Impact of the Cosmic Magnetic Field on the Formation of the Early Stars in the Universe

Abstract

The investigation of the formation of the first stars in the Universe, termed Population III (Pop III), has until now largely neglected the impact of the cosmic magnetic field on their formation. Our present work focuses on the fragmentation properties of primordial protostellar discs and how they are modified by including magneto-hydrodynamic (MHD) aspects. We start from cosmological initial con- ditions at z = 100, then analyse the dynamics inside the most massive minihalo at a redshift close to z ~ 25. A magnetic field is inserted at an intermediate evolutionary stage, normalized to a fraction of the equipartition value. In order to explore the parameter space, different magnetic field geometries are considered including uniform, radial, toroidal, and poloidal configurations. The collapse of the gas is followed for ~8 orders of magnitude in density after the field was inserted, until a maximum of 10^15 cm^- 3 is reached. We find that the magnetic field leads to a delay in the collapse of the gas and inhibits the fragmentation of the emerging disc surrounding the central core. Moreover, the geometry of the field affects the collapse and the multiplicity of Pop III stars. The full understanding of the formation of these stars and their mass distribution thus needs to take into account the effect of magnetic fields. We further conclude that ideal MHD is only a first step in this endeavor, to be followed-up with a comprehensive treatment of dissipative effects, such as ambipolar diffusion and Ohmic dissipation. This is worth doing in future works.