The influence of magnetic fields on the thermodynamics of primordial star formation

TL;DR

This paper investigates how magnetic fields influence the thermodynamics and collapse processes in primordial star formation, revealing that even weak initial fields can significantly affect the evolution and feedback mechanisms of early stars.

Contribution

It provides a detailed analysis of magnetic field effects on primordial chemistry, collapse dynamics, and potential feedback in the first star formation, highlighting the amplification of weak magnetic fields during collapse.

Findings

Magnetic fields of >0.1 nG delay halo collapse by increasing Jeans mass.

Temperature at high densities rises with stronger magnetic fields, affecting collapse timing.

Weak initial magnetic fields can be amplified to influence jet formation and IGM magnetization.

Abstract

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 $\sim10^{12}$ cm$^{-3}$ in a one-zone model. In the intergalactic medium (IGM), comoving field strengths of $\gtrsim0.1$ nG lead to Jeans masses of $10^8 M_\odot$ 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 $\sim10^3$ 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 $\gtrsim0.1$ nG. This may delay gravitational collapse, in particular at densities of $\sim10^9$ cm$^{-3}$, where the proton abundance drops rapidly and the main contribution to the ambipolar diffusion resistivity is due to collisions with Li$^+$. After the formation of the protostar, the increased gas temperatures may enhance the protostellar accretion rate. 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.