Theory of Cluster Formation: Effects of Magnetic Fields

TL;DR

This paper reviews recent numerical simulations and observations showing how magnetic fields influence star cluster formation in dense molecular cloud clumps, affecting collapse rates and magnetic structures.

Contribution

It provides new insights into the role of magnetic fields in star formation, highlighting their impact on collapse dynamics and magnetic field configurations in cluster-forming regions.

Findings

Magnetic fields slow down star formation in dense clumps.

Turbulence amplifies magnetic fields to equipartition strength.

Polarization maps reveal well-ordered magnetic fields in star-forming regions.

Abstract

Stars form predominantly in clusters inside dense clumps of molecular clouds that are both turbulent and magnetized. The typical size and mass of the cluster-forming clumps are $\sim 1$ pc and $\sim 10^2 - $ 10$^3$ M$_\odot$, respectively. Here, we discuss some recent progress on numerical simulations of clustered star formation in such parsec-scale dense clumps with emphasis on the role of magnetic fields. The simulations have shown that magnetic fields tend to slow down global gravitational collapse and thus star formation, especially in the presence of protostellar outflow feedback. Even a relatively weak can retard star formation significantly, because the field is amplified by supersonic turbulence to an equipartition strength. However, in such a case, the distorted field component dominates the uniform one. In contrast, if the field is moderately strong, the uniform component remains dominant. Such a difference in the magnetic structure is observed in simulated polarization maps of dust thermal emission. Recent polarization measurements show that the field lines in nearby cluster-forming clumps are spatially well-ordered, indicative of a rather strong field. In such strongly-magnetized clumps, star formation should proceed relatively slowly; it continues for at least several global free-fall times of the parent dense clump ($t_{\rm ff}\sim $ a few $\times 10^5$ yr).