Shells and bubbles around compact clusters of massive stars: 3D MHD simulations

TL;DR

This paper uses 3D MHD simulations to study how winds from massive stars in young clusters shape their environment, forming dense shells with amplified magnetic fields, especially in neutral interstellar media.

Contribution

It presents the first detailed 3D MHD simulations of cluster wind bubbles around Wolf-Rayet star clusters, analyzing magnetic field amplification and shell formation in different interstellar media.

Findings

Cluster winds can sweep up over 10^4 solar masses of gas in 200,000 years.

Shells formed are thin, dense, and exhibit cellular magnetic structures due to instabilities.

Magnetic fields in shells are amplified to over 50 microGauss.

Abstract

We present the results of three-dimensional magnetohydrodynamic (3D MHD) simulations of the plasma flow structure in the vicinity of a compact cluster of young massive stars. The cluster is considered at the evolutionary stage dominated by Wolf-Rayet stars. This stage occurs in clusters with ages of several million years, close to the onset of supernova explosions; the well-known objects Westerlund 1 and 2 are the prototypes. The collisions of powerful winds from massive stars in the cluster core, calculated as interactions of individual outflows, are accompanied by their partial thermalization and produce a collective cluster wind. The MHD dynamics of the cluster wind bubble expansion into the interstellar medium is considered, depending on the density of the surrounding medium with a uniform magnetic field. We show that when expanding into a cold neutral medium, the cluster wind is able to reshape its surrounding environment over the Wolf-Rayet star lifetime, sweeping up more than $10^4$ $M_{\odot}$ of gas in $\sim 2 \times 10^5$ yr and producing extended, thin and dense shells with an amplified magnetic field. In a cold neutral medium with a density of $\sim 20$ cm$^{-3}$ and a magnetic field of $\sim 3.5$ $\mu$G, a thin shell forms around the cluster wind bubble, characterized by a cellular structure in its density and magnetic field distributions. The cellular magnetic field structure appears in parts of the shell expanding transversely to the orientation of the external magnetic field. Magnetic fields in the shell are amplified to strengths $\gtrsim 50$ $\mu$G. The formation of the cellular structure is associated with the development of instabilities. The expansion of the bubble into a warm neutral interstellar medium also leads to the formation of a shell with an amplified magnetic field.