The Bimodal Colors of Centaurs and Small Kuiper Belt Objects

TL;DR

This study reveals that the bimodal surface colors of Centaurs and small Kuiper Belt Objects are size-dependent phenomena, with larger KBOs showing a different bimodal pattern influenced by collisional families, challenging previous formation and surface evolution theories.

Contribution

It demonstrates that the bimodal color distribution is related to object size and includes new data for small objects, providing insights into surface composition and evolution.

Findings

Bimodal color distribution is size-related among Centaurs and small KBOs.

Large KBOs also show bimodal colors, influenced by collisional families.

Colors form a peculiar N shape in B-R versus H(α) space.

Abstract

Ever since the very first photometric studies of Centaurs and Kuiper Belt Objects (KBOs) their visible color distribution has been controversial. That controversy gave rise to a prolific debate on the origin of the surface colors of these distant icy objects of the Solar System. Two different views attempt 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. In this letter, we readdress this issue using a carefully compiled set of B-R colors and H({\alpha}) magnitudes (as proxy for size) for 253 objects, including data for 10 new small objects. We find that the bimodal behavior seen among Centaurs is a size related phenomenon, common to both Centaurs and small KBOs, i.e. independent of dynamical classification. Further, we find that large KBOs also exhibit a bimodal behavior of surface colors, albeit distinct from the small objects and strongly dependent on the `Haumea collisional family' objects. When plotted in B-R, H({\alpha}) space, the colors of Centaurs and KBOs display a peculiar N shape.