The period of rotation, shape, density, and homogeneous surface color of the Centaur 5145 Pholus

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 h for Pholus, consistent with periods of 9.9825±0.004 and 9.9823±0.0012 h by Buie and Bus (1992, Icarus 100, 288-294) and Farnham (2001, Icarus 152, 238-245). We find a lightcurve peak-to-peak amplitude of 0.60 mag, significantly larger than peak-to-peak amplitude determinations of 0.15 and 0.39 mag by Buie and Bus and Farnham. We use the three observed amplitudes and an amplitude-aspect model to derive four possible rotational pole positions 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 310×160×150 km. If we assume Pholus is a strengthless rubble-pile and its non-spherical shape is due to rotational distortion, our axial ratios and period measurements indicate Pholus has a density of 0.5 g cm⁻³, suggestive of an ice-rich, porous interior. 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 mag (i.e., much smaller than the 1.0<B−R<2.0 range seen among the Centaur and Kuiper belt object 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. Observations of the Kuiper belt object 2004 DW (90482) over a time interval of seven hours show no color variation Our 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 Kuiper belt object populations.