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Colors of Centaurs

Authors: S. C. Tegler, J. M. Bauer, W. Romanishin, and N. Peixinho

Ref.: The Solar System Beyond Neptune. Edited by M. A. Barucci; H. Boehnhardt; D. P. Cruikshank; A. Morbidelli; With Assistance from Renee Dotson. The University of Arizona Press., 105-114 (2008)

Abstract: Minor planets on outer planet-crossing orbits, called Centaur objects, are important members of the solar system in that they dynamically link Kuiper belt objects to Jupiter-family comets. In addition, perhaps 6% of near-Earth objects have histories as Centaur objects. The total mass of Centaurs (10^(–4) M_Earth) is significant, about one-tenth of the mass of the asteroid belt. Centaur objects exhibit a physical property not seen among any other objects in the solar system, their B–R colors divide into two distinct populations: a gray and a red population. Application of the dip test to B–R colors in the literature indicates there is a 99.5% probability that Centaurs exhibit a bimodal color distribution. Although there are hints that gray and red Centaurs exhibit different orbital elements, application of the Wilcoxon rank sum test finds no statistically significant difference between the orbital elements of the two color groups. On the other hand, gray and red Centaurs exhibit a statistically significant difference in albedo, with the gray Centaurs having a lower median albedo than the red Centaurs. Further observational and dynamical work is necessary to determine whether the two color populations are the result of (1) evolutionary processes such as radiation-reddening, collisions, and sublimation or (2) a primordial, temperature-induced, composition gradient.