Superbubbles in magnetized interstellar media --- Blowout or confinement ? ---

Abstract

The importance of the interstellar magnetic field is studied in relation to the evolutions of superbubbles with a three-dimensional (3D) numerical magnetohydrodynamical (MHD) simulation. A superbubble is a large supernova remnant driven by sequential supernova explosions in an OB association. Its evolution is affected by the density stratification in the galactic disk. After the size reaches 2--3 times the density scale-height, the superbubble expands preferentially in the $z$-direction. Finally it can punch out the gas disk (blow-out). On the other hand, the magnetic field running parallel to the galactic disk has an effect to prevent from expanding in the direction perpendicular to the field. The density stratification and the magnetic fields have completely opposite effects on the evolution of the superbubble. We present results of 3D MHD simulation in which both effects are included. As a result, it is concluded that when the magnetic field has a much larger scale-height than the density, even for a model that the bubble would blow out from the disk if the magnetic field were absent, the magnetic field with the strength of 5 $μ$G can confine the bubble in $|z| \alt 300$ pc for $\simeq$20 Myr (confinement). In a model that the field strength decreases in the halo in proportion to $B \propto ρ^{1/2}$, the superbubble eventually blows out like a model of B=0 even if the magnetic field in the mid-plane is as strong as $B=5μ$G.