Magnetic field generation in mergers of massive main-sequence stars

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Abstract
Magnetic fields are found in many astrophysical objects, ranging from galaxy clusters to the interstellar medium of galaxies and neutron stars. Strong surface magnetic fields are also observed in about 7% of OBA-type stars, and stellar mergers are the likely origin of at least some of them. We investigated magnetic-field amplification during the merger of a 9 and an 8 M ⊙ main-sequence star using 3D magnetohydrodynamic simulations from our previous work. We focused on the magnetic-field amplification mechanisms, field geometry, and the structure and properties of the resulting merger, in particular its rotational configuration. The merger produces a star–torus structure in which the core of the initially less massive star is surrounded by material from the primary. The torus contains ≈3 M ⊙ and stores about 60% of the initial orbital angular momentum, and dynamic mass loss is negligible. Initially, turbulent motions driven by Kelvin–Helmholtz and magneto-rotational instabilities generate small-scale (≈0.1 R ⊙ ) magnetic fields. Subsequently, large-scale ordered azimuthal flows drive a larger-scale dynamo that amplifies and redistributes the magnetic energy to larger spatial scales (≈5 R ⊙ ), producing a remnant threaded by a strong large-scale magnetic field. The final magnetic configuration consists of intertwined poloidal and toroidal components, with a residual small-scale structure that resembles previously identified stable magnetic field equilibria. The amplification process is largely insensitive to the initial binary separation, numerical resolution, and seed magnetic-field strength, indicating that the final field loses memory of the initial magnetic conditions. The central regions of the merger remnant rapidly approach solid-body rotation, transitioning to a Keplerian-like profile within the surrounding torus. Our results support stellar mergers as a viable pathway for the formation of strongly magnetic massive stars and potentially highly magnetized compact remnants, such as magnetic white dwarfs and magnetars.

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