Ballistic mixing: stellar mergers drive internal mixing in extremely massive stars, with implications for proto-globular cluster pollution

Extremely massive stars, with masses of 10 cubed - 10 4 M, have been proposed as potential sources of the chemical enrichment associated with multiple populations in globular clusters. Early on the main sequence, these stars reach central temperatures of 65 to 75 MK, enabling nucleosynthesis through the CNO, NeNa, and MgAl cycles, producing abundance patterns similar to those observed in the so-called cluster's second population. Stellar mergers are among the leading formation channels for extremely massive stars. In this work, we follow the formation of a about6700 M star via 253 stellar mergers, drawn from a direct N-body star cluster simulation with properties similar to the densest star clusters observed at redshift z about10. We evolve each star as a 1D stellar model and construct the merger products from mixing and shock heating prescriptions calibrated to 3D simulations of parabolic collisions. We find that entropy-based mixing efficiently transports nuclear-processed material from the convective core to the envelope, while merger-driven heating promotes chemical homogenization. As a result, the stellar surface becomes enriched with H-burning products, producing abundance patterns consistent with those observed in globular cluster multiple populations and supporting the role of merger-formed extremely massive stars as potential sources of early chemical enrichment in proto-globular cluster-like conditions.