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 $1000$ $1$ $M_\odot$ stars embedded in a uniform cluster of dynamically relaxed 0.5 $M_\odot$ 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 nuclear star clusters. 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 have a high probability of experiencing a collision over their main-sequence lifetime, perhaps with implications for their observed youth and properties.

💡 Research Summary

This research investigates the intricate coupling between stellar evolution and dynamical collisions within Nuclear Star Clusters (NSCs) surrounding Supermassive Black Holes (SMBHs). NSCs represent some of the most extreme and dense stellar environments in the cosmos, where the high frequency of stellar encounters can fundamentally alter the stellar mass function and create exotic stellar populations. The study focuses on how the internal evolutionary tracks of stars—specifically changes in mass and radius—interact with the collision rates driven by the cluster’s dynamical environment.

To explore this, the authors conducted a sophisticated simulation involving a sample of 1,000 stars, each with an initial mass of 1 $M_\odot$, embedded within a dynamically relaxed background of 0.5 $M_\odot$ stars. By employing the COSMIC stellar evolution code, the researchers were able to track the real-time evolution of mass, radius, and evolutionary state of the primary stars as they underwent collisions within the cluster.

The findings reveal a significant spatial bifurcation in collision mechanisms. Within the innermost 0.1 parsec (pc) of the SMBH, the extreme stellar density ensures that most stars experience collisions while still in the main-sequence phase. In contrast, at distances greater than 0.1 pc, the dominant collision mode shifts to the red giant phase. This shift is driven by the dramatic expansion of stellar radii during the red giant stage, which significantly increases the collision cross-section, making these expanded stars much more susceptible to encounters even in slightly less dense regions.

Furthermore, the study demonstrates that the overall collisional history of the cluster—whether it is dominated by main-sequence or red giant collisions—is heavily dependent on the “stellar cusp” steepness. The cusp profile determines the spatial distribution of stars, and thus, the density gradient that dictates which evolutionary phase is most vulnerable to collisions. This implies that the large-scale dynamical structure of a galaxy’s nucleus is a primary driver of the microscopic evolutionary outcomes of its constituent stars.

Finally, the paper applies these findings to the well-known S-stars orbiting the Milky Way’s SMBH. The simulation suggests that several of these S-stars have a high probability of having undergone collisions during their main-sequence lifetimes. Such collisional events could lead to “rejuvenation,” where the merger or mass-stripping processes make the stars appear younger or possess different chemical properties than their true age would suggest. This provides a potential physical explanation for the observed “youth” of S-stars and offers a new framework for interpreting stellar populations in the vicinity of supermassive black holes.