# Colors of Trans-Neptunian Contact Binaries

## TL;DR

This study analyzes the surface colors of contact binaries in the Kuiper belt to infer their origins and formation regions, revealing color patterns linked to their dynamical history and potential in-situ formation.

## Contribution

It provides new color measurements of Kuiper belt contact binaries and explores their implications for understanding their formation and dynamical evolution.

## Key findings

- Cold Classical contact binaries are very-red/ultra-red, indicating in-situ formation.
- Resonant contact binaries show a range of colors, suggesting diverse origins.
- An anti-correlation between inclination and color was observed in the population.

## Abstract

The g'r'i' colors of seven likely and potential contact binaries in the Kuiper belt were acquired with the Magellan-Baade telescope and combined with colors from the literature to understand contact binary surfaces. The likely and potential contact binaries discovered in the dynamically Cold Classical population display very-red/ultra-red colors. Such a color is common in this sub-population and infers that the Cold Classical contact binaries were formed in-situ. The likely contact binaries found in several mean motion resonances with Neptune have colors from moderately to ultra-red suggesting different formation regions. Among the nine contact binaries discovered in resonances, five have very-red/ultra-red colors and four have moderately-red surfaces. Based on the very-red/ultra-red colors and low to moderate inclination of the contact binaries in resonances, these contact binaries are maybe escaped dynamically Cold Classicals that are now trapped in resonances. Moderately-red surfaces are common in diverse sub-populations of the Kuiper belt and thus pinpointing their origin is difficult though they are most likely captured objects formed in the giant planet area. Finally, for the contact binary population we report an anti-correlation between inclination and g'-r', as noticed in the rest of this belt. We also have hints for trends between eccentricity, perihelion distance, rotational period and g'-r', but as we are still dealing with a limited sample, additional data are required to confirm them.