The role of magnetic fields in the formation of multiple massive stars

Abstract

Context. Most massive stars are located in multiple stellar systems. Magnetic fields are believed to be essential in the accretion and ejection processes around single massive protostars. Aims. Our aim is to unveil the influence of magnetic fields in the formation of multiple massive stars, in particular on the fragmentation modes and properties of the multiple protostellar system. Methods. Using RAMSES, we follow the collapse of a massive pre-stellar core with (non-ideal) radiation-(magneto-)hydrodynamics. We choose a setup which promotes multiple stellar system formation in order to investigate the influence of magnetic fields on the multiple system's properties. Results. In the purely hydrodynamical models, we always obtain (at least) binary systems following the fragmentation of an axisymmetric density bump in a Toomre-unstable disk around the primary sink: this sets the frame for further studying stellar multiplicity. When more than two stars are present in these early phases, their gravitational interaction triggers mergers until there are two stars left only. The following gas accretion increases their orbital separation and hierarchical fragmentation occurs so that both stars host a comparable disk and stellar system which then form similar disks as well. Disk-related fragmenting structures are qualitatively resolved when the finest resolution is ≈1/20 of the disk radius. We identify several modes of fragmentation: Toomre-unstable disk fragmentation, arm-arm collision and arm-filament collision. Disks grow in size until they fragment and become truncated as the newly-formed companion gains mass. When including magnetic fields, the picture evolves: the primary disk is initially elongated into a bar, it produces less fragments, disk formation and arm-arm collision are captured at comparatively higher resolution, and arm-filament collision is absent. Magnetic fields reduce the initial orbital separation but do not affect its further evolution, which is mainly driven by gas accretion. With magnetic fields, the growth of individual disks is regulated even in the absence of fragmentation or truncation. Conclusions. Hierarchical fragmentation is seen in unmagnetized and magnetized models. Magnetic fields, including non-ideal effects, are important because they remove certain fragmentation modes and limit the growth of disks, which is otherwise only limited through fragmentation.