Large-scale Dynamo in a Primordial Accretion Flow: An Interpretation from Hydrodynamic Simulation

Abstract

Abstract Without an existing large-scale coherent magnetic field in the early universe, Population III stars would likely rotate at or near breakup speed. In this work, focusing on the accretion phase of Population III stars, we investigate the possibility of generating a coherent magnetic field through large-scale dynamo processes, as well as the corresponding field saturation level. Using results from hydrodynamic simulations performed with a cylindrical grid, we demonstrate that primordial accretion disks are turbulent with a Shakura–Sunyaev disk parameter α ss ≳ 10−3 and evidence for helical turbulence with a dynamo number ∣D αΩ∣ ≫ 10. The presence of helical turbulence at these levels allows large-scale dynamo modes to grow, and the saturation level is determined by the amount of net helicity remaining in the dynamo active regions (i.e., the quenching problem). We demonstrate that if the accretion could successfully alleviate the quenching problem, the magnetic field can reach approximate equipartition with B/B eq ∼ 3.