The bimodal colors of Centaurs and small Kuiper belt objects^⋆,⋆⋆

¹ Center for Geophysics of the University of Coimbra, Av. Dr. Dias da Silva, 3000-134 Coimbra, Portugal
e-mail: peixinho@mat.uc.pt; gafeira@mat.uc.pt
² Astronomical Observatory of the University of Coimbra, Almas de Freire, 3040-004 Coimbra, Portugal
³ Laboratoire d’Astrophysique de Marseille, Université d’Aix-Marseille, CNRS, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France
e-mail: Audrey.Delsanti@oamp.fr; Audrey.Delsanti@obspm.fr
⁴ Observatoire de Paris, Site de Meudon, 5 place Jules Janssen, 92190 Meudon, France
⁵ UCLA, Department of Earth and Space Sciences, 595 Charles E. Young Drive East, Los Angeles CA 90095, USA
e-mail: aguilbert@ucla.edu
⁶ Queen’s University Belfast, Astrophysics Research Centre, Belfast BT7 1NN, UK
e-mail: p.lacerda@qub.ac.uk

Received: 16 February 2012
Accepted: 14 June 2012

Abstract

Ever since the very first photometric studies of Centaurs and Kuiper belt objects (KBOs) their visible color distribution has been controversial. This controversy has triggered to a prolific debate on the origin of the surface colors of these distant icy objects of the solar system. Two scenarios have been proposed to interpret and explain the large variability of colors, hence surface composition. Are the colors mainly primordial and directly related to the formation region, or are they the result of surface evolution processes? To date, no mechanism has been found that successfully explains why Centaurs, which are escapees from the Kuiper belt, exhibit two distinct color groups, whereas KBOs do not. We readdress this issue using a carefully compiled set of B − R colors and H_R(α) magnitudes (as proxy for size) for 253 objects, including data for 10 new small objects. We find that the bimodal color distribution of Centaurs is a size-related phenomenon, common to both Centaurs and small KBOs, i.e. independent of dynamical classification. Furthermore, we find that large KBOs also have a bimodal distribution of surface colors, albeit distinct from the small objects and strongly dependent on the “Haumea collisional family” objects. When plotted in B − R, H_R(α) space, the colors of Centaurs and KBOs display a peculiar 𝒩 shape.