EXPANSION OF A SUPERBUBBLE IN A UNIFORM MAGNETIC-FIELD

We investigate the effects of interstellar magnetic fields on the evolution and structure of superbubbles. We first derive the governing equations for propagation of a radiative shock, and solve them analytically in the limit of high expansion velocity. We then present a numerical code able to solve them in the general case and discuss the numerical results. We find that magnetic fields of the strength present in the Galactic disk do not significantly modify the overall shape and dimensions of a superbubble. They reduce the volume of the interior cavity by one-third on average during the expansion phase. The shell elongates slightly in the direction of the external field. Magnetic pressure dominates over gas pressure in most of the shell and forces it to thicken substantially. Finally, because of the transverse mass motions which take place from the magnetic poles to the equator, the column density at the poles is reduced by typically a factor of 10.