3D MHD astrospheres: applications to IRC-10414 and Betelgeuse
  A significative fraction of all massive stars in the Milky Way move
supersonically through their local interstellar medium (ISM), producing bow
shock nebulae by wind-ISM interaction. The stability of these observed
astrospheres around cool massive stars challenges precedent two-dimensional
(magneto-)hydrodynamical simulations of their surroundings. We present
three-dimensional magneto-hydrodynamical (3D MHD) simulations of the
circumstellar medium of runaway M-type red supergiant stars moving with
velocity v_star= 50 km/s. We treat the stellar wind with a Parker spiral and
assume a 7 microG magnetisation of the ISM. Our free parameter is the angle
theta_mag between ISM flow and magnetisation, taken to 0, 45 and 90 degrees. It
is found that simulation dimension, coordinate systems and grid effects can
greatly affect the development of the modelled astrospheres. Nevertheless, as
soon as the ISM flow and magnetisation directions differs by more than a few
degrees (theta_mag>5 degree), the bow shock is stabilised, most clumpiness and
ragged structures vanishing. The complex shape of the bowshocks induce
important projection effects, e.g. at optical Ha line, producing complex of
astrospheric morphologies. We speculate that those effects are also at work
around earlier-type massive stars, which would explain their diversity of their
observed arc-like nebula around runaway OB stars. Our 3D MHD models are fitting
well observations of the astrospheres of several runaway red supergiant stars.
The results interpret the smoothed astrosphere of IRC-10414 and Betelgeuse
aOri) are stabilised by an organised, non-parallel ambient magnetic field. Our
findings suggest that IRC-10414 is currently in a steady state of its
evolution, and that Betelgeuse's bar is of interstellar origin.
