190 research outputs found
Can the magnetic field in the Orion arm inhibit the growth of instabilities in the bow shock of Betelgeuse?
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 -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. 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
-Orionis. We used the code MPI-AMRVAC to make magneto-hydrodynamic
simulations of a circumstellar bow shock, using the wind parameters derived for
-Orionis and interstellar magnetic field strengths of , and G, which fall within the boundaries of the observed
magnetic field strength in the Orion arm of the Milky Way. 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. 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 -Orionis.Comment: 5 pages, including 7 figures. The published version will include 4
animations. Accepted for publication in A&
Computing the dust distribution in the bowshock of a fast moving, evolved star
We study the hydrodynamical behavior occurring in the turbulent interaction
zone of a fast moving red supergiant star, where the circumstellar and
interstellar material collide. In this wind-interstellar medium collision, the
familiar bow shock, contact discontinuity, and wind termination shock
morphology forms, with localized instability development. Our model includes a
detailed treatment of dust grains in the stellar wind, and takes into account
the drag forces between dust and gas. The dust is treated as pressureless gas
components binned per grainsize, for which we use ten representative grainsize
bins. Our simulations allow to deduce how dust grains of varying sizes become
distributed throughout the circumstellar medium. We show that smaller dust
grains (radius <0.045 micro-meters) tend to be strongly bound to the gas and
therefore follow the gas density distribution closely, with intricate
finestructure due to essentially hydrodynamical instabilities at the
wind-related contact discontinuity. Larger grains which are more resistant to
drag forces are shown to have their own unique dust distribution, with
progressive deviations from the gas morphology. Specifically, small dust grains
stay entirely within the zone bound by shocked wind material. The large grains
are capable of leaving the shocked wind layer, and can penetrate into the
shocked or even unshocked interstellar medium. Depending on how the number of
dust grains varies with grainsize, this should leave a clear imprint in
infrared observations of bowshocks of red supergiants and other evolved stars.Comment: Accepted for publication in ApJL, 4 figure
Using numerical models of bow shocks to investigate the circumstellar medium of massive stars
Many massive stars travel through the interstellar medium at supersonic
speeds. As a result they form bow shocks at the interface between the stellar
wind. We use numerical hydrodynamics to reproduce such bow shocks numerically,
creating models that can be compared to observations. In this paper we discuss
the influence of two physical phenomena, interstellar magnetic fields and the
presence of interstellar dust grains on the observable shape of the bow shocks
of massive stars.
We find that the interstellar magnetic field, though too weak to restrict the
general shape of the bow shock, reduces the size of the instabilities that
would otherwise be observed in the bow shock of a red supergiant. The
interstellar dust grains, due to their inertia can penetrate deep into the bow
shock structure of a main sequence O-supergiant, crossing over from the ISM
into the stellar wind. Therefore, the dust distribution may not always reflect
the morphology of the gas. This is an important consideration for infrared
observations, which are dominated by dust emission.
Our models clearly show, that the bow shocks of massive stars are useful
diagnostic tools that can used to investigate the properties of both the
stellar wind as well as the interstellar medium.Comment: 7 pages, 4 figures, to be published in the Journal of Physics:
Conference Series (JPCS) as part of the proceedings of the 13th Annual
International Astrophysics Conferenc
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