Bow shock nebulae of hot massive stars in a magnetized medium

Abstract

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