Double bow shocks around young, runaway red supergiants: application to
  Betelgeuse
  A significant fraction of massive stars are moving supersonically through the
interstellar medium (ISM), either due to disruption of a binary system or
ejection from their parent star cluster. The interaction of their wind with the
ISM produces a bow shock. In late evolutionary stages these stars may undergo
rapid transitions from red to blue and vice versa on the Hertzsprung-Russell
diagram, with accompanying rapid changes to their stellar winds and bow shocks.
Recent 3D simulations of the bow shock produced by the nearby runaway red
supergiant (RSG) Betelgeuse, under the assumption of a constant wind, indicate
that the bow shock is very young (<30000 years old), hence Betelgeuse may have
only recently become a RSG. To test this possibility, we have calculated
stellar evolution models for single stars which match the observed properties
of Betelgeuse in the RSG phase. The resulting evolving stellar wind is
incorporated into 2D hydrodynamic simulations in which we model a runaway blue
supergiant (BSG) as it undergoes the transition to a RSG near the end of its
life. We find that the collapsing BSG wind bubble induces a bow shock-shaped
inner shell around the RSG wind that resembles Betelgeuse's bow shock, and has
a similar mass. Surrounding this is the larger-scale retreating bow shock
generated by the now defunct BSG wind's interaction with the ISM. We suggest
that this outer shell could explain the bar feature located (at least in
projection) just in front of Betelgeuse's bow shock.
