Turbulent dynamos in a collapsing cloud

Abstract: The amplification of magnetic fields is crucial for understanding the observed magnetization of stars and galaxies. Turbulent dynamo is the primary mechanism responsible for that but the understanding of its action in a collapsing environment is still rudimentary and relies on limited numerical experiments. We develop an analytical framework and perform numerical simulations to investigate the behavior of small-scale and large-scale dynamos in a collapsing turbulent cloud. This approach is also applicable to expanding environments and facilitates the application of standard dynamo theory to evolving systems. Using a supercomoving formulation of the magnetohydrodynamic (MHD) equations, we demonstrate that dynamo action in a collapsing background leads to a super-exponential growth of magnetic fields in time, significantly faster than the exponential growth seen in stationary turbulence. The enhancement is mainly due to the increasing eddy turnover rate during the collapse, which boosts the instantaneous growth rate of the dynamo. We also show that the final saturated magnetic field strength exceeds the expectation from considerations of pure flux-freezing or energy equipartition with the turbulence, scaling as $B \propto \rho^{5/6}$, where $\rho$ is the cloud density. Apart from establishing a formal framework for the studies of magnetic field evolution in collapsing (or expanding) turbulent plasmas, these findings have significant implications for early star and galaxy formation, suggesting that magnetic fields can be amplified to dynamically relevant strengths much earlier than previously thought.