How interacting winds shape the mechanical feedback of massive star clusters over millions of years

Abstract

In recent years, massive star cluster environments have proved to be bright sources of very-high energy gamma-rays, in particular young clusters which are powered by the winds interacting in their cores. In order to understand how these winds can accelerate particles up to very-high energies, it is necessary to model their interactions from small (sub-pc) to large (10s of pc) scales over several millions of years. A key open question concerns the structure and properties of the resulting wind termination shock. By performing 3D magnetohydrodynamic simulations of clustered winds embedded in a superbubble cavity, we demonstrate that the dynamics of stellar wind interactions and the resulting shock structure solely depends on the density and pressure of the cavity. This implies that the initial conditions of the simulation can be tuned in order to simulate star clusters of arbitrary age at a reduced computational cost. This novel method is validated using a toy cluster hosting 30 identical stars. We discuss the properties of the resulting cluster-wind termination shock under various assumptions. In particular, we are able for the first time to obtain a fully decoupled spherical wind termination shock for a 5 Myr old cluster. We further show that radiative cooling increases the sphericity of the shock. In general, the morphology of the outflow depends on the number of dominant stars, on the power of the stars sitting at the edge of the cluster core, and on the compactness of the cluster. We additionally show how a semi-analytical model can be used in order to estimate key morphological properties of the outflow without relying on large-scale simulations.

Figures (16)

• Figure 1: Evolution of the stellar feedback over 1 Myr in a homogeneous ISM. First row: density slices without cooling. Second row: density slices with cooling. Third row: Mach number with cooling. Timestamps from left to right: 10kyr, 103 kyr, 298 kyr, 1.003 Myr. The striations aligned with the Cartesian axes visible at 1 Myr are numerical artifacts due to the stretching of the grid.
• Figure 2: Detailed structure of the cluster outflow and cluster termination front at 1 Myr. Strong supersonic contours (Mach number = 3) are shown in red. Transsonic contours (Mach number = 1) are shown in orange.
• Figure 3: Evolution of the mean pressure in the subsonic superbubble medium.
• Figure 4: Evolution of the stellar feedback over 200 kyr starting with a superbubble ansatz (3 first columns), compared with the solution obtained in the full run starting at $t=0$ (last column).
• Figure 5: Comparison between the solution obtained at 1.2 Myr in the full run and the solution obtained at 200 kyr in the run started with the 1 Myr old superbubble ansatz. The white surface is the cluster termination front. The blue sheets are the trans-sonic sheets escaping the cluster core. The turquoise interface renders the outer shell of the superbubble.