Hydrodynamic simulation of Cygnus OB2: the absence of a cluster wind termination shock

TL;DR

This study uses hydrodynamic simulations to show that Cygnus OB2's stellar winds do not produce a large-scale termination shock, challenging previous assumptions about energy emission mechanisms.

Contribution

It provides the first detailed hydrodynamic simulation of Cygnus OB2 that resolves small-scale structures and demonstrates the absence of a cluster wind termination shock.

Findings

Cluster excavates a spherical superbubble cavity.

No large-scale cluster wind termination shock forms.

Wind-wind interactions are less efficient than previously thought.

Abstract

We perform a large-scale hydrodynamic simulation of a massive star cluster whose stellar population mimics that of the Cygnus OB2 association. The main-sequence stars are first simulated during 1.6 Myr, until a quasi-stationary state is reached. At this time the three Wolf-Rayet stars observed in Cygnus OB2 are added to the simulation, which continues to 2 Myr. Using a high-resolution grid in the centre of the domain, we can resolve the most massive stars individually, which allows us to probe the kinetic structures at small (parsec) scales. We find that, although the cluster excavates a spherical "superbubble" cavity, the stellar population is too loosely distributed to blow a large-scale cluster wind termination shock, and that collective effects from wind-wind interactions are much less efficient than usually assumed. This challenges our understanding of the ultra-high energy emission observed from the region.