Unveiling the excess of slow rotators in the small Main Belt Asteroids

The Small Main Belt Asteroids population displays a significant excess of slow rotators. With a sophisticated Monte Carlo model, considering the effects of YORP and collisions, we have been able to reproduce the slow rotators excess without introducing any ad-hoc latency time in the large rotation period states. A deeper analysis of these results shows how this excess is due to a non-linear interaction between the YORP effect and the collisions. No significant excess can be found by simulating the two effects alone. On the other hand, with the effects acting at the same time, whenever the rotation rate of an object is slowed down by YORP, the amount of angular momentum change imparted by the collisions against larger projectile becomes comparable to the total angular momentum of the body. This keeps the asteroid "lingering" in a slow rotation state for a comparatively large time, leading to the observed excess. This effect happens mainly for the larger objects in our sample (around 5 km of diameters). For smaller objects the YORP cycles are so short and the perturbation induced by collision so frequent and comparatively large, that the body is rapidly jumping from the two extreme states of slow and fast rotation and no accumulation is observed. Further investigations are ongoing and will be reported at the meeting.