The growth of massive stars via stellar collisions in ensemble star clusters

Recent simulations and observations suggest that star clusters form via the assembling of smaller subclusters. Because of their short relaxation time, subclusters experience core collapse much earlier than virialized solo clusters, which have similar properties of the merger remnant of the assembling clusters. As a consequence, it seems that the assembling clusters result in efficient multiple collisions of stars in the cluster core. We performed a series of N-body simulations of ensemble and solitary clusters including stellar collisions and found that the efficiency of multiple collisions between stars is suppressed if subclusters assemble after they experience core collapse individually. In this case, subclusters form their own multiple collision stars which experienced a few collisions, but they fail to collide with each other after their host subclusters assemble. The multiple collision stars scatter each other and escape, and furthermore the central density of the remnant clusters had already been depleted for the stars to experience more collisions. On the other hand, if subclusters assemble before they experience core collapse, the multiple collisions of stars proceed efficiently in the remnant cluster, and the collision products are more massive than virialized solo clusters and comparable in mass to cold solo clusters.