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A Study of Short Term Rotational Variability in Tnos and Centaurs from Sierra Nevada Observatory

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A Study of Short Term Rotational Variability in Tnos and Centaurs from Sierra Nevada Observatory A&A 407, 1149–1155 (2003) Astronomy DOI: 10.1051/0004-6361:20030972 & c ESO 2003 Astrophysics A study of short term rotational variability in TNOs and Centaurs from Sierra Nevada Observatory J. L. Ortiz1,P.J.Guti´errez1;2, V. Casanova1,andA.Sota1 1 Instituto de Astrof´ısica de Andaluc´ıa, CSIC, Apt 3004, 18080 Granada, Spain 2 Laboratoire d’Astrophysique de Marseille, Traverse du Siphon, BP 8, 13376 Marseille Cedex 12, France Received 9 April 2003 / Accepted 19 June 2003 Abstract. Broad band CCD observations focused on short-term rotational variability have been carried out for six TNOs: 1999 TD10, 1999 TC36, 2000 EB173, (20000) Varuna, (28978) Ixion, 2002 CR46, and four Centaurs: 2000 QC243, 2001 PT13, 2002 PN34 and 2002 GO9 using the 1.5 m telescope at Sierra Nevada Observatory (Granada, Spain) since mid 2001. Three of the bodies exhibit periodic double-peaked lightcurves with amplitudes larger than 0.4 mag while another four show periodic variability with amplitudes below 0.20 mag. Basic physical properties of these objects can be derived or constrained from the observations. Here we present a summary of the main results obtained for these objects. Key words. minor planets, asteroids – Kuiper Belt 1. Introduction link needed to check heterogeneity of surfaces as suggested by spectroscopy data (Barucci et al. 2002; Lazzarin et al. 2003). Trans-Neptunian Objects (TNOs) and Centaurs are outer Solar However, time–resolved photometry requires considerable System minor bodies from which Jupiter family comets orig- observing time devoted to each object and therefore it is hard to inate. All these groups seem to share the same origin in carry out due to the difficulty of getting long observing runs in the Kuiper or Edgeworth-Kuiper Belt (e.g. Fern´andez 1980; large telescopes. Smaller telescopes can be used in longer runs, Duncan et al. 1988, etc.). At present, it is believed that these but restricted to sufficiently bright TNOs. objects are among the least evolved in the Solar System. In 2001, we started a CCD photometry program devoted Therefore, the study of these bodies will help us in understand- to studying the short–term variability of some of the bright- ing the formation and evolution of the Solar System. Many of est TNOs and Centaurs from Sierra Nevada observatory. This the current observational studies of TNOs and Centaurs are fo- paper summarizes part of the results already obtained by the cused on color photometry or spectroscopy (e.g. Jewitt & Luu ongoing program. 2001; Boehnhardt et al. 2002; Barucci et al. 2002; Hainaut & Delsanti 2002; Doressoundiram et al. 2002; etc.). By using sta- tistical analysis, most of these studies try to find likely correla- 2. Observations and data reduction tions between color trends and orbital parameters. The observations were carried out by means of the Instituto Other works related to TNOs and Centaurs are focused on the study of their short–term variability (e.g. Davies et al. de Astrof´ısica de Andaluc´ıa 1.5m telescope at Sierra Nevada Observatory, in Granada, Spain. One-week observing runs 1998; Hainaut et al. 2000; Guti´errez et al. 2001; Farnham 2001; were allocated in August 2001, September 2001, October 2001 Ortiz et al. 2002; Sheppard & Jewitt 2002; Bauer et al. 2002; Sekiguchi et al. 2002; Peixinho et al. 2002; etc.). These studies and February 2002, March 2002, May 2002 and August 2002, but weather or technical problems prevented us from observ- can give us hints on the shape, density and other basic physical properties of these bodies (e.g. Sheppard & Jewitt 2002) as well ing in some of the nights of each run. The bodies observed are as on the original size-shape distribution (Lacerda et al. 2003). listed in Table 1 along with other relevant data. The dates with usable data are also listed in Table 1. Besides, short-term variability studies of the largest members of the Kuiper Belt are very important because some knowl- As Centaurs and TNOs have typical drift rates ranging from edge on the rotational state is needed in order to apply the one to a few arcsecs per hour, we used the shortest exposure appropriate thermophysical models to derive the albedos of time possible in order not to get elongated images of either the these bodies. Also, accurate rotation periods can provide the object or the field stars (depending on whether the telescope is tracked at sidereal or nonsidereal rate respectively). An expo- Send offprint requests to: J. L. Ortiz, sure time of 100 s was short enough to avoid noticeable trailing e-mail: [email protected] under the best foreseeable seeing conditions, but long enough Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20030972 1150 J. L. Ortiz et al.: CCD photometry from Sierra Nevada Observatory Table 1. Dates and geometric data (range) of the objects observed. Object rh(AU) ∆ (AU) α(deg) No of images Observing dates TNOs 1999 TC36 31.3 30.57–30.49 1.30–1.14 205 13, 15, 17–20, AUG. (2001) 1999 TD10 12.7 11.75–11.77 0.86–1.10 450 23–26, OCT. (2001) 2000 EB173 29.6 29.18–28.71 1.72–0.87 503 9–10, FEB. (2002); 8–10, MAR. (2002) (2000 WR106) Varuna 43.1 42.33–43.34 0.81–0.83 343 8–9, FEB. (2002) (2001 KX76) Ixion 43.1 42.81–42.87 1.30–1.32 180 15, 17–19, AUG. (2001) 2002 CR46 18.1 17.37–17.40 2.15–2.23 190 8–10, MAR. (2002) Centaurs 2000 QC243 19.2 18.21–18.17 1.12–0.77 290 13, 15, 17–20, AUG. (2001) 2001 PT13 8.9 7.93–8.45 2.67–5.89 681 10–11, 13–14, SEP. (2001); 23–28, OCT. (2001) 2002 PN34 13.4 12.50–12.52 2.30-2.43 260 28–30, AUG. (2002) 2002 GO9 14.4 13.17-13.18 2.15–2.26 263 18–19, MAY (2002) so that the sky background was the dominating noise source. In “approximate V magnitude” in the plots presented here. The order to get a good enough signal to noise ratio, most of the ob- absolute calibration is irrelevant for this investigation, in which servations were obtained with no filter. Therefore, unless oth- short-term variability of the object is obtained by using the stars erwise noted, the observations consisted of sequences of 100 s in the field of view. integrations with no filter. The number of images obtained is The time-resolved observations were inspected for period- therefore very high, in the order of a few thousands. The typ- icities by means of the Lomb technique (Lomb 1976) as imple- ical seeing during the observations ranged from 1.1 arcsec to mented in Press et al. (1992). Typical 1σ errors in the period 2.5 arcsec, with median around 1.5 arcsec. A fast readout CCD determination for our typical four-day observing windows are was used in order not to spend large fractions of the nights read- in the order of 0.02 h (neglecting the interfering effects of the ing out the CCD. The CCD chip format is 1024 1024 pixels aliases). In the cases in which we were able to use longer time × and the total field of view is 7 arcmin 7 arcmin. The images frames, the period determination is more precise. The reference × were bias subtracted in the standard way and flat-fielded using stars were also inspected for short term variability but none was a flatfield frame (the median of a large set of dithered twilight found. images of the sky at blank fields). No cosmic ray removal al- gorithms were used and we simply rejected the few images in which a cosmic ray hit was close to the objects. Relative pho- 3. Results tometry using seven field stars was carried out by means of The main photometric results are listed by object and summa- Daophot routines. The synthetic aperture used was typically 8 rized in Table 2. to 12 pixels in diameter (the smallest possible in order to get the (20000) Varuna (formerly 2000 WR106). The short-term highest signal to noise). Care was taken not to introduce spuri- variability of this very large TNO has been studied by several ous signals of faint background stars or galaxies in the aperture. authors. Farnham (2001) found a 3:17 h 0:01 h periodicity and In cases where the TNOs or Centaurs were close to faint stars Jewitt & Sheppard (2002) reported a double-peaked± lightcurve or galaxies, the data were rejected. with a period of 6:3442 0:0002 h, consistent with the pre- Since the TNOs and Centaurs move relatively slowly, we vious observations. Our own± data, taken in coordination with were able to use the same field stars within each observing the IRAM-30 m radiotelescope in order to obtain a new deriva- run. The typical error bars of the individual 100 s integrations tion of Varuna’s albedo (Lellouch et al. 2002) are shown in are 0.15 mag for the faintest targets we observed, and around Fig. 1. The periodogram analysis of our data showed a clear 0.06 mag for the brightest ones. Both figures were considerably periodicity peak that implied a rotation period of 6:35 0:02 h, improved by averaging or “median averaging” the large amount which was consistent with the rotation period given by± Jewitt of relative photometry data points.
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