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

TL;DR

This work tackles how winds from a compact, Wolf-Rayet–dominated star cluster interact with the surrounding ISM before the first supernovae. Using 3D MHD simulations with a scaling strategy, the authors model a cluster of 60 stars driving a collective wind into either cold CNM or warmer WNM, yielding a hot bubble ($T\gtrsim10^7$ K) and a dense, magnetized shell. In CNM, the shell develops a cellular, highly magnetized structure with $B\gtrsim50$–$70\mu$G, while in WNM the shell remains smoother, and magnetic-field amplification occurs mainly in the shell due to compression; conduction can further modify density and temperature by evaporating shell material. The results quantify the bubble’s expansion as $R(t)\propto t^{0.57}$ and show that the WR-dominated phase can sweep $\gtrsim10^4$ M$_{\odot}$ of gas into a fragmented shell on $\sim2\times10^5$ yr timescales, providing crucial insights for interpreting X-ray emission and particle acceleration in young clusters before core-collapse supernovae.

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 $\sim 3.5$ $μ$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 $\gtrsim 50$ $μ$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.

Figures (7)

• Figure 1: Left: Absolute value of the cooling/heating function for an optically thin plasma in the two interstellar medium phases, CNM and WNM. The equilibrium temperatures $T_{\rm cnm}=160$ K (blue dashed) and $T_{\rm wnm}=6400$ K (red solid) correspond to the inflection points where the functions change sign. Right: Absolute value of the cooling/heating function for an optically thin, ionized medium (red solid) bounded by the bubble shell and filled with cluster wind material. Dashed curves show individual components (Meyer et al. 2014): contributions from hydrogen and helium line cooling (magenta dash-dotted); metal line cooling (blue dotted; Wiersma et al. 2009); hydrogen recombination (yellow dashed; Hummer 1994); forbidden metal lines (green dash-dotted; Henney et al. 2009); and hydrogen photoionization heating (cyan dashed). The equilibrium temperature is $T_{\rm ion}=8300$ K.
• Figure 2: Positions of randomly distributed massive stars of different spectral classes within the computational domain representing the cluster core. Black asterisks mark O-stars; purple crosses mark WR stars. Marker opacity indicates line-of-sight depth. Red, green, and blue dots on the domain faces show projections of the stellar positions onto the yz, xz, and xy planes, respectively.
• Figure 3: Comparison of the bubble expansion dynamics $R(t)$ with well-known theoretical expansion laws. Left: WNM case. Right: CNM case. Black points show the growth of the bubble's average forward shock radius over time $t_{\rm evol}$. Three specific times are shown as magnetic field slices for both ISM cases in Fig. \ref{['fig5']}. A black straight line approximates the data with a power law $R \propto t^{\alpha}$. For comparison, colored dashed lines show well-known analytic expansion laws (see, e.g., Sedov 1946; Weaver et al. 1977).
• Figure 4: Comparison of physical quantities in the bubble, shown in a central $x_2$--$x_3$ plane slice. Top row: the WNM case at $t_{\rm evol}=63$ kyr. Bottom row: the CNM case at $t_{\rm evol}=235$ kyr. Each row displays, from left to right, maps of the velocity field, density, and temperature. Black arrows on the velocity maps indicate the local flow direction.
• Figure 5: Comparison of magnetic field maps in a central $x_2$--$x_3$ plane slice of the bubble. Top row: the WNM case. Bottom row: the CNM case. Each row displays the field strength at three different evolutionary times $t_{\rm evol}$, ordered chronologically from left to right as the simulation domain is rescaled from 5 to 10 to 20 pc. Black arrows show the local magnetic field direction.