Abstract
A recent examination of K2 lightcurves indicates that similar to 15% of Jupiter Trojans have very slow rotation (spin periods P-s > 100 hr). Here we consider the possibility that these bodies formed as equal-size binaries in the massive outer disk at similar to 20-30 au.
Prior to their implantation as Jupiter Trojans, tight binaries tidally evolved toward a synchronous state with P-s similar to P-b, where P-b is the binary orbit period. They may have been subsequently dissociated by impacts and planetary encounters with at least one binary component retaining its slow rotation.
Surviving binaries on Trojan orbits would continue to evolve by tides and spin-changing impacts over 4.5 Gyr. To explain the observed fraction of slow rotators, we find that at least similar to 15%-20% of outer disk bodies with diameters 15 D a(b)/R less than or similar to 30, where a(b) is the binary semimajor axis and R = D/2.
The mechanism proposed here could also explain very slow rotators found in other small-body populations.