Stellar-wind feedback and magnetic fields around young compact star clusters: 3D magnetohydrodynamics simulations

Context . The environments of young star clusters are shaped by the interactions of the powerful winds of massive stars and their feedback on the cluster birth cloud. Several young star clusters show diffuse γ -ray emission on the degree scale, which hints at ongoing particle acceleration. Aims . To date, particle acceleration and transport in star-cluster environments are not well understood. A characterisation of magnetic fields and flow structures is necessary to progress towards physical models. Previous work has largely focused on 100 pc scale feedback or detailed modelling of wind interaction of just a few stars. We aim to bridge this gap. We focus in particular on compact clusters in order to study collective effects arising from stellar-wind interaction. Objects in this class include Westerlund 1 and R136. Methods . We performed 3D ideal-magnetohydrodynamics simulations of compact young massive star clusters. We kinetically injected stellar winds for 46 individual very massive stars ( M > 40 M ⊙ ) distributed in a spherical region of radius ≤1 pc. We included a sub-population of five magnetic stars with increased dipole field strengths of 0.1–1 kG, and we studied the evolving superbubble over several hundred thousand years. Results . The bulk flow and magnetic fields show an intricate non-uniform morphology that is critically impacted by the relative position of individual stars. The cluster wind terminates in a strong shock that is non-spherical, and similar to the flow, it has non-uniform properties. The magnetic field is composed of both highly tangled sections and coherent quasi-radial field-line bundles. Steep particle spectra in the teraelectronvolt domain arise naturally from the variation of magnetic field magnitude over the cluster-wind termination shock. This finding is consistent with γ -ray observations. We deem the scenario of petaelectronvolt particle acceleration as unlikely.