.1992) which made it one of the reddest objects in the solartem (Buie and Bus 1992). Also, the spectra published for thKBOs (1993 SC: Luu and Jewitt 1996a, Brownet al.1997; 1996TL66: Luu and Jewitt 1998; 1996 TO66: Brown et al. 1999)are completely different, implying different surface compotion. Many authors (Luu and Jewitt 1996b, Greenet al. 1997,Tegler and Romanishin 1997, Jewitt and Luu 1998) found a wrange of color values among the KBO population and theygue for significant compositional diversity. However, Tegler aRomanishin (1998) found that the Centaur and KBO populaare split in two distinct groups with neutral and very red colorespectively.

To shed further light on the degree of diversity in the KBpopulation it is necessary to obtain additional observationa much larger sample. In this paper we presentV and R pho-tometry of 16 KBOs obtained during eight observing periobetween 1997 and 2000. The observations and data reduare described in the next section. The results are presentediscussed in Section 3.

2. OBSERVATIONS AND DATA REDUCTION

2.1. Observations

Observations were obtained during nine nights (23–October 1997 UT; 30 August 1998 UT; 30 September 19UT; 10 and 12 March 1999 UT; 10 June 1999 UT; 11 a13 November 1999 UT) using the Complejo Astron´omico ElLeoncito (CASLEO) 2.1-m telescope on San Juan, Argent

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and during three more nights (22 April 2000 UT; 27–28 M2000 UT) using the 3.6-m Telescopio Nazionale Galileo (TNof the El Roque de los Muchachos Observatory on La Pa(Canary Islands), Spain. In the observing runs at CASLEOused a 1k× 1k pixel TEK CCD chip with Bessell (1990)V andR filters and a focal reducer yielding a useful circular area wa diameter of 700 pixels, covering in the sky an area≈5 arcminin radius at 0.88 arcsec pixel−1. For the runs at TNG we usedmosaic of two 2k× 4k pixels EEV CCD in a 2× 2 binning modeallowing an image scale of 0.144 arcsec pixel−1 and BesselVandR filters. The exposure time ranged between 420 and 78for both filters and we made two to six measurements throeach filter at intervals of 40 min to 2 h. This procedure minimizpotential systematic errors due to faint background source ctamination and allows to identify the objects. The nights wephotometric with stable seeing of≈2 arcsec for CASLEO and≈1.2 arcsec for TNG, with the exception of the two nightsMarch 1999 when thin cirrus were present. The telescopes wtracked at sidereal rate. Transformation equations were derfrom several Landolt (1992) standard fields containing fourmore stars spanning a range in colors. The total calibration efor any given observation is mostly contained in the errorthe magnitude zero point, which appears to be accurate to0.01 mag level.

2.2. Data Reduction

The CCD images were processed in the usual manner uthe IRAF CCDRED package. All the frames obtained with tCASLEO telescope were bias-subtracted and divided by afield which was a composite of dome and twilight flats. TNframes were corrected by overscan, bias, and flatfielded by utwilight flats. The dark current of the CCDs was checked afound negligible and was accordingly not corrected for. Afbias subtraction and flatfielding, the regions around the objecinterest were searched for nearby cosmic rays. In the cases wa cosmic ray hit the detector near the standard stars or KBOwas replaced by the average of neighboring good pixels.

To increase the S/N ratio, groups of individual frames taktogether in the same night were averaged. Because the motithe KBOs, two averaged images of each group were produone averaged image had the individual frames shifted sothe stars were registered, the other so that the images omoving KBO were registered. The motion of the KBO durineach individual image did contribute to smearing the image,this motion was small, less than 1 pixel.

Aperture photometry was done with the Phot task in the IRDIGIPHOT package. The Landolt standard stars were measusing an aperture of 4 FWHM (5–9 arcsec, depending the nand instrument), which encompassed all the light of the starsthe given typical seeing. Since the KBOs are so faint, thecertainty in their magnitudes is dominated by sky noise. Unsuch conditions, for each averaged KBO image we measure

object with a small aperture (3 pixels for CASLEO’s data an5 pixels for the TNG) on the image with the KBO registered

OMETRY 247

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To obtain the correction between the small aperture, usedthe KBOs, and the large one, used for the standard starsmeasure through a series of apertures several field stars oaveraged KBO image with the stars registered (Howell 19Stetson 1990). The sky value was found in an annulus fromto 22-pixel radius. The sky value was the peak of a Gaussfit to the histogram of pixel values in the sky annulus. If necsary, the sky annulus was first cleaned of objects which mcontaminate the measurement.

Since the nights on March 1999 were non-photometric,observations were corrected by differential photometry usingsecondary standards some of the stars used to find the apecorrection. Those stars were standardized using a complemtary observing run on the night of 11 April 1999 UT, with th0.76-m telescope of F´elix Aguilar Observatory and a CCD witha TI-215 chip.

3. RESULTS AND DISCUSSION

A summary of the aspect data and measurements is giveTable I, where the date, heliocentric and geocentric distanphase angle, filter, number of frames combined, exposureof each individual frame, and the observed magnitude for eobject are shown. Quoted uncertainties are based on statiserrors from photon counting combined with calibration transfThe values ofHv, the (V − R) color and the adopted diameteare listed in Table II. Where separate measurements are avaifor a particular object, we have listed the weighted means.calculate the diameters we adopted a canonical albedo ofpv =0.04, which was confirmed for KBOs by the thermal IR mesurements obtained with ISO (Thomaset al.1999). Within theuncertainties, our measurements for 1994 EV3, 1995 HM5, 1995QY9, and 1996 TO66 are consistent with the magnitudes ancolors listed in the literature (Luu and Jewitt 1996b, Tegler aRomanishin 1998, Jewitt and Luu 1998, Romanishin and Te1999, Barucciet al. 1999, Hainautet al. 1999, Barucciet al.2000, Davieset al. 2000), but a few exceptions exist. TheHV

for 1993 SB, 1994 EV3, and 1997 CQ29are fainter than the valuereported for each object by Davieset al. (2000) (8.26± 0.05),Luu and Jewitt (1996b) (6.99± 0.08), and Barucciet al.(2000)(7.38± 0.03), respectively. The differences could be explainif these KBOs are elongated bodies with light-curve amptudes of∼0.3–0.4 (Romanishin and Tegler 1999). Moreovthe quoted amplitudes agree with those listed by these autfor objects with similarHv. On the other hand, the (V − R)color for 1997 CQ29 is larger than the value reported by Barucet al. (2000) (0.68± 0.06), but variation in (V − R) was ob-served for other KBOs (for example, 1993 RO with1(V − R) =0.24 in few days [Luu and Jewitt 1996b], and 1996 TO66 with1(V − R) = 0.16 in a month [Hainautet al.1999]) and couldbe the result of patches with different colors produced by resfacing due to impacts (Luu and Jewitt 1996b) or to comet

d.activity (Hainautet al. 1999). TheHv and (V − R) for 1994GV9, 1994 JQ1, 1995 QZ9, 1995 SM55, 1996 TC68, 1998 KG62,

248

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GIL-HUTTON AND LICANDRO

TABLE IAspect Data and Magnitudes

Object Date R [AU] 1 [AU] α [deg] Filter N Exp (s) Magnitude

1993 SB 1998 Aug 30 31.14 30.29 1.0 V 3 600 23.77± 0.08R 3 600 23.33± 0.06

1994 EV3 2000 May 28 44.69 43.93 0.9 V 3 600 24.02± 0.09R 3 600 23.43± 0.07

1994 GV9 1999 Mar 10 42.22 41.25 0.3 V 2 600 23.76± 0.07R 2 600 23.02± 0.07

1994 JQ1 1999 Mar 12 42.94 42.24 0.9 V 5 600 23.88± 0.06R 6 600 22.89± 0.04

2000 May 27 42.89 42.01 0.7 V 4 600 23.76± 0.07R 4 600 22.80± 0.06

2000 May 28 42.89 42.02 0.7 V 3 600 23.98± 0.08R 3 600 23.12± 0.07

1995 HM5 1999 Mar 10 32.08 31.18 0.8 V 3 600 23.30± 0.08R 2 600 22.92± 0.07

1999 Jun 10 32.02 31.59 1.7 V 2 600 23.40± 0.09R 2 600 22.98± 0.07

2000 May 27 31.83 31.17 1.4 V 3 600 23.20± 0.07R 3 600 22.77± 0.06

1995 QY9 1997 Oct 23 29.66 28.96 1.4 V 4 600 22.63± 0.05R 4 600 22.02± 0.04

1997 Oct 24 29.66 28.97 1.4 R 2 600 22.05± 0.061995 QZ9 1998 Sep 30 34.98 33.99 0.3 V 2 600 23.27± 0.07

R 3 600 22.77± 0.041995 SM55 1999 Nov 13 39.50 38.59 0.6 V 3 600 20.62± 0.03

R 2 600 20.18± 0.021996 TO66 1997 Oct 24 45.77 44.92 0.7 V 4 600 21.58± 0.03

R 4 600 21.17± 0.031996 TQ66 1998 Sep 30 34.79 33.86 0.6 V 2 780 23.02± 0.04

R 2 720 22.42± 0.041996 TC68 1999 Nov 11 38.78 38.11 1.1 V 3 600 23.20± 0.06

R 3 600 22.60± 0.051997 CQ29 2000 Apr 22 41.41 40.80 1.1 V 3 420 23.94± 0.10

R 3 420 22.99± 0.071997 QJ4 1998 Aug 30 34.79 33.86 0.6 V 3 600 23.40± 0.07

R 4 600 22.94± 0.051998 KG62 1999 Jun 10 45.25 44.55 0.9 V 3 600 23.20± 0.07

R 3 600 22.59± 0.041998 SN165 1999 Nov 11 38.21 37.64 1.2 V 3 600 21.71± 0.04

R 3 600 21.24± 0.031998 UR43 1999 Nov 11 31.78 30.81 0.3 V 3 600 23.71± 0.08

R 2 600 23.07± 0.061999 Nov 13 31.78 30.81 0.4 V 3 600 23.80± 0.09

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1998 SN165, and 1998 UR43, and (V − R) for 1993 SB, 1996TQ66, and 1997 QJ4 are reported for the first time.

The (V − R) colors listed in Table II are all redder than thcolor of the Sun (0.36, Muelleret al.1992) and show a dispersiolarger than can be attributed to the errors of measurement.classical KBOs 1997 CQ29 and 1994 JQ1 are the reddest objectin our sample with (V − R) = 0.95 and 0.94, respectively, an1995 HM5 and 1996 TO66 are the bluest with the same colo(V − R) = 0.41.

near constant albedos, there does not aptrend of redness with size in our KBO sa

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ple. However, the Plutinos observed by us appear to be ccentrated in a region of (V − R) between 0.4 and 0.6, whileclassical objects are dispersed in a wider range. This is mclear in Fig. 1, where we show a plot of semi-major axis(V − R) for our sample, which includes objects observedother authors (Luu and Jewitt 1996a, Greenet al.1997, Teglerand Romanishin 1997, Jewitt and Luu 1998, Hainautet al.1999, Barucciet al. 2000). When many values for the samobject exist we obtain an average. There are only two discrep

pearm-Plutinos, 1994 JV and 1994 TB, both observed by Luu andJewitt (1996a), but the last object was also observed by Tegler

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TABLE IIHv Values, (V − R) Colors, and Adopted Diameters

Object Hv (V − R) Diameter [km]

1993 SB 8.74± 0.08 0.44± 0.10 1201994 EV3 7.41± 0.09 0.59± 0.11 2221994 GV9 7.48± 0.07 0.74± 0.10 2151994 JQ1 7.46± 0.12 0.94± 0.16 2171995 HM5 8.12± 0.14 0.41± 0.18 1601995 QY9 7.77± 0.05 0.60± 0.09 1881995 QZ9 7.82± 0.07 0.50± 0.08 1841995 SM55 4.59± 0.03 0.44± 0.04 8131996 TO66 4.89± 0.03 0.41± 0.04 7091996 TQ66 7.55± 0.04 0.60± 0.06 2081996 TC68 7.19± 0.06 0.60± 0.08 2461997 CQ29 7.64± 0.10 0.95± 0.12 2001997 QJ4 7.93± 0.07 0.46± 0.09 1751998 KG62 6.53± 0.07 0.61± 0.08 3331998 SN165 5.75± 0.04 0.47± 0.05 4771998 UR43 8.72± 0.12 0.62± 0.15 121

and Romanishin (1997) and Barucciet al.(2000), who reporteda different (V − R) (0.68 and 0.74, against 0.85 for Luu aJewitt). Assuming that the collisional resurface is the main pcess to produce the color diversity in the Kuiper Belt, this behior could be explained if the collisional process for the Plutinhas a time scale very short or very long in comparison wthe time scale to form an irradiation mantle, producing in bcases almost uniform colors, while the wider range of (V − R)for classical objects could be the result of similar time scalesboth processes. Since there is another mechanism to explacolor diversity, the cometary activity proposed by Hainautet al.(1999), this explanation is not unique but we think that it issimplest one.

FIG. 1. (V − R) color vs semimajor axis. The KBOs presented in tpaper are indicated by boxes and those reported by other authors by diam

The Plutinos appear to be clustered in a narrow range between (V − R) = 0.4and 0.6.

TOMETRY 249

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Unless the albedo value is variable for different objects, 19SM55 becomes one of the largest KBO so far identified, makitself a future target for spectroscopy and rotational studies.the other hand, 1998 SN165 becomes the largest Plutino, apafrom Pluto (diameter= 2400 km) and Charon (1200 km).

ACKNOWLEDGMENTS

We thank J. K. Davies and an anonymous referee for their helpful commto improve this paper. We also thank Marco Pedani for taking some of the Tframes in service time. RGH acknowledges the use of the CCD and data asition system at CASLEO supported under U.S. National Science FoundaGrant AST-90-15827 to R. M. Rich.

This paper uses observations made with the Italian Telescopio NazioGalileo (TNG) operated at the island of La Palma by the Centro Galileo Gaof the Consorzio Nazionale per l’Astronomia e l’Astrofisica (CNAA) at thSpanish Observatorio del Roque de los Muchachos of the Instituto de Astrofide Canarias.

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