Col-OSSOS: The BrightIR and FaintIR Taxonomy For Kuiper Belt Object

After 7 years of observations, the Colours of the Outer Solar System Origins Survey has gathered near simultaneous UV-optical-NIR, of nearly 100 Kuiper Belt Objects (KBOs). This has allowed the completion of one of the main goals of the survey: determine the number of surface types of small KBOs. The full spectrophotometric color distribution, which includes observations in the u, g, r, z, and J filters, presents some expected properties, including the long known bimodal optical color distribution, and correlated optical-NIR colors. The color distribution also shows some unexpected features such as a region in the (g-r) and (r-J) space that parallels the reddening curve, and is totally devoid of objects. We present a non-linear projection of the color data, which we call the reddening curve projection, that utilizes basis vectors that are the instantaneous tangential and perpendicular vectors to the reddening curve. This projection rectifies the non-linear features that appear in the color space allowing for greatly simplified statistical analysis. The results of our analysis reveal the presence (>3-sigma significance) of only two classes of object that significantly overlap in the optical color space. The BrightIR and FaintIR classes can be broadly described as those KBOs that fall along the reddening curve (BrightIR), and those that fall significantly below it in NIR bands (FaintIR). Both classes are relatively agnostic of orbital class, with FaintIR and BrightIR objects being found throughout all observed dynamical classes, including the cold classicals. These two classes – and only these two classes – appear in the reddening curve projections of multiple past optical-NIR datasets, including the Minor Bodies in the Outer Solar System (MBOSS) and Hubble/WFC3 Test of Surfaces in the Outer Solar System (H/WTSOSS) datasets, but vanish in datasets that do not include NIR photometry (>0.9 microns). Importantly, with a very simple toy spectral model containing only these two classes, we can fully account for virtually all features of the color distribution of KBOs, including, but not limited to, the bimodal optical colors, the higher albedos of cold classicals, and the correlated optical-NIR colors of past surveys.