The Interplay of Stellar Evolution and Collisions in Galactic Nuclei

Nuclear star clusters (NSCs) surrounding supermassive black holes (SMBHs) are among the densest stellar environments in the Universe. In these environments, collisions can shape the stellar mass function and produce exotic stellar populations. In this work, we investigate how stellar collisions couple with stellar evolution in the inner parsec of an NSC. We simulate the evolution of a sample of 1000 1 M _⊙ stars embedded in a uniform cluster of dynamically relaxed 0.5 M _⊙ stars. Using COSMIC to evolve stellar properties in time, we track the mass, radius, and evolutionary state of the stars as they collide in the cluster. Our results show that most stars within 0.1 pc of the SMBH experience a collision while on the main sequence. However, outside of this distance, stars collide during the red giant phase, when the stellar radius increases dramatically. We find that the most common type of collision—main sequence or red giant—over the lifetime of the cluster depends on the steepness of the stellar cusp, which determines the spatial distribution of the stars in the cluster. These results show that stellar evolution plays a fundamental role in shaping the collisional history of stars in NSCs. Lastly, we consider whether the closest known stars to the Milky Way’s SMBH have experienced a collision. We estimate that several of the S-stars may experience a collision over their main-sequence lifetime, perhaps with implications for their observed youth and properties.