Time-resolved photometry of Kuiper belt objects: Rotations, shapes, and phase functions

We present a systematic investigation of the rotational light curves of trans-Neptunian objects based on extensive optical data from Mauna Kea. Four of 13 objects (corresponding to 31%) in our sample [(33128) 1998 BU48, 2000 GN(171), (20000) Varuna, and (40314) 1999 KR16] were found to exhibit light curves with peak-to-peak range greater than or equal to0.15 mag. In a larger sample obtained by combining our data with reliably determined light curves from the literature, seven of 22 objects (32%) display significant (greater than or equal to0.15 mag range) light curves. About 23% of the sampled objects have light-curve ranges greater than or equal to0.4 mag. Curiously, the objects are very large (greater than or similar to250 km diameter, assuming an albedo of 0.04) and, in the absence of rotation, should be nearly spherical as a result of self-compression. We propose that the large-amplitude, short-period objects are rotationally distorted, low-density rubble piles. Statistically, the trans-Neptunian objects are less spherical than their main-belt asteroid counterparts, indicating a higher specific angular momentum, perhaps resulting from the formation epoch. In addition to the rotational light curves, we measured phase darkening for seven Kuiper belt objects in the 0degrees to 2degrees phase-angle range. Unlike Pluto, the measured values show steep slopes and moderate opposition surge, indicating backscatter from low-albedo porous surface materials.