Context : Many evolved stars travel through space at supersonic velocities , which leads to the formation of bow shocks ahead of the star where the stellar wind collides with the interstellar medium ( ISM ) . Herschel observations of the bow shock of \textalpha -Orionis show that the shock is almost free of instabilities , despite being , at least in theory , subject to both Kelvin-Helmholtz and Rayleigh-Taylor instabilities .

Aims : A possible explanation for the lack of instabilities lies in the presence of an interstellar magnetic field .
We wish to investigate whether the magnetic field of the interstellar medium ( ISM ) in the Orion arm can inhibit the growth of instabilities in the bow shock of \textalpha -Orionis .

Methods : We used the code MPI-AMRVAC to make magneto-hydrodynamic simulations of a circumstellar bow shock , using the wind parameters derived for \textalpha -Orionis and interstellar magnetic field strengths of B = 1.4 , 3.0 , and 5.0 \textmu G , which fall within the boundaries of the observed magnetic field strength in the Orion arm of the Milky Way .

Results : Our results show that even a relatively weak magnetic field in the interstellar medium can suppress the growth of Rayleigh-Taylor and Kelvin-Helmholtz instabilities , which occur along the contact discontinuity between the shocked wind and the shocked ISM .

Conclusions : The presence of even a weak magnetic field in the ISM effectively inhibits the growth of instabilities in the bow shock .
This may explain the absence of such instabilities in the Herschel observations of \textalpha -Orionis .