Magnetic Field Amplification during Stellar Collisions between Low-mass Stars

Abstract Blue straggler stars in stellar clusters appear younger and bluer than other cluster members, offering a unique opportunity to understand the stellar dynamics and populations within their hosts. In the collisional formation scenario, excessive angular momentum in the collision product poses a challenge, as the consequent significant mass loss during transition to a stable state leads to a star with too low of a mass to be a blue straggler, unless it spins down efficiently. While many proposed spin-down mechanisms require magnetic fields, the existence or strength of these magnetic fields has not been confirmed. Here, we present 3D moving-mesh magnetohydrodynamical simulations of collisions between low-mass main-sequence stars and investigate magnetic field amplification. Magnetic field energy is amplified during collisions by a factor of 10 8 –10 10 , resulting in the magnetic field strength of 10 7 –10 8 G at the core of the collision product, independent of collision parameters. The surface magnetic field strengths increase up to 10–10 4 G. In addition, a distinctly flattened, rotating gas structure appears around the collision products in off-axis collisions, suggesting potential disk formation. These findings indicate that magnetic braking and disk locking could facilitate spin-down, enabling the formation of blue straggler stars.