Graze-and-merge Collisions under External Perturbers

Abstract

Graze-and-merge collisions are common multi-step mergers occurring in low-velocity, off-axis impacts between similar-sized planetary bodies. The first impact happens at somewhat faster than the mutual escape velocity; for typical impact angles this does not result in immediate accretion, but the smaller body is slowed down so that it loops back around and collides again, ultimately accreting. The scenario changes in the presence of a third major body, i.e., planets accreting around a star, or satellites around a planet. We find that when the loop-back orbit remains inside roughly one third of the Hill radius from the target, then the overall process is not strongly affected. As the loop-back orbit increases in radius, the return velocity and angle of the second collision become increasingly random, with no record of the first collision's orientation. When the loop-back orbit gets to about three quarters of the Hill radius, the path of smaller body is disturbed up to the point that it will usually escape the target.