Col-OSSOS: The Two Types of Kuiper Belt Surfaces

Abstract

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 μm ≲ λ ≲ 1.4 μm), 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.

Col-OSSOS: Evidence for a Compositional Gradient Inherited from the Protoplanetary Disk?

“…Previous works have shown that outer solar system color classes correlate with orbital distributions (e.g., Tegler & Romanishin 2000;Peixinho et al 2008;Tegler et al 2016;Marsset et al 2019;Ali-Dib et al 2021), which was interpreted as evidence for the existence of multiple classes of objects with distinct origins (Marsset et al 2019). This dichotomy also exists in the FaintIR/BrightIR taxonomy (Fraser et al 2023), which is explained by the fact that the BrightIR/FaintIR classification overlaps with the previously used less red/very red classification. Motivated by these earlier results, we investigate here the existence of additional correlations within the new FaintIR/BrightIR taxonomic scheme, using a combined data set of TNO color measurements from Col-OSSOS and previous observations (Section 2).…”

“…The observing conditions of the Col-OSSOS data set analyzed in this work are provided in Fraser et al (2023). In order to increase our sample size and strengthen any existing correlation in our data set, we combined this data set with additional, previously published measurements of similar photometric quality acquired with the Hubble Space Telescope (HST) as part of the Hubble/WFC3 Test of Surfaces in the Outer Solar System (H/WTSOSS; Fraser & Brown 2012;Fraser et al 2015).…”

“…Objects were then classified into the BrightIR and FaintIR classes using the color projection system of Fraser et al (2023), i.e., using their location distance on the parallel (PC 1 ) and perpendicular (PC 2 ) vectors along the so-called reddening curve. The reddening curve is the curve of equal spectral slope in all colors, meaning that, in a given color-color plot, an object located below the reddening curve has a concave (frown) spectrum, whereas an object located above is spectrally convex (smile).…”

“…The reddening curve is the curve of equal spectral slope in all colors, meaning that, in a given color-color plot, an object located below the reddening curve has a concave (frown) spectrum, whereas an object located above is spectrally convex (smile). The reprojection of the color-color plot in PC 1 versus PC 2 parameter space proposed by Fraser et al (2023) is such that the reddening curve becomes a flat line along the abscissa. A visual demonstration is provided in Appendix C of their paper.…”

“…Based on observation of 102 individual TNOs, we reported evidence for only two composition classes in the bulk population of the Kuiper Belt (excluding large TNOs and members of the waterrich Haumea family), thereby pointing toward limited radial compositional stratification in the early outer solar system. In the new taxonomy proposed by Fraser et al (2023), TNOs are assigned either to the BrightIR or FaintIR classes based on their reflectivity in the NIR. Specifically, BrightIR objects have large albedos in NIR wavelengths with respect to their values in the optical region, while FaintIR objects have low albedos in the NIR compared to BrightIR objects with similar optical colors.…”