Shape and evolution of wind-blown bubbles of massive stars: on the
  effect of the interstellar magnetic field
  The winds of massive stars create large (>10 pc) bubbles around their
progenitors. As these bubbles expand they encounter the interstellar coherent
magnetic field which, depending on its strength, can influence the shape of the
bubble. We wish to investigate if, and how much, the interstellar magnetic
field can contribute to the shape of an expanding circumstellar bubble around a
massive star. We use the MPI-AMRVAC code to make magneto-hydrodynamical
simulations of bubbles, using a single star model, combined with several
different field strengths: B = 5, 10, and 20 muG for the interstellar magnetic
field. This covers the typical field strengths of the interstellar magnetic
fields found in the galactic disk and bulge. Furthermore, we present two
simulations that include both a 5 muG interstellar magnetic field and a 10,000
K interstellar medium and two different ISM densities to demonstrate how the
magnetic field can combine with other external factors to influence the
morphology of the circumstellar bubbles. Our results show that low magnetic
fields, as found in the galactic disk, inhibit the growth of the circumstellar
bubbles in the direction perpendicular to the field. As a result, the bubbles
become ovoid, rather than spherical. Strong interstellar fields, such as
observed for the galactic bulge, can completely stop the expansion of the
bubble in the direction perpendicular to the field, leading to the formation of
a tube-like bubble. When combined with a warm, high-density ISM the bubble is
greatly reduced in size, causing a dramatic change in the evolution of
temporary features inside the bubble. The magnetic field of the interstellar
medium can affect the shape of circumstellar bubbles. This effect may have
consequences for the shape and evolution of circumstellar nebulae and supernova
remnants, which are formed within the main wind-blown bubble.
