Massive Star Cluster Formation II. Runaway Stars as Fossils of Sub-Cluster Mergers

Two main mechanisms have classically been proposed for the formation of runaway stars. The binary supernova scenario (BSS) suggests that a massive star in a binary explodes as a supernova, ejecting its companion. The dynamical ejection scenario suggests that a star is ejected through a strong dynamical encounter between multiple stars. We propose a third mechanism for the formation of runaway stars: the sub-cluster ejection scenario (SCES), where an infalling sub-cluster of stars is ejected out of the cluster by a slingshot interaction with the contracting gravitational potential of the assembling cluster. We demonstrate the SCES in a star-by-star simulation of a young massive cluster forming from a 10^6 M_⊙ gas cloud using the Torch framework. This star cluster forms hierarchically through a sequence of sub-cluster mergers, determined by the initial turbulent, spherical initial conditions of the gas. We find that these mergers drive the formation of runaway stars in our model. Late-forming sub-clusters fall into the central potential, where they are ejected on a slingshot trajectory, forming groups of runaway stars that are distributed highly anisotropically. Runaways formed by the same SCES share similar ages, velocities, and ejection directions. Surveying observations, we identify several SCES candidate groups with anisotropic ejection directions. The SCES is capable of producing runaway binaries: two wide dynamical binaries in infalling sub-clusters were tightened through ejection. This allows for another velocity kick via subsequent BSS, which is a promising avenue for producing hypervelocity stars unbound to the Galaxy. The SCES occurs when sub-cluster formation is resolved. We expect non-spherical initial gas distributions to increase runaway star numbers. The observation of groups of runaway stars formed via SCES thus reveals the assembly history of their natal clusters.