Accretion in stellar clusters and the collisional formation of massive stars

We present results from a numerical simulation of gas accretion in a cluster containing 1000 stars. The accretion forces the cluster to contract, leading to the development of a high-density core with a maximum density 10(5) times the mean stellar density. This density is sufficient for a significant number of stellar collisions to occur, resulting in the most massive stars being formed through a combination of gas accretion and stellar mergers. In the simulation, 19 mergers occur, generally where a binary is forced to merge owing to the interaction with another star in a small- N group. These small-N groups form owing to the self-gravity of the gas and constitute the highest-density regions in the cluster. Binary formation in these groups is common, occurring through dynamical three-body capture. The massive stars are thus generally in binary systems, which can be relatively wide. The self-gravity of the gas also forms significant structure in the vicinity of the cluster core, while continuing infall forms a circumbinary disc around the most massive star. This structure may be the source of collimation for the observed outflows from young massive stars. Finally, the resultant initial mass function from the combination of gas accretion and stellar mergers is indistinguishable from a Salpeter mass function.