Colors of Trans-Neptunian Contact Binaries

Abstract

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 colors are common in this sub-population and imply 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 inclinations of the contact binaries in resonances, these contact binaries are possibly escaped dynamically cold classicals that are now trapped in resonances. Moderately red surfaces are common in diverse sub-populations of the Kuiper Belt, thus pinpointing their origin is difficult though they are most likely captured objects that 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 find hints of 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.