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S. C. Tegler (N. Arizona U.), W. Romanishin (U. Oklahoma), G. J. Consolmagno (Vatican Obs.), J. Rall (N. Arizona U.), M. Nelson (Steward Obs.)
We present optical photometry of the Centaur 5145 Pholus during 2003 May and 2004 April using the facility CCD camera on the 1.8-m Vatican Advanced Technology Telescope on Mt. Graham, Arizona. We derive a double-peaked lightcurve and a rotation period of 9.980 ± 0.002 hr for Pholus, consistent with periods of 9.9825 ± 0.004 and 9.9823 ± 0.0012 hr by Buie and Bus (1992) and Farnham (2001). We find a lightcurve amplitude of 0.60 magnitude, significantly larger than amplitude determinations of 0.15 magnitude and 0.39 magnitude by Buie and Bus and Farnham.
We use the three observed amplitudes and an amplitude-aspect model to derive two possible rotational pole positions, lambda = 155 deg, beta = +5 deg and lambda = 335 deg, beta = -5 deg, as well as axial ratios of a/b = 1.9 and c/b = 0.9. If we assume an albedo of 0.04, we find Pholus has dimensions of 307 x 161 x 145 km.
By combining B-Band and R-Band lightcurves, we find B-R = 1.94 ± 0.01 and any B-R color variation over the surface of Pholus must be smaller than 0.06 magnitude (i.e. much smaller than the 1.0 < B-R < 2.0 range seen among the Centaur and KBO populations). By combining our V-R measurements with values in the literature, we find no evidence for any color variegation between the northern and southern hemispheres of Pholus. Our Pholus observations add to the growing body of evidence that individual Centaurs and KBOs exhibit homogeneous surface colors and hence gray impact craters on radiation reddened crusts are probably not responsible for the surprising range of colors seen among the Centaur and KBO populations.
We thank the NASA Planetary Astronomy Program and the Vatican Observatory for support of this research.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.