Similar to Figs. 2 and 5 but for Simulation C with an external magnetic field of 10 μG. This plot shows both the logarithm of the density in [cgs] and the magnetic field lines. The scale of the central and right panels in this figure is 80 × 80 pc. As for simulation B, the swept-up shell of shocked ISM is relatively thick, which prevents the formation of thin-shell instabilities; and the contact discontinuity between shocked wind and swept-up ISM is strongly asymmetric. Although the RSG shell can form freely against the spherically symmetric wind termination shock (right side of centre panel), the expanding WR nebula is strongly affected by the asymmetry of the bubble (left side of right panel).