  A significant fraction of OB-type, main-sequence massive stars are classified
as runaway and move supersonically through the interstellar medium (ISM). Their
strong stellar winds interact with their surroundings where the typical
strength of the local ISM magnetic field is about 3.5-7 micro-G, which can
result in the formation of bow shock nebulae. We investigate the effects of
such magnetic fields, aligned with the motion of the flow, on the formation and
emission properties of these circumstellar structures. Our axisymmetric,
magneto-hydrodynamical simulations with optically-thin radiative cooling,
heating and anisotropic thermal conduction show that the presence of the
background ISM magnetic field affects the projected optical emission our bow
shocks at Ha and [OIII] lambda 5007 which become fainter by about 1-2 orders of
magnitude, respectively. Radiative transfer calculations against dust opacity
indicate that the magnetic field slightly diminishes their projected infrared
emission and that our bow shocks emit brightly at 60 micron. This may explain
why the bow shocks generated by ionizing runaway massive stars are often
difficult to identify. Finally, we discuss our results in the context of the
bow shock of Zeta Ophiuchi and we support the interpretation of its imperfect
morphology as an evidence of the presence of an ISM magnetic field not aligned
with the motion of its driving star.
