Quantifying collision-driven mass loss in supermassive star formation: the role of stellar structure and accretion

Abstract: Observations of high-redshift galaxies with JWST have renewed interest in scenarios where supermassive stars form via runaway stellar collisions in dense clusters, yet the impact of collision-driven mass loss on their growth remains uncertain. In this work, we perform a post-processing analysis of 3D hydrodynamical simulations of the formation of a supermassive star, applying an analytic mass-loss prescription to stellar collisions while exploring different assumptions for the internal stellar structure. We consider a polytropic main-sequence model, a semi-analytic accreting protostar model, and structures derived from stellar evolution calculations. We find that the cumulative mass-loss fraction depends sensitively on the adopted stellar structure, ranging from $\lesssim 10-25\%$ for more compact configurations to $\gtrsim 30-40\%$ for more extended protostellar models. The importance of mass loss further depends on the dynamical state of the system, including the ratio of stellar velocity dispersion to the stellar surface escape velocity. We find significant uncertainty depending on the prescription used. As a result, collision-driven mass loss could significantly limit the growth of the central object, at least in some cases. Overall, our results indicate that uncertainties in the internal structure of rapidly accreting protostars represent a major source of systematic uncertainty and must be better constrained to robustly assess the viability of runaway-collision pathways for forming massive black hole seeds.