Rotation and Color Studies of Centaurs, KBOs and Comets

We present results from an ongoing study of the rotational and color properties of Centaurs, Kuiper belt objects and distant comets. Measurements of the rotation states and the shapes of these objects can lead to constraints on their density and composition, while colors and color variations as a function of rotational phase lead to information about their surface properties. Our most interesting object to date is 20000 Varuna (2000 WR₁₀₆), a classical KBO discovered on 28 November 2000. We obtained CCD images of Varuna in January, March and September 2001, and performed photometry to measure variations in the lightcurve as a function of time. Our data show periodic variations with a peak-to-peak amplitude of 0.5 mag. Because Varuna is very large (900 km diameter; Jewitt et al. 2001), we may assume that it is spherical and the lightcurve variations are produced by albedo features on its surface, with one hemisphere reflecting 60% more light than the other. If this interpretation is correct, then the lightcurve is single-peaked with a period of 3.17 hr, (though secondary periods of 2.79 and 3.66 hr cannot be ruled out). With this period, Varuna is very close to the critical rotation rate for a body with a density around 1 g cm^-3 (P_crit=(3π /Gρ )^1/2=3.3 hr), indicating that it either has a density greater than 1 or it has internal strength. An alternative case is that the lightcurve is double-peaked, with a 6.34 hr period. This is also interesting because either it is the largest known elongated body in the solar system, with a minimum axial ratio a/b=1.6, or it is a binary KBO. Among the other objects to be addressed is 2001 PT₁₃, a Centaur with an 8.4 hr period. The 0.15 mag amplitude in its lightcurve indicates that this object has a minimum axial ratio a/b=1.2. Results for other objects will also be presented, including those for which no rotational variation was found. This work was funded by NASA.