THE INFLUENCE OF MAGNETIC FIELDS ON THE THERMODYNAMICS OF PRIMORDIAL STAR FORMATION

We explore the effects of magnetic energy dissipation on the formation of the first stars. For this purpose, we follow the evolution of primordial chemistry in the presence of magnetic fields in the post-recombination universe until the formation of the first virialized halos. From the point of virialization, we follow the protostellar collapse up to densities of similar to 10(12) cm(-3) in a one-zone model. In the intergalactic medium (IGM), comoving field strengths of greater than or similar to 0.1 nG lead to Jeans masses of 10(8) M-circle dot or more and thus delay gravitational collapse in the first halos until they are sufficiently massive. During protostellar collapse, we find that the temperature minimum at densities of similar to 10(3) cm(-3) does not change significantly, such that the characteristic mass scale for fragmentation is not affected. However, we find a significant temperature increase at higher densities for comoving field strengths of greater than or similar to 0.1 nG. This may delay gravitational collapse, in particular at densities of similar to 10(9) cm(-3), where the proton abundance drops rapidly and the main contribution to the ambipolar diffusion resistivity is due to collisions with Li+. We further explore how the thermal evolution depends on the scaling relation of magnetic field strength with density. While the effects are already significant for our fiducial model with B proportional to rho(0.5-0.57), the temperature may increase even further for steeper relations and lead to the complete dissociation of H-2 at densities of similar to 10(11) cm(-3) for a scaling with B proportional to rho(0.6). The correct modeling of this relation is therefore very important, as the increase in temperature enhances the subsequent accretion rate onto the protostar. Our model confirms that initial weak magnetic fields may be amplified considerably during gravitational collapse and become dynamically relevant. For instance, a comoving field strength above 10(-5) nG will be amplified above the critical value for the onset of jets which can magnetize the IGM.