The Color Distribution in the Edgeworth-Kuiper Belt

In 1997 we began the Meudon Multicolor Survey of Outer Solar System Objects with the aim of collecting a large and homogeneous set of color data for trans-Neptunian objects (TNOs) and Centaurs. Here we present our latest B-V, V-R, and R-I color measurements obtained with the CFH12K mosaic camera of the 3.6 m Canada-France-Hawaii Telescope. With the colors of 30 objects reported in this work, we have a combined sample of 52 B-R color measurements for eight Centaurs, 22 classical TNOs, 13 Plutinos, eight scattered objects, and one object of unidentified dynamical class. This is the largest single and homogeneous data set published to date, and it is large enough to search for compositional structures, interrelations between dynamical classes of objects, and correlations with physical and orbital parameters. The color-color diagrams show that all the classes of objects share the same wide color diversity. No significant correlations are seen for the whole population of TNOs and Centaurs, or for individual subpopulations, except for the classical objects. Indeed, we found a significant and strong correlation of the colors of classical TNOs with inclination, eccentricity, and perihelion, but nothing with semimajor axis and absolute magnitude. Most of these results are common to previous works and do not seem to be due to sampling bias. Moreover, a strong correlation with mean excitation velocity [VK(e2 + i2)1/2] points toward a space weathering or impact origin for the color diversity. However, thorough modeling of the collisional/dynamical environment in the Edgeworth-Kuiper belt needs to be done in order to confirm this scenario. We found also that the classical TNOs are made up of a superposition of two distinct populations: the dynamically cold classical TNOs (red colors, low i, small sizes) and the dynamically hot classical TNOs (diverse colors, moderate and high i, larger sizes). Furthermore, the latter population displays a strong correlation between color and mean excitation velocity. The dynamically cold classical TNOs may be primordial, while the dynamically hot classical TNOs, whose surface colors may be the result of space weathering and/or impact processes, have possibly been injected from the inner regions of the disk. Our specific observation strategy to repeat color measurements with no rotation artifacts have permitted us to highlight a few objects suspected to have true compositional and/or texture variation on their surfaces. These TNOs are 1998 HK151, 1999 DF9, 1999 OY3, 2000 GP183, 2000 OK67, and 2001 KA77, and they should be prime targets for further observations in order to study and confirm the color variation with the rotation. Finally, our survey has also highlighted some peculiar objects, such as 1998 SN165, whose colors and dynamical properties put this object in a new dynamical class distinct from the classical TNOs, its previously assigned dynamical class.