An extended parameter space study of the effect of cohesion in gravitational aggregates through spin-up simulations

Asteroids are subject to the YORP effect, a slow change in spin caused by reflection and reemission of solar radiation. There is an upper limit to how fast an asteroid can spin before some form of failure takes place, depending on the strength of the asteroid. The maximum spin rate for a particle to remain at the equator of a rigid body depends only on the overall shape of the asteroid and its bulk density, not on its size. However, observations show that many smaller asteroids are spinning faster than the theoretical limit for a particle to remain on the equator, while most larger asteroids are not. Using the PKDGRAV simulation package, we conducted simulations modeling the spin-up of simulated rubble-pile asteroids, over a broad parameter space. First the effect of cohesion was tested, and we found that the stronger the cohesive force, the faster a simulated asteroid can spin before failing, confirming previous analytical and numerical work, but for a wider range of body shapes that included oblate and prolate bodies. Next, the effect of overall size of asteroids was tested, at low and high resolution (fewer and more particles, respectively). Results show that while cohesion has an effect on simulated asteroids of all sizes, it is less important for larger bodies, for which gravity dominates, also as noted in previous studies, but also confirmed in our study for objects with a size distribution in particle size. Spin-up outcomes for our tested set included reshaping and fission, as well as satellite formation from gravitational reaccumulation of material shed from the progenitor equator.