Col-OSSOS: The Two Types of Kuiper Belt Surfaces

The Colors of the Outer Solar System Origins Survey (Col-OSSOS) has gathered a high-quality, near-simultaneous, and brightness-complete sample of ( g − r ) and ( r − J ) colors for 102 Kuiper Belt objects (KBOs) with ( u − g ) and ( r − z ) gathered for some. We present the current state of the survey and data analysis. Recognizing that the optical colors of most icy bodies broadly follow the reddening curve, we present a new projection of the optical−near-IR (NIR) colors, which rectifies the main nonlinear features in the optical−NIR along the ordinates. We find evidence for a bifurcation in the projected colors that presents itself as a diagonal empty region in the optical−NIR. A reanalysis of past color surveys reveals the same bifurcation. We interpret this as evidence for two separate surface classes: the BrightIR class spans the full range of optical colors and broadly follows the reddening curve, while the FaintIR objects are limited in optical color and are less bright in the NIR than the BrightIR objects. We present a two-class model. Objects in each class consist of a mix of separate blue and red materials and span a broad range in color. Spectra are modeled as linear optical and NIR spectra with different slopes that intersect at some transition wavelength. The underlying spectral properties of the two classes reproduce the main observed structures in the UV−optical−NIR color space (0.4 μ m ≲ λ ≲ 1.4 μ m), including the bifurcation observed in the Col-OSSOS and H/WTSOSS data sets, including the tendency for cold classical KBOs to have lower ( r − z ) colors than excited objects, and the well-known bimodal optical color distribution.