Optical and Infrared Photometry of Kuiper Belt Object 1993SC

ICARUS

125, 61–66 (1997) IS965591

ARTICLE NO.

Optical and Infrared Photometry of Kuiper Belt Object 1993SC JOHN K. DAVIES Joint Astronomy Centre, 660 N A’ohoku Pl., Hilo, Hawaii 96720 E-mail: [email protected] AND

NEIL MCBRIDE AND SIMON F. GREEN Unit for Space Sciences and Astrophysics, Physics Laboratory, University of Kent, Canterbury CT2 7NR, United Kingdom Received May 2, 1996; revised August 12, 1996

from their discovery observations to suggest that 1993SC had a lightcurve amplitude of approximately 0.5 magnitude. They also used these data, some 11 photometric points taken over eight nights, to infer a possible rotation period for 1993SC of about 15 hr, although they noted that there was an 80% chance that this result was due to noise. Luu and Jewitt (1996) presented a low-resolution optical spectrum which they suggested may show evidence for some spectral features. In order to compare their spectrum to broadband photometric colors they fitted these data with a relative reflectance spectral index and used this to deduce approximate BVRI colors. They compared these data with VRI colors of other distant objects, from which they concluded that the colors of 1993SC lie between those of the spectrally neutral 2060 Chiron (see Hartmann et al. 1990) and the extremely red object 5145 Pholus (Fink et al. 1992, Buie and Bus 1992, Mueller et al. 1992), these objects being the best observed of the six currently known Centaur objects. It is of interest to extend this spectral information into the near IR since this is where the only diagnostic spectral features yet detected on any of these distant objects have been found (Davies et al. 1993). Such data may establish a link between objects in the Kuiper Belt and transition objects in unstable orbits closer to the Sun, such as Centaurs 1993HA2 and 5145 Pholus. However, at the present level of technology, IR spectroscopy of these objects is barely feasible and so it is necessary to obtain broadband VJK colors in order to make this comparison. The visual and IR data are generally obtained at two different epochs, so any attempt to determine visual to IR colors requires the extrapolation of data in the visual. Since Williams et al. (1995) suggest that 1993SC may have a significant lightcurve, this behavior must be investigated in order to interpret our visual and infrared data. This paper reports

Minor planet 1993SC, with a semi-major axis of 39.67 AU, is one of the brightest of numerous recently discovered objects with orbits close to or beyond Neptune. It is a member of the Kuiper Belt, a planetesimal population remaining from the formation of the Solar System. We present optical photometry which indicates a lightcurve amplitude of less than 0.2 magnitude for 1993SC and which does not support the 0.5 magnitude lightcurve of I. P. Williams et al. (Icarus 116, 180–185, 1995). We derive (Kron-Cousins photometric system) V 2 R 5 0.54 6 0.14, V 2 I 5 0.97 6 0.14, and V 2 J 5 2.08 6 0.15, which confirm that 1993SC has optical/infrared colors closer to Centaur 1993HA2 than to the extremely red 5145 Pholus. We also find that VRI colors published by J. X. Luu and D. C. Jewitt (Astron. J. 111, 499–503, 1996) are inconsistent with their reflectance spectrum of 1993SC and we derive new values from their reflectance spectrum of V 2 R 5 0.56 6 0.08 and V 2 I 5 1.19 6 0.18, which give reasonable agreement with our results.  1997 Academic Press 1. INTRODUCTION

The outer Solar System object 1993SC was discovered in September 1993 (Williams et al. 1993) and was subsequently re-observed in 1994 and 1995. This has led to a fairly secure orbit which has a semi-major axis of 39.67 AU, an eccentricity of 0.19 (hence a perihelion distance of 32.14 AU), and an inclination of 58. The object can cross the orbit of Neptune but is presumably stabilized by being in a 2 : 3 resonance in the same fashion as Pluto (Marsden, personal communication). It is hypothesized that 1993SC and p30 currently known other objects are members of the Kuiper Belt (Edgeworth 1949, Kuiper 1951), planetesimals remaining from the formation of the Solar System (see, for example, Luu 1994, Jewitt and Luu 1995, Weissman 1995). Due to the extreme faintness of these objects physical studies are difficult, but Williams et al. (1995) used data 61

0019-1035/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.

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DAVIES, MCBRIDE, AND GREEN

TABLE I Optical Photometry from the INT

a further study of the lightcurve amplitude of 1993SC and combines these data with non-simultaneous J data to deduce VRIJ colors. 2. LIGHTCURVE OBSERVATIONS

We observed 1993SC using the 2.5-m Isaac Newton Telescope of the Observatorio del Roque de Los Muchachos, La Palma, on the nights of 1995 October 26–30 with a TEK3 thinned CCD chip mounted at prime focus and a Kitt Peak R broadband filter (Krons-Cousins photometric system). The final images were 1024 3 1024 pixels with an image scale of 0.59 arcsec per pixel. The weather throughout the run was not ideal, with all nights being affected to a greater or lesser extent by either ridge cloud or thin cirrus cloud. Although there were periods that may have been photometric we present here only relative optical photometry as we would not have full confidence in using our standard star fields for absolute calibration. The relative photometry, however, is adequate for the conclusions of this paper. The data presented amount to 33 frames obtained on five nights. Each image was exposed for 1000 sec. No attempt was made to track the object, since in this time the target motion was only p1.4 pixels. Each image was bias subtracted and divided by a flat field image using the Starlink software package KAPPA (Currie 1992) in the usual way. Flat field frames were obtained using the twilight sky (on clear nights) and compared with dome flats. The dome flats compared very well to the twilight flats, suggesting a minimum of color anomalies, and so a median frame of multiple dome flat images was used in the reduction. Relative photometry of the resulting images was carried out with the DAOPHOT II (and ALLSTAR) software package (Stetson 1992) using profile fitting to ensure optimum signal to noise on 1993SC. Instrumental magnitudes for 1993SC were determined relative to a mean of six comparison stars on the same frame as 1993SC. Each of the six stars’ instrumental magnitudes was checked for consistency relative to the other five stars throughout the 33 frames, and seen to be well behaved. The 1s uncertainty in the comparison stars’ mean relative instrumental magnitude was 0.01. In addition to relative photometry of 1993SC being obtained, two other stars of similar magnitude were used as check stars to assess any overall systematic errors in the photometry of 1993SC. The relative magnitude Dm data for 1993SC and the two check stars are given in Table I. Quoted errors are those output by the DAOPHOT II package, which are the estimated standard errors of the stars’ magnitudes. The data are plotted in Fig. 1 as differences in magnitude Dm from the mean value obtained from all 33 frames. Magnitude corrections due to aspect

1993SC

Star 1

Star 2

Day (1995 Oct UT)

Dm

Err

Dm

Err

Dm

Err

26.9660 26.9799 26.9938 27.0097 27.0250 27.0389 27.0528 27.0674 27.0819 27.9083 27.9215 27.9354 27.9493 27.9944 28.0090 28.0236 28.0722 28.0854 28.1000 28.1139 28.1271 28.8979 28.9111 28.9236 28.9854 28.9993 29.0125 29.0431 29.1208 30.1000 30.1132 30.8674 30.8958

2.934 2.940 2.801 2.642 2.948 2.836 2.715 2.744 2.719 2.781 2.790 2.541 2.877 2.714 2.797 2.819 2.915 2.696 2.738 2.700 2.788 2.779 2.809 2.780 2.796 2.861 2.745 2.847 2.750 2.846 2.915 2.909 2.930

0.082 0.078 0.065 0.065 0.084 0.054 0.063 0.062 0.077 0.074 0.063 0.063 0.070 0.058 0.073 0.059 0.072 0.059 0.063 0.043 0.105 0.075 0.105 0.091 0.079 0.077 0.072 0.143 0.123 0.130 0.147 0.104 0.111

2.558 2.538 2.385 2.370 2.535 2.465 2.462 2.570 2.552 2.565 2.563 2.344 2.495 2.447 2.506 2.682 2.537 2.643 2.612 2.561 2.388 2.426 2.523 2.307 2.578 2.490 2.347 2.503 2.457 2.541 2.527 2.517 2.521

0.054 0.053 0.034 0.053 0.042 0.039 0.039 0.041 0.051 0.049 0.049 0.040 0.037 0.034 0.061 0.060 0.064 0.081 0.076 0.072 0.067 0.081 0.073 0.065 0.061 0.061 0.049 0.085 0.083 0.094 0.101 0.073 0.091

2.763 2.718 2.574 2.677 2.959 2.769 2.582 2.655 2.690 2.801 2.710 2.685 2.719 2.685 2.811 2.862 2.777 2.830 2.800 2.732 2.681 2.503 2.778 2.787 2.574 2.875 2.685 2.760 2.650 2.905 2.508 — —

0.069 0.039 0.069 0.081 0.080 0.059 0.052 0.062 0.060 0.046 0.049 0.098 0.039 0.045 0.054 0.053 0.065 0.053 0.059 0.053 0.076 0.072 0.075 0.087 0.064 0.095 0.079 0.123 0.083 0.116 0.095 — —

mean 2.800

s 0.092

mean 2.500

s 0.086

mean 2.726

s 0.107

Note. The relative magnitudes Dm and the mean and standard deviation s of 1993SC and two check stars of similar brightness are given. The magnitudes are relative to a mean of the same six comparison stars on each frame. The second check star was not on the last two frames.

changes are negligible compared to the errors of the photometry. At first glance, the 1993SC data appear to vary by up to 0.3 magnitude, but these changes occur over a short time scale (less than 0.5 hr) which does not seem physically reasonable for a rotating solid object. We conclude that the 1993SC data vary randomly about a constant value (rather than displaying a regular lightcurve behavior) for the following reason. The typical 1s error for individual 1993SC data points is 60.08 magnitude, compared with the standard deviation of all the data which is 0.09 magnitude. It is therefore clear that the errors given by the DAOPHOT II package represent the true 1s errors in the

PHOTOMETRY OF 1993SC

63

FIG. 1. Relative magnitudes for 1993SC and two comparison stars for the five nights, 1995 October 26–30. The data are plotted as magnitudes from the mean Dm values, which are 2.800, 2.500, and 2.726 for 1993SC, star 1 and star 2, respectively. Note the typical error of a 1993SC data point is 60.08 magnitude, and the spread of the data about its mean is 60.09. We conclude that the spread is random. The second check star was not on the last two frames.

photometry and the distribution of data about the mean is random. So we could not expect to determine any lightcurve variation of less than around 60.1 magnitude. Additionally, the spread of the data for the check stars is comparable to that for 1993SC, with 1s values being 0.09 and 0.10 magnitude for stars 1 and 2, respectively. We searched for correlation of the check stars with the 1993SC data, but found none. We conclude that a lightcurve variation of 0.5 magnitude as reported by Williams et al. (1995) is not supported by our data (and thus we cannot identify a rotation period), and indeed any lightcurve variation of 1993SC must have an amplitude of less than p0.2 magnitude. 3. OPTICAL PHOTOMETRY

VRI photometry of 1993SC was obtained with the 3.9-m Anglo-Australian Telescope, New South Wales, on 1995 October 21. Images of 1024 3 1024 pixels were obtained using a TEK CCD chip with Kitt Peak V, R, and I filters at prime focus, providing an image scale of 0.39 arcsec per pixel. Exposures were for 1000 sec in R and V and 500 sec in I to prevent sky background saturation. The

weather was clear for the first half of the night when the 1993SC data (and standard frames) were obtained with seeing p2.5 arcsec. Processing was performed as for the INT observations and calibrated using equatorial standard fields PG 2331 and PG 0231 (Landolt 1992) providing Kron-Cousins standard magnitudes. Calibration was performed using PHOTOM (Eaton 1989). (DAOPHOT profile fitting could not be used for calibration since the instrumental magnitudes it gives are relative to stars on the frame; i.e., each frame has a different zero point.) Multiaperture photometry using PHOTOM (Eaton 1989) was performed separately for the standards and a selection of about 15 field stars on the 1993SC frames. This allowed calibration of the field stars in a relatively small aperture (therefore increasing the signal to noise ratio) despite the different PSF and possible variations in seeing, by determining an aperture correction for the average of all the field stars. These aperture corrections were ,0.05 magnitude in all cases. Calibration of 1993SC could then be performed using either profile fitting or aperture photometry of the field stars and object. In general, profile fitting provides a smaller statistical error in the photometry, but care must be exercised in ensuring that the image profile

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DAVIES, MCBRIDE, AND GREEN

TABLE II Optical Photometry of 1993SC Reduced Using Profile Fitting and Aperture Photometry Time (1995 Oct UT)

Filter

21.5048 21.5173 21.5299 21.5367 21.5434

R V I I R

Adopted magnitude

V 5 22.35 6 0.1

Magnitude (profile fitting) 21.85 22.94 21.25 21.40 21.74

6 6 6 6 6

Magnitude (aperture photometry)

0.03 0.08 0.09 0.10 0.04

21.70 22.35 21.43 21.43 21.93

R 5 21.81 6 0.1

6 6 6 6 6

0.05 0.08 0.18 0.18 0.07

I 5 21.38 6 0.1

Note. The adopted magnitudes are means of the two techniques except for the V filter, for which the PSF of 1993SC was very different from the field stars in the frame. Errors quoted for each technique are statistical errors output by the reduction package and may not be representative (see text). Errors of 0.1 for the adopted magnitudes are based on the scatter in results from the two techniques plus a contribution for small calibration uncertainties.

of the target object is similar to the field stars, which is not always the case for slightly trailed images. In aperture photometry, the sky noise dominates the error, due to numerous barely detected faint stars and galaxies. Examination of magnitudes determined using both techniques, given in Table II, illustrates the potential difficulties. The derived statistical errors can be considerably smaller than the differences in magnitudes derived using the two techniques, for the reasons given above. For instance, a significant difference in the two methods is apparent for the single V frame and closer examination of the target profile showed a very poor fit to the PSF derived from the field stars in this case. Since aperture photometry, while noisier, provides the overall object counts, independent of the image profile (as long as the aperture is large enough to ensure that the same proportion of the signal is detected as for the field stars), we have used only the aperture photometry magnitude for this V frame. We emphasize that, from our experience of very faint moving objects observed against a non-uniform background, observers must guard against relying solely on the statistical errors of the data when assessing the accuracy of their photometry. 4. INFRARED PHOTOMETRY

Infrared observations of 1993SC were made with the IRCAM3 common user (facility) camera on the United Kingdom Infrared Telescope. The camera, which has a 256 3 256 pixel InSb array, has a plate scale of 0.286 arcsec per pixel and a field of view of 73.2 arcsec. For each filter a basic observation comprised a mosaic of nine separate frames, offset from each other by 8 arcsec to minimize the effect of bad pixels. After an appropriate dark frame was subtracted from the object frames, each set of nine frames

was median filtered to produce a flat field, which was then divided into each frame. The resulting frames were coadded with appropriate offsets to produce a single image. Each such image has a total exposure time of 540 sec. In some of these images the target was visible but close to the detection threshold; in others it was not. To improve the signal to noise, 4–6 of the nine-frame mosaics were coadded with due allowance for the object motion. Software aperture photometry was performed on these images and calibrated relative to images of standard stars taken at similar airmasses within an hour of the 1993SC observations. The final errors quoted combine internal errors in the images with uncertainties in photometric calibration. IR observations were made on three nights. On 1994 November 5 the conditions started photometric but deteriorated during the observations due to the onset of cirrus cloud. Only some of the images were usable, although the target and a nearby star were detected in a number of frames allowing photometry relative to the nearby star. The photometric calibration of the resulting image was made by taking an image of the same field on a later date, which by then no longer contained 1993SC. On 1995 September 20 observations were again affected by light cirrus cloud but once again there were sufficient stars in the field (three in this case) to obtain photometry of 1993SC relative to field stars. The following night was photometric and this was used to obtain additional data on 1993SC and to calibrate the field observed the night before. Data are listed in Table III. 5. OPTICAL/INFRARED COLORS

Although the optical and infrared photometry were not simultaneous, we have derived colors by correcting for changes in aspect (see Table III) assuming

65

PHOTOMETRY OF 1993SC

TABLE III The Aspect Data and Photometry for 1993SC Date (UT) 1994 1995 1995 1995

1995 1995 1995 1995 1995

Nov 05.4 Sep 21.4 Sep 22.4 Oct 21.5

Oct Oct Oct Oct Oct

26.0 27.0 28.0 29.0 30.0

Telescope

r (AU)

D (AU)

a (deg)

UKIRT UKIRT UKIRT AAT

34.046 34.176 34.176 34.188

33.314 33.176 33.175 33.288

1.14 0.17 0.14 0.72

INT INT INT INT INT

34.190 34.191 34.191 34.191 34.192

33.327 33.337 33.346 33.356 33.366

0.8 0.9 0.9 0.9 0.9

Photometry J 5 20.48 6 0.2 J 5 20.11 6 0.1 J 5 20.18 6 0.1 V 5 22.35 6 0.1 R 5 21.81 6 0.1 I 5 21.38 6 0.1 R relative R relative R relative R relative R relative

V used for colors

Colors

22.38 22.27 22.26

V 2 J 5 1.90 6 0.24 V 2 J 5 2.16 6 0.17 V 2 J 5 2.08 6 0.17

22.35 22.35

V 2 R 5 0.54 6 0.14 V 2 I 5 0.97 6 0.14

Note. In order to derive V 2 J colors, we use the Oct 21 V magnitude and correct it according to the change in r, D, and a to produce the ‘‘V used for colors’’ values shown for the UKIRT nights. The Nov 1994 field was calibrated using frames from 1994 Dec 10, and the 1995 Sep 21 field was calibrated using frames from 1995 Sep 22.

(i) the lightcurve amplitude is ,0.2 magnitude as indicated from the INT data, and (ii) the slope parameter G 5 0.15. The uncertainties in the data themselves are p0.1 magnitude. If we consider G values of 0.05 or 0.25 this will introduce an error of only 60.02 magnitude between the dates of the optical and infrared observations, which is small compared with the other errors. Aspect data for the observations, plus the V magnitude corrected to the phase angle and distance of the J data (as G is defined in V, not J), are listed in Table III. Using the average R and I data, we obtain V 2 R 5 0.54 6 0.14 and V 2 I 5 0.97 6 0.14 on 1995 October 21. We obtain V 2 J 5 1.90 6 0.24 on 1994 November 5, V 2 J 5 2.16 6 0.17 on 1995 September 21, and V 2 J 5 2.08 6 0.17 on 1995 September 22, which gives a weighted mean result of V 2 J 5 2.08 6 0.15. Although the V 2 J data are consistent within the errors, it is possible that a color change could occur between the two dates due to differences in the scattering phase function at the V and J wavelengths since these data are obtained at very small phase angles where any phase effect would be most apparent. It is also possible that there is some lightcurve variation with amplitude less than 0.2 magnitude. 6. DISCUSSION

Luu and Jewitt (1996, Table 1) present an R 2 I color from photometry of 0.86 6 0.10 (redder than 5145 Pholus) in apparent disagreement with our result of R 2 I 5 0.43 6 0.14. They also derive colors from their reflection spectrum of V 2 R 5 0.57 6 0.09 and R 2 I 5 1.05 6 0.16 (also redder than Pholus), but then conclude that 1993SC is intermediate in color between Pholus and 2060 Chiron. Inspection of their results (Table 1) and their spec-

trum suggests that they may have in fact erroneously used the V 2 I color of the Sun (0.69; Hartmann et al. 1990) in deducing the R 2 I color from their reflectance spectrum. ˚ )21 Luu and Jewitt’s spectral index S9 5 0.2 6 0.09 (103 A can be used to determine the relative reflectance (i.e., flux of the object divided by solar analog flux, normalized at the V filter) using R 5 1 1 S9

(l 2 lV) , 1000

(1)

˚ . We obtain R (V) 5 1 by definition, where l and lV are in A R (R) 5 1.20 6 0.08, and R (I) 5 1.56 6 0.25 (using lV 5 550 nm, lR 5 650 nm, and lI 5 830 nm for the Krons-Cousins system). The colors are then given by mV 2 ml 5 2.5 log R (l) 1 (mV 2 ml )( ,

(2)

where (mV 2 mR)( 5 0.36, (mV 2 mI)( 5 0.71, and (mV 2mJ)( 5 1.05 (Meech et al. 1995). We obtain V 2 R 5 0.56 6 0.08 and V 2 I 5 1.19 6 0.18 for a spectral index S9 5 0.2 6 0.09, in reasonable agreement with our results. No observational details are given by Luu and Jewitt for their R 2 I photometry, the colors of which do not agree with their spectrum (or quoted conclusions) or our data. Mueller et al. (1992) report Pholus V 2 R values of 0.75 for 1992 January 9, and 0.66 for 1992 January 23 (no errors given), whereas Buie and Bus (1992) report V 2 R 5 0.810 6 0.006. The reflectance spectra reported in Fink et al. (1992) and Binzel (1992) give V 2 R 5 0.81 6 0.02 and V 2 R 5 0.78 6 0.02, respectively. Davies et al. (1996) report that in 1992, Pholus had V 2 J of 2.5 6 0.06 and V 2 K of 2.94 6 0.06 (taking into account its known lightcurve).

66

DAVIES, MCBRIDE, AND GREEN STARLINK is funded by PPARC. We thank Duncan Steel, Gordon Gerradd, and Andrew Taylor for obtaining the AAT observations, Tim Hawarden at UKIRT, and Kathryn Hudson for some of the data reduction. N. McBride acknowledges the financial support of PPARC. We thank Alan Stern and Anita Cochran for helpful comments on the manuscript.

REFERENCES

FIG. 2. Color diagram comparing Pholus, Chiron, 1992HA2 , and 1993SC. The grid region for Pholus encompasses the range of values (including errors) reported by Binzel (1992), Buie and Bus (1992), Fink et al. (1992), Mueller et al. (1992), and Davies et al. (1996). The shaded region for Chiron similarly encompasses the range of values reported by Hartmann et al. (1990) (when Chiron was relatively inactive). The 1992HA2 curve uses R p 0.7 from Luu (1994) (we have assigned 60.15 error) and the V 2 J reported by Davies et al. (1996).

Luu (1994) describes 1993HA2 as having V 2 R p 0.7. Data presented by Davies et al. (1996, Table 1) show that between July 1993 and July 1994 1993HA2 had V 2 J in the range 1.8–2.3, giving a mean of 2.07 6 0.09, and V 2 K between 2.0 and 2.4, giving a mean of 2.28 6 0.07 (five determinations with no knowledge of the lightcurve), although we note that the errors in the individual K magnitudes are of order 60.3, which may artificially increase the spread of the V-K color. We have plotted these data as reflectance spectra in Fig. 2, where we have shown Pholus and Chiron as regions encompassing the range of values (including errors) described above. Our data suggest that 1993SC has the very red colors characteristic of the Centaur objects 1993HA2 and 5145 Pholus, with optical–infrared colors closer to those of 1993HA2 than to 5145 Pholus. ACKNOWLEDGMENTS The Isaac Newton telescope is operated on the island of La Palma by the Royal Greenwich Observatory in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. The AngloAustralian Telescope is operated by staff of the Anglo-Australian Observatory on behalf of the UK Particle Physics and Astronomy Research Council (PPARC) and the Australian Department of Employment, Education, and Training. The UK Infrared Telescope is operated by the Joint Astronomy Centre on behalf of PPARC. Image processing and data reduction were performed using the STARLINK network and software.

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