Bimodal Star Formation in Simulations of Strongly Magnetized Giant Molecular Clouds

TL;DR

This study uses radiation magnetohydrodynamic simulations to show that strong magnetic fields in giant molecular clouds can cause star formation to become bimodal, with two distinct episodes affecting star formation efficiency and stellar populations.

Contribution

It demonstrates that magnetic field strength can induce bimodal star formation in GMCs, a novel insight into the role of magnetic fields in star formation processes.

Findings

Stronger magnetic fields lead to bimodal star formation.

Second star formation episode produces only low-mass stars.

Magnetic trapping prolongs star formation and increases efficiency.

Abstract

We present the results of a set of radiation magnetohydrodynamic simulations of turbulent molecular clouds in which we vary the initial strength of the magnetic field within a range ($1 \lesssim \mu \lesssim 5$) consistent with observations of local giant molecular clouds (GMCs). We find that as we increase the strength of the magnetic field, star formation transitions from unimodal (the baseline case, $\mu=5$, with a single burst of star formation and Salpeter IMF) to bimodal. This effect is clearest in the most strongly magnetized GMCs ($\mu=1$): a first burst of star formation with duration, intensity and IMF comparable to the baseline case, is followed by a second star formation episode in which only low-mass stars are formed. Overall, due to the second burst of star formation, the strongly magnetized case results in a longer star formation period and a higher efficiency of star formation. The second burst is produced by gas that is not expelled by radiative feedback, instead remaining trapped in the GMC by the large-scale B-field, producing a nearly one-dimensional flow of gas along the field lines. The trapped gas has a turbulent and magnetic topology that differs from that of the first phase and strongly suppresses gas accretion onto protostellar cores, reducing their masses. We speculate that this star formation bimodality may be an important ingredient to understand the origin of multiple stellar populations observed in massive globular clusters.