The Formation of Massive Stars through Stellar Collisions

In this review, I present the case for how massive stars may form through stellar collisions. This mechanism requires very high stellar densities, up to 4 orders of magnitude higher than are observed in the cores of dense young clusters. In this model, the required stellar densities arise due to gas accretion onto stars in the cluster core, including the precursers of the massive stars. This forces the core to contract until the stellar densities are sufficiently high for collisions to occur. Gas accretion is also likely to play a major role in determining the distribution of stellar masses in the cluster as well as the observed mass segregation. One of the main advantages of this mechanism is that it explicitly relies on the cluster environment in order to produce the massive stars. It is thus in a position to explain the relation between clustered and massive star formation which is not an obvious outcome of an isolated accretion mechanism. A recent numerical simulation supports this model as the cluster core increases its density by $10^5$ during gas accretion. Approximately 15 stellar collisions occur (with $R_{coll}=1$ au) in the cluster core, making a significant contribution to the mass of the most massive star