Comparing the collision and merger products of stars using MHD

Pavan Vynatheya (University of Toronto)

September 22, 2025

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Abstract: In dense star cluster cores, frequent gravitational encounters often lead to stellar collisions. Additionally, close binaries in these dense environments can harden over time, eventually undergoing mass transfer and merging due to orbit shrinkage. Using the moving-mesh magnetohydrodynamics code \texttt{AREPO}, we conduct the first simulations comparing collisions and mergers of massive (5 – 10 Msun) main-sequence stars to examine similarities and differences in the properties of their resulting products. Our simulations show that both collisions and mergers lead to significant internal mixing, replenishing the stellar core with hydrogen from the envelope, and extending the main-sequence lifetime. This mixing also amplifies the stellar magnetic field by up to 10 orders of magnitude. The resulting stars rotate rapidly and differentially as a result of angular momentum transfer from the orbits. Key differences emerge between the two channels: merger products have noticeable axial outflows and develop more distinct disk-like structures in the merger plane compared to collision products. In addition, collision products are mixed more thoroughly, while merger products are more segregated.