The role of magnetic fields in the structure and interaction of supershells

Abstract

Large-scale shocks formed by clustered feedback of young OB stars are considered an important source of mechanical energy for the ISM and a trigger of molecular cloud formation. Their interaction sites are locations where kinetic energy and magnetic fields are redistributed between ISM phases. In this work we study the effect of the magnetic field on the expansion and fragmentation of supershells and look for the signatures of supershell collisions on dense structures and on the kinetic and magnetic energy distribution of the ISM. We performed a series of high-resolution, three-dimensional simulations of colliding supershells. The shocks are created by time-dependent feedback and evolve in a diffuse turbulent environment that is either unmagnetized or has different initial magnetic field configurations. In the hydrodynamical situation, the expansion law of the superbubbles is consistent with the radius-time relation that is theoretically predicted for wind-blown bubbles. The supershells fragment over their entire surface into small dense clumps that carry more than half of the total kinetic energy in the volume. However, this is not the case when a magnetic field is introduced, either in the direction of the collision or perpendicular to the collision. In magnetized situations, the shell surfaces are more stable to dynamical instabilities. When the magnetic field opposes the collision, the expansion law of the supershells also becomes significantly flatter than in the hydrodynamical case. Although a two-phase medium arises in all cases, in the MHD simulations the cold phase is limited to lower densities.