Shells and Bubbles around Compact Clusters of Massive Stars: 3D MHD Simulations

Abstract

We present the results of our three-dimensional magnetohydrodynamic (3D MHD) simulations of the plasma flow structure in the vicinity of a compact young massive star cluster at the cluster evolution phase governed by Wolf–Rayet stars. This phase takes place for a cluster with an age of a few million years close to the onset of supernova explosions; the well-known objects Westerlund 1 and 2 are the prototype. The collisions of powerful winds from massive stars in the cluster core calculated as the interactions of individual outflows from stars are accompanied by their partial thermalization and produce a collective cluster wind. We consider the MHD expansion dynamics of the bubble blown by the cluster wind as a function of the density of the surrounding interstellar medium with a uniform magnetic field. We show that when expanding in a cold neutral medium, the collective wind of the cluster is able to change the topology of its parent cloud in the time of the Wolf–Rayet phase, sweeping up more than \(10^{4}\;M_{\odot}\) of gas in \({\sim}2\times 10^{5}\) yr and producing thin dense extended shells with amplified magnetic fields. In a cold neutral medium with a density \({\sim}20\) cm\({}^{-3}\) and a magnetic field \({\sim}3.5\) \(\mu\)G a thin shell with a characteristic cellular structure of the density and magnetic field distributions is formed around the cluster wind bubble. The cellular magnetic field structure manifests itself in the parts of the shell that expand in a direction transverse to the orientation of the external magnetic field. The magnetic fields in the shell are amplified to strengths \({\gtrsim}50\) \(\mu\)G. The cellular structure formation is associated with the development of instabilities. The bubble expansion in a warm neutral interstellar medium is also accompanied by the formation of a shell with an amplified magnetic field.