Very Slow Rotators from Tidally Synchronized Binaries

A recent examination of K2 lightcurves indicates that ∼15% of Jupiter Trojans have very slow rotation (spin periods Ps > 100 hr). Here we consider the possibility that these bodies formed as equal-size binaries in the massive outer disk at ∼20–30 au. Prior to their implantation as Jupiter Trojans, tight binaries tidally evolved toward a synchronous state with Ps ∼ Pb, where Pb 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 ∼15%–20% of outer disk bodies with diameters 15 < D < 50 km would have to form as equal-size binaries with 12 ≲ ab/R ≲ 30, where ab 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.