A&A 371, 753-759 (2001)
N. Peixinho1 - P. Lacerda2 - J. L. Ortiz3 - A. Doressoundiram4 - M. Roos-Serote1 - P. J. Gutiérrez3
1 - Centre for Astronomy and Astrophysics of the University of Lisbon/Lisbon Astronomical Observatory, Tapada da Ajuda, 1349-018 Lisbon, Portugal
2 - Leiden Observatory, The Netherlands
3 - Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain
4 - Paris-Meudon Observatory, France
Received 9 November 2000 / Accepted 6 March 2001
We present the results of visible broad band photometry of two Centaurs, 10199 Chariklo (1997CU) and 1999UGfrom data obtained at the 1.52 meter telescope of the National Astronomical Observatory at Calar Alto, Spain, during 2 separate runs in April 1999 and February 2000 and at the 1.5 meter telescope of the Sierra Nevada Observatory, Spain, in March of 1999. For Chariklo, the absolute magnitudes determined from the February 2000 data are found to be higher by about 0.27 mag than the average in the 1999 run. This may indicate long period rotational variability and possibly a G parameter higher than the assumed value of 0.15. From the best sampled R-lightcurve obtained in the February 2000 run, no short term rotational variability was found. The V-R colours for this object in all runs are similar to previously published values. For 1999UG, colours were found to be very red: , and . These results place this object in the group of the reddest known bodies in the Solar System. HR and HV are found to be and respectively, and its diameter is estimated to be on the order of km.
Key words: Minor bodies - Centaurs - photometry - rotation - colours
The Centaurs are a family of minor bodies, usually defined as those objects having orbits between Jupiter and Neptune, even though a strict dynamical definition does not exist. They are thought to be in transition from Kuiper Belt Objects into short period comets, and their orbits are dynamically unstable on a 106-107 year time scale (Asher & Steel 1993; Levison & Duncan 1997; Morbidelli 1997). Therefore, the study of the chemical and physical properties of these objects together with those of the Kuiper Belt, which probably contain some of the most pristine material in the Solar System, is of great importance for the study of the earlier stages of the formation of the Solar System. At the time of writing about 20 Centaur objects have been identified.
Centaur 10199 Chariklo (1997CU) was discovered on February 15, 1997 (Scotti et al. 1997) and has been observed on several occasions since then (Brown 2000; Brown et al. 1998; Davies et al. 1998; McBride et al. 1999; Tegler & Romanishin 1998). It is one of the few distant minor bodies for which a spectrum has been obtained, and it shows signatures of water ice (Brown 2000; Brown et al. 1998). Also, infrared and visible measurements indicate an albedo of about 0.045 and a corresponding size of km diameter (Jewitt & Kalas 1998). No cometary activity has been detected (McBride et al. 1999). The rotation period has been difficult to determine, due to the apparently very weak variations in the lightcurve, which suggest either a very long period or one close to 24 hours (Davies et al. 1998; McBride et al. 1999, from observations taken in 1997). Another possibility is that the object is (nearly) spherical without significant albedo variations on the surface, or that one of its poles was facing the Sun in 1997. In this paper we discuss these possibilities.
Centaur 1999UG was discovered on October 29, 1999. This is the first report on photometry of this object.
The observations were obtained during three separate runs:
The detector at NAO was a Tektronix
CCD with square
0.4arcsec pixels and a
arcmin field of view.
We used standard Johnson BVRI filters.
The typical seeing conditions during the photometric nights
were about 2-3 arcsec in 1999 and 2 arcsec in 2000.
|Night||Telescope||Landolt Fields||Extinction Coeficients||Colour Terms||Filter|
|1999 Mar. 19-20||SNO||V|
|1999 Mar. 20-21||SNO||pg1657+078, 104325||V|
|SNO||pg1657+078, 104325||assumed kR = 0.15||R|
|1999 Mar. 21-22||SNO||V|
|1999 Apr. 14-15||NAO||V|
|1999 Apr. 17-18||NAO||pg1657+078, pg0918+029||V|
|2000 Feb. 02-03||NAO||pg0231+051, pg0918+029||V|
|2000 Feb. 03-04||NAO||pg0231+051, pg0918+029||R|
|2000 Feb. 04-05||NAO||pg0231+051, pg0918+029||B|
|2000 Feb. 06 - 07||NAO||pg0918+029||R|
The reduction of the 1999 data was carried out following standard processing of average bias subtraction and flatfield correction using high signal-to-noise flatfields for each filter (Johnson R and V). The DAOPHOT package was used for the synthetic aperture calculations. The synthetic aperture used for the Centaur and field stars had a diameter of six pixels for the SNO run and 2 times the Full Width at Half Maximum (FWHM) of the stellar profiles for the NAO run. These small apertures allow us to maximize the signal to noise on faint objects. The photometry was computed relatively to two field stars (observed on all nights) and was placed in an absolute scale by means of observations of the Landolt standard pg1657+078 and 104325 (Landolt 1992) for which we used a large enough aperture to get all the flux. The fraction of flux loss in the relatively small apertures used for the Centaur and field stars was estimated by measuring the percentage of flux loss in the brightest stars. The final results are therefore corrected for this effect. Extinction coefficients were derived for the V band from strömgren photometry which was being carried out at the 0.9 meter telescope in the SNO on the same nights, whereas for R, it was assumed to have a slightly smaller value (the resulting uncertainty is very small because the difference in airmasses between the calibration star and the centaur was lower than 0.2). The uncertainty in the absolute calibration is estimated to be 0.03 mag. For the NAO 1999 run, the extinction coefficients were derived from our own data in R and V, as we had adequate sky coverage. The uncertainty in the absolute calibration is estimated as 0.04.
For the February 2000 run, we used IRAF's CCDRED package and averaged biases and high signal-to-noise flat fields taken frequently during the run. We observed Landolt fields (Landolt 1992) close to our objects, and tracked them during the night for absolute calibration. Synthetic aperture photometry was performed using IRAF's PHOT routine. We used an aperture of 2 times the FWHM of the field stars' profiles for 10199 Chariklo (1997CU) and the standard stars, and 1.5 times the FWHM for the fainter 1999UG. Growth curves were derived using the brightest stars in each frame, and the correction for the flux loss was determined with IRAF's MKAPFILE routine, considering the profile out to 4 times the FWHM, where the profile reached its plateau. The colour terms on this run turned out to be considerably high and we took them into account.
Table 1 lists all the details of the calibration parameters.
Table 2 lists the observations for 1997CU, together with the values for the calibrated magnitudes. Since our observations were focused on the determination of a possible rotation period, we mainly observed through the R filter. In order to monitor the correctness of our data, we did some observations in the V filter to check the V-R colour.
From the March 1999 run (SNO), we obtain an average V value of and an average R value of . The above values translate into absolute magnitudes and , assuming a phase correction parameter G = 0.15. The V-R colour as calculated from these values is .
From the April 1999 run (NAO), the average V value is , and the average R value is , so that the absolute magnitudes are and , assuming the same phase correction parameter. The V-R colour is .
For the February 2000 run (NAO), we find an average value for V of and for R of . Considering only consecutive V and R measurements from the first night, we obtain . The values for HV and HR are and respectively, again assuming G = 0.15.
In all runs, the V-R colour indices are consistent and similar to previously published values (Table 7; Davies et al. 1998; McBride et al. 1999; Tegler & Romanishin 1998).
|UT date||Julian date||R (AU)||(AU)||Exp.(s)||Filter||Mag|
|1999 Mar. 19-20||2451257.49236||13.49||12.72||2.74||600||V||18.014|
|1999 Mar. 19-20||2451257.50208||1000||V||18.070|
|1999 Mar. 19-20||2451257.51458||1500||V||18.025|
|1999 Mar. 19-20||2451257.53472||1500||V||17.968|
|1999 Mar. 19-20||2451257.55417||1000||R||17.515|
|1999 Mar. 20-21||2451258.46806||13.49||12.73||2.80||1000||V||18.007|
|1999 Mar. 20-21||2451258.48750||1000||V||17.947|
|1999 Mar. 20-21||2451258.50764||1000||R||17.499|
|1999 Mar. 20-21||2451258.52361||1000||R||17.465|
|1999 Mar. 20-21||2451258.53681||1000||V||17.940|
|1999 Mar. 20-21||2451258.55000||1000||R||17.475|
|1999 Mar. 20-21||2451258.56389||1500||V||18.140|
|1999 Mar. 20-21||2451258.57778||1500||V||18.010|
|1999 Mar. 20-21||2451258.58700||1000||V||18.074|
|1999 Mar. 21-22||2451259.39236||13.49||12.74||2.86||1000||V||18.197|
|1999 Mar. 21-22||2451259.40764||1000||R||17.519|
|1999 Mar. 21-22||2451259.42361||1000||V||17.944|
|1999 Mar. 21-22||2451259.43611||1000||R||17.521|
|1999 Mar. 21-22||2451259.44931||500||R||17.641|
|1999 Mar. 21-22||2451259.45625||500||V||17.995|
|UT date||Julian Date||R (AU)||(AU)||Exp. (s)||Filter||Mag|
|1999 Apr. 14-15||2451283.457||13.48||13.05||3.90||500||R||17.625|
|1999 Apr. 14-15||2451283.470||1000||R||17.621|
|1999 Apr. 14-15||2451283.481||1000||V||18.107|
|1999 Apr. 14-15||2451283.490||1000||V||18.137|
|1999 Apr. 17-18||2451286.336||13.48||13.08||3.99||500||R||17.466|
|1999 Apr. 17-18||2451286.352||500||R||17.476|
|1999 Apr. 17-18||2451286.358||1000||V||17.976|
|1999 Apr. 17-18||2451286.366||1000||V||17.963|
|1999 Apr. 17-18||2451286.373||500||R||17.496|
|1999 Apr. 17-18||2451286.414||500||R||17.561|
|1999 Apr. 17-18||2451286.421||500||R||17.525|
|1999 Apr. 17-18||2451286.428||1000||V||17.938|
|1999 Apr. 17-18||2451286.435||1000||V||18.009|
|1999 Apr. 17-18||2451286.457||500||R||17.618|
|UT date||Julian date||R (AU)||(AU)||Exp. (s)||Filter||Mag||Error|
|2000 Feb. 02.94132||2451577.44132||13.36||12.43||1.48||300||R||17.524||0.023|
|2000 Feb. 02.94571||2451577.44571||300||V||18.057||0.023|
|2000 Feb. 03.02298||2451577.52298||500||R||17.551||0.021|
|2000 Feb. 03.02965||2451577.52965||500||V||18.002||0.019|
|2000 Feb. 03.08629||2451577.58629||500||R||17.526||0.021|
|2000 Feb. 03.09727||2451577.59727||500||R||17.560||0.014|
|2000 Feb. 03.10402||2451577.60402||500||V||18.065||0.019|
|2000 Feb. 03.13865||2451577.63865||500||V||18.010||0.018|
|2000 Feb. 03.14532||2451577.64532||13.36||12.43||1.47||500||R||17.547||0.021|
|2000 Feb. 03.17434||2451577.67434||500||R||17.553||0.021|
|2000 Feb. 03.18398||2451577.68398||500||V||18.051||0.020|
|2000 Feb. 03.96915||2451578.46915||13.36||12.42||1.42||500||R||17.519||0.016|
|2000 Feb. 04.00981||2451578.50981||500||R||17.515||0.015|
|2000 Feb. 04.05909||2451578.55909||500||R||17.531||0.016|
|2000 Feb. 04.10560||2451578.60560||500||R||17.509||0.016|
|2000 Feb. 04.15458||2451578.65458||500||R||17.545||0.016|
|2000 Feb. 04.94554||2451579.44554||13.36||12.42||1.37||500||R||17.521||0.043|
|2000 Feb. 04.96629||2451579.46629||500||R||17.574||0.043|
|2000 Feb. 05.01117||2451579.51117||500||R||17.565||0.043|
|2000 Feb. 05.03236||2451579.53236||500||R||17.579||0.043|
|2000 Feb. 05.06306||2451579.56306||13.36||12.42||1.36||500||R||17.520||0.043|
|2000 Feb. 05.08058||2451579.58058||500||R||17.578||0.043|
|2000 Feb. 05.10391||2451579.60391||500||R||17.565||0.043|
|2000 Feb. 05.12286||2451579.62286||500||R||17.544||0.044|
|2000 Feb. 06.97955||2451581.47955||13.36||12.41||1.26||500||R||17.574||0.016|
|2000 Feb. 07.02205||2451581.52205||500||R||17.556||0.016|
|2000 Feb. 07.04083||2451581.54083||500||R||17.597||0.016|
|2000 Feb. 07.06536||2451581.56536||500||R||17.569||0.014|
|2000 Feb. 07.08455||2451581.58455||500||R||17.548||0.016|
|2000 Feb. 07.14320||2451581.64320||13.36||12.41||1.25||500||R||17.563||0.017|
|2000 Feb. 07.16938||2451581.66938||500||R||17.560||0.017|
|Figure 1: The Lomb-Normalized Power Spectrum as determined from the February 2000 data set in the R-filter using Lomb's (1976) method (Press et al. 1992) for unevenly spaced data. The maximum power at 0.769 cycles/day, corresponding to a 31.2 hours period, has no significance|
|Open with DEXTER|
Also presented in Table 5 are the ecliptic latitude and longitude of the object as seen from Earth at the dates of the observations, as well as the ) mag. The maximum difference in ) mag is 0.23, between the May 1997 (McBride et al. 1999) and the April 1999 data. From this table, no obvious relation is seen between the different magnitudes and the geometry of the observations.
Taking G values of 0.15 and 0.30 results in maximum differences between HV's (April 1999 and February 2000 data) of 0.34 and 0.31 respectively.
In the case of G = 0.30 McBride et al.'s (1999) value comes much closer to our February 2000 value, although the difference is still slightly larger than the error bars. They find G = 0.36 for J band, so it appears that G = 0.30 for the V band would be better than 0.15 and would not require the large phase redding they introduce.
|Period||lat.||lon.||)||HV (G = 0.15)||HV (G = 0.30)|
|May 1997||4.1||125||-3.6||7.02 0.04||6.64 0.04||6.70 0.04|
|(McBride et al. 1999)|
|Mar. 1999||2.8||139||-9.6||6.85 0.07||6.56 0.07||6.60 0.07|
|Apr. 1999||3.94||139||-9.6||6.79 0.07||6.42 0.07||6.48 0.07|
|Feb. 2000||1.4||150||-12.4||6.94 0.03||6.76 0.03||6.79 0.03|
We judge it unlikely that large calibration errors are present, and so the observed differences can argue for rotational variability, although other possibilities exist, such as an outburst of activity in 1999.
Applying the formula used by Jewitt & Kalas (1998):
Concerning the rotation rate of 10199 Chariklo (1997CU), Davies et al. (1999) have reported no clear variable periodic signal. Since they observed the object only a few times in three days, they argued that a rotation period might exist but would be very long or close to 24 hours. The lack of variability in their data set would also be consistent with a nearly spherical body or with the possibility that one of its poles was facing the Sun in 1997.
We tried to explore our data to find if any periodicity could be detected. We applied the Lomb method (Lomb 1976) as described in Press et al. (1992), for the 1999 runs and the February 2000 runs separately.
The Lomb periodogram of the 1999 R-band data shows a weak signal at 18.5 hours which is below the 50% confidence level. Its confidence level was overestimated in Peixinho et al. (1999), rating it as 70%. Besides, the 18.5 periodic signal is absent in the V lightcurve. We therefore conclude that the 18.5 hour period is very likely spurious.
The 2000 Calar Alto data set is better suited for period analysis because of the larger amount of data in R band. The highest peak at a frequency of 0.769 cycles/day, corresponding to a period of 31.2 hours, has a false period probability of 67.6%, hence it is not significant (Fig. 1).
Assuming the results for the variation in the absolute magnitude from 1999 to 2000 are real and due to a change in cross section, the minimum value of the ratio of the major to minor axis can be calculated from the amplitude of the variation. This is in the order of 1.33 (peak to peak amplitude of 0.31 mag). This probably indicates that the object has a slow rotation rate rather than being pole on.
At the time of our February 2000 observing run 1999UG was visible only in the first few hours of the astronomical night. Therefore, not enough observations could be obtained through the same filter for the determination of a possible rotation period. We focused on the determination of the colours for this object.
In order to avoid errors induced by possible rotational variability, the colours were determined using only consecutive images, or almost consecutive, on the corresponding filters, with the exception of B-V.
Table 6 lists the observations for 1999UG,
values for the absolute magnitudes.
|UT date||Julian date||R (AU)||(AU)||Exp. (s)||Filter||Mag||Error|
|2000 Feb. 02.81945||2451577.31945||7.66||7.78||7.28||300||R||19.370||0.053|
|2000 Feb. 02.82188||2451577.32188||500||V||19.967||0.060|
|2000 Feb. 03.81248||2451578.31248||7.66||7.79||7.25||500||V||20.062||0.048|
|2000 Feb. 03.81898||2451578.31898||500||R||19.476||0.044|
|2000 Feb. 03.85439||2451578.35439||900||V||20.082||0.041|
|2000 Feb. 03.86737||2451578.36737||900||R||19.570||0.038|
|2000 Feb. 04.81285||2451579.31285||7.66||7.80||7.23||500||I||18.582||0.055|
|2000 Feb. 04.83004||2451579.33004||500||R||19.428||0.059|
|2000 Feb. 04.84458||2451579.34458||500||I||18.943||0.068|
|2000 Feb. 04.85185||2451579.35185||500||R||19.538||0.063|
|2000 Feb. 04.87417||2451579.37417||500||B||21.085||0.14|
|2000 Feb. 04.88096||2451579.38096||500||R||19.396||0.066|
|2000 Feb. 04.89412||2451579.39412||500||B||20.750||0.13|
|2000 Feb. 04.90148||2451579.40148||500||R||19.591||0.078|
|1999UG5||0.88 0.18||0.60 0.08||0.72 0.13||this paper|
|5415 Pholus||-||0.75 0.04||0.76 0.06||Luu & Jewitt (1996)|
|1.19 0.1||0.78 0.04||-||Romanishin et al. (1997)|
|0.75 0.02||0.84 0.03||Davies et al. (1998)|
|7066 Nessus||0.88 0.07||0.72 0.05||-||Luu & Jewitt (1996)|
|-||0.80 0.07||0.69 0.12||Davies et al. (1998)|
|1995 GO||0.75 0.04||0.47 0.04||-||Romanishin et al. (1997)|
|0.78 0.05||0.73 0.04||-||Brown & Luu (1997)|
|-||0.41 0.02||0.55 0.04||Davies et al. (1998)|
|1997 CU26||-||0.46 0.02||0.55 0.03||Davies et al. (1998)|
|-||0.48 0.02||0.56 0.03||Tegler & Romanishin (1998)|
|-||0.46 0.02||-||McBride et al. (1999)|
|0.50 0.09||-||March 1999, this paper|
|0.47 0.09||-||April 1999, this paper|
|0.49 0.04||-||February 2000, this paper|
|1995 DW2||-||0.41 0.10||0.46 0.18||Davies et al. (1998)|
|2060 Chiron||0.67 0.06||0.34 0.03||-||Luu & Jewitt (1996)|
|Sun||0.67||0.36||0.53||Hartmann et al. (1990)|
From V and R filters, disregarding any rotational variations, average values for and are obtained, and assuming a phase parameter value G = 0.15, this implies and .
The diameter of 1999UG as estimated from Eq. (1), assuming the same value for pV as for Chariklo, is km.
The authors acknowledge the use of the National Astronomical Observatory 1.52 meter telescope at the Calar Alto Observatory, Spain. This work is partially based on observations made with the 1.5 meter telescope of the Observatorio de Sierra Nevada Observatory, Granada, Spain, which is operated by the Instituto de Astrofísica de Andalucía (CSIC).
We also would like to thank Dr. Luisa Almeida, from CAAUL, for her valued help with the subtleties of IRAF's PHOTCAL routines and her very careful method for standard calibration.