Col-OSSOS: Investigating the Origins of Different Surfaces in the Primordial Kuiper Belt

TL;DR

Col-OSSOS colours of TNOs reveal a bimodal optical distribution linked to a bi- or multi-component primordial disk. The authors couple Col-OSSOS data with two Neptune-migration scenarios (condensed and extended disks) to locate a surface-colour transition line, testing Red/Neutral and BrightIR/FaintIR classifications via binomial likelihoods and dynamical cuts. They employ the OSSOS survey simulator and a CFEPS L7 intrinsic-population model to estimate intrinsic colour fractions, finding that inner-neutral/outer-red or inner-BrightIR/outer-FaintIR disk configurations best explain the observations, with transition positions around 27–32 au depending on the model. The work provides constraints on early solar-system structure, informs interpretations of Neptune Trojans and blue binaries, and demonstrates a framework ready for upcoming LSST data to further refine the primordial-color distribution in the Kuiper belt.

Abstract

The Colours of the Outer Solar System Origins Survey (Col-OSSOS) measured the optical/NIR colours of a brightness-complete sample of Trans-Neptunian Objects (TNOs). Like previous surveys, this one found a bimodal colour distribution in TNOs, categorised as red and very red. Additionally, this survey proposed an alternative surface classification scheme: FaintIR and BrightIR. Cold classical TNOs mostly have very red or FaintIR surfaces, while dynamically excited TNOs show a mixture of surfaces. This likely indicates that formation locations and proximity to the Sun influenced surface characteristics and color changes. Our study combines the data from Col-OSSOS with two dynamical models describing the formation of the Kuiper belt during Neptune's migration. We investigate the proposed surface-colour changing line and explore the distribution of different surfaces within the primordial disk. By comparing radial colour transitions across various scenarios, we explore the origins of surface characteristics and their implications within the context of BrightIR and FaintIR classifications. Moreover, we extend our analysis to examine the distribution of these surface classes within the present-day Kuiper Belt, providing insights into the configuration of the early solar system's planetesimal disk prior to giant planet migration. We find that the most likely primordial disk compositions are inner neutral / outer red (with transition $30.0^{+1.1}_{-1.2}$ au), or inner BrightIR / outer FaintIR (with transition $31.5^{+1.1}_{-1.2}$ au).

Figures (8)

• Figure 1: Optical and NIR colours of the full Col-OSSOS dataset with 102 TNOs. The shapes and colours of each point show the dynamical classifications, as described in the legend. The solar colours, with $g-r=0.45$ and $r-J=0.97$, are shown by the yellow star. The red dashed line shows the solar reddening line, while the blue dot-dashed line shows the $PC^2$$= -$0.13 split between BrightIR and FaintIR surfaces. The vertical green dotted line shows the well known bifurcation in the optical colour distribution of the dynamically excited populations at $(g-r) = 0.75$. For TNOs with multiple surface colour observations in the survey, the colour points are joined with dashed lines. Some of these TNOs only had repeat measurements in the optical $(g-r)$, in which case the same $(r-J)$ colour is used for both and the missing NIR colour indicated by a lack of error bars.
• Figure 2: A schematic of the disk in the model of nesvorny_neptunes_2016, where the massive planetesimal disk spans from $\sim$24 au to $\sim$30 au. Although the cold classical TNOs are not included in the simulation, a representation of their outer solar system position (beyond $\sim$50 au) position is shown. In this dynamical model there is no contribution material between the edge of the massive planetesimal disk at $\sim$30 au and the cold classical TNOs.
• Figure 3: A schematic of the disk in the model of 2020AJ....160...46N, where a massive planetesimal disk spans from $\sim$24 au to $\sim$30 au and beyond this is a lower mass disk extension that spans to near the cold classical region. Although the cold classical TNOs are not included in the simulation, a representation of their outer solar system position (beyond $\sim$50 au) position is shown.
• Figure 4: The projection values $PC^1$ and $PC^2$ of the $(g - r)$ and $(r - J)$ colours of Col-OSSOS in the right-hand panel. In this projection, the reddening curve would be a horizontal line with $PC^2 = 0$. The blue dot-dashed line is at a value of $PC^2$$= -$0.13, the value the split between the FaintIR and BrightIR classes. Similarly to Figure \ref{['fig:colossosallcolours']}, TNOs with multiple measurements have dashed lines between their points. The left-hand panel shows a histogram of the distribution in PC$^2$, illustrating the bimodal distribution from which the BrightIR and FaintIR classes are defined.
• Figure 5: Col-OSSOS photometry of observed non-resonant, non-Centaur, dynamically excited objects for comparison with the two dynamical models. In the upper plot the red/neutral colour split is placed at a $(g - r)$ magnitude of 0.75. The star shows solar colours. In the lower plot the split between the BrightIR and FaintIR classes is shown by the red dashed horizontal line at $PC^{2}_{grJ}$ of -0.13.