The rarity of very red TNOs in the scattered disk

Abstract

We investigate the origins of the photometrically Very Red and Less Red Trans-Neptunian Objects. We first reanalyse the dataset of Marsset et al. 2019 and find that, in addition to the known color-inclination correlation in hot TNOs, a similar trend exists for color-eccentricity. We show that VR TNOs are sharply constrained to eccentricities < 0.42 and inclinations < 21 deg, leading to a paucity of VR scattered disk and distant MMR objects. We then interpret these findings using N-body simulations accounting for Neptune's outward migration into a massless particles disk, and find that these observations are best reproduced with a LR-to-VR color transition line between 38 and 42 AU in the primordial disk, separating the objects' formation locations. For an initial surface density profile $\Sigma \propto 1/r^2$, a color transition around 38 AU is needed to explain the high abundance of VR plutinos but creates too many VR scattered disk objects, while a transition line around 42 AU seems to better reproduces the scattered disk colors but creates virtually no VR plutinos. Our simulations furthermore show that the rarity of VR particles at high eccentricity is possibly due to the absence of sweeping higher order MMRs, and secular resonances, beyond 42 AU. Inspecting individual populations, we show that the majority of VR SDOs originate as objects trapped in Neptune's second and third order MMRs. These then evolve due to diffusion, scattering, Kozai-Lidov cycles, and secular resonances into their current orbits. Future unbiased color surveys are crucial to better constrain the TNOs dynamical origins.