Time-resolved Photometry of Kuiper Belt Objects: Rotations, Shapes, and Phase Functions

Abstract

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 GN171, (20000) Varuna, and (40314) 1999 KR16] were found to exhibit light curves with peak-to-peak range � 0.15 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 (� 0.15 mag range) light curves. About 23% of the sampled objects have light-curve ranges � 0.4 mag. Curiously, the objects are very large (e250 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 0 � to 2 � phase-angle range. Unlike Pluto, the measured values show steep slopes and moderate opposition surge, indicating backscatter from low-albedo porous surface materials.