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A revised rotation period for asteroid (2903) Zhuhai.

The BAA Handbook gives a list, prepared by Richard Miles, of favourable asteroid observing opportunities throughout the year. The asteroids in the list have an opposition magnitude brighter than 15.0 and have very uncertain or unknown rotation periods. For the uncertain rotation periods, the period and magnitude amplitude are given with the source of this information being taken from the Minor Planet Observer website. (1)

As a first attempt at measuring the rotation period of an asteroid I wanted to select an object with a rotation period of just a few hours and reasonably large magnitude amplitude. The rotation period was chosen so that a complete rotation was shorter than one night and the large magnitude amplitude was selected so that the asteroid rotation could be easily detected.

As none of the asteroids listed in the Handbook around 2010 November included any rotation period information, a more extensive list from the MPO site (1) was searched and asteroid (2903) Zhuhai selected. This had an opposition date on 2010 November 28 at magnitude 15.0 with a reported period of 6.152 hours and amplitude of 0.53 mag. Its uncertainity flag (U) of 2 indicates that the rotation period error could be up to 30% or the reported period could be ambiguous.

[FIGURE 1 OMITTED]

An initial set of 5 observations was acquired with the Sierra Stars Observatory Network (SSON) (2) remotely operated 0.61m f/10 Cassegrain telescope (IAU code G68) on 2010 November 03 with exposure times of 30 seconds each and time separation of 15 minutes (i.e. a total elapsed time of 1 hour). These observations showed that the V = 15.8 mag asteroid exhibited a 0.5 mag variation and that the period was longer than the hour time interval between the first and last image. A second set of 8 images over a total of 3.5 hours was acquired using the SSON University of Iowa's OMI 0.37m f/14 Rigel Telescope (857) on 2010 November 10 using 60 second exposures and a clear filter. Now more of the rotation period was shown but not a complete cycle. Two further sets of observations with the Rigel telescope were obtained on 2010 November 13 (Figure 1) and December 08, with each set comprising 18 images taken over a period of almost 6 hours (20 minutes between images). The exposure times for these two longer time series were 60 seconds and 50 seconds and again a clear filter was used. Both of these longer time series showed a complete rotation period.

[FIGURE 2 OMITTED]

Initial lightcurve analysis was performed using Astrometrica (3) by manually extracting the magnitude estimation for each image using the CMC-14 catalogue. This enabled a preliminary lightcurve to be plotted and superimposing a sine curve gave an approximate rotation period of just over 5 hours. A more detailed analysis was performed using the MPO Canopus v10 software (4) which enables accurate photometry to be obtained using comparison stars with a similar colour to that of the Sun, and then the combining of several nights observations to derive a composite lightcurve. Figure 2 shows the derived lightcurve from 3 nights observations relative to the observations from November 13 when the asteroid phase angle was 7.2[degrees] (the V magnitude band was chosen for the derived magnitudes). The 5 observations from November 03 were not used due to poorer photometry. The MPO lightcurve analysis gives a good fit to the 44 measurements used in the analysis and a rotation period of 5.264 hours (synodic) and amplitude of 0.41 magnitudes. The corresponding period spectrum is shown in Figure 3; note that the original reported period of 6.152 hours does not appear as a possible period in the spectrum.

[FIGURE 3 OMITTED]

In summary the derived 5.264 hour rotation period is shorter and the rotation amplitude of 0.41 magnitudes is slightly smaller than the rotation information provided on the Minor Planet Observer website. (1)

The support of the BAA Robotic Telescope Project for the SSON observations is acknowledged.

Address: 6 Chelmerton Avenue, Great Baddow, Chelmsford, Essex, CM2 9RE [peter@petermeadows.com]

References

(1) Minor Planet Observer: http://minorplanetobserver.com/

(2) Sierra Stars Observatory Network: http://www.sierrastars.com/

(3) Astrometrica: http://www.astrometrica.at/

(4) MPO Canopus: http://www.minorplanetobserver.com/ MPOSoftware/MPOCanopus.htm

Received 2010 December 20; accepted 2011 March 30
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Title Annotation:Short paper
Author:Meadows, Peter
Publication:Journal of the British Astronomical Association
Geographic Code:4EUUK
Date:Apr 1, 2012
Words:733
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