Galactic-scale magnetic fields could ‘smooth out’ the stellar bow shocks

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Posted December 23, 2013

The stars we see in our nigh sky are not really stationary. It only seems to us that these celestial objects remain in the same place year after year, as we simply cannot notice any observable differences due to small timescale and vast distances. However, the modern astronomy makes it possible to measure the velocities, at which stars move through space. Sometimes this velocities are high enough to cause a strong side effect called the stellar bow shock.

Herschel PACS image of the bow shock of a-Orionis. The bow shock consists of multiple smooth arcs, that lack large-scale instabilities. Image courtesy of the researchers.

Herschel PACS image of the bow shock of α-Orionis. The bow shock consists of multiple smooth arcs, that lack large-scale instabilities. Image courtesy of the researchers.

This phenomenon happens when star’s velocity is high enough to create a spherical-shaped area of increased density between its own stellar wind colliding with the particles contained inside the interstellar medium.

It should be noted that the existing analytic models and astrophysical computer simulations suggest the formation of instabilities, when star’s velocity exceeds the velocity of the stellar winds it creates. However, the recent Herschel observations of Betelgeuse (α-Orionis) in 2012 clearly demonstrated that the bow shock for this object is smooth and does not contain any substantial instabilities that should be present there according to theoretical predictions. And this is not the only case: similar outcomes were observed for two other stars, UU Aurigae and X Pavonis in the same year.

The previous attempts to explain the absence of instabilities were based on assumption that the observed bow shocks are still relatively ‘young’ and the structural disturbances have not yet had time to emerge. And now, one of the most recent studies formulated an alternative hypothesis that the growth of bow shock instabilities may be inhibited by a large-scale interstellar magnetic field.

A team of scientists from Belgium and France performed an analysis of Betelgeuse bow shock in a paper published at Betelgeuse resides in the Orion arm of our galaxy at a distance of approximately 8000 kpc from the galactic center. As estimated by earlier researchers, this region is also influenced by a weak magnetic field stretching over a distance of more than 100 parsecs. The authors note that, according to earlier analytical predictions, this uniform magnetic field aligned with the motion of the particle flow (in parallel to the direction of motion), could theoretically inhibit the formation of bow shock instabilities.

In order to verify the possibility of such effect, the team created a 2.5-D model of the Betelgeuse bow shock for two opposite scenarios: with and without the interstellar magnetic field.

Density of the circumstellar medium in g cm 3 after 20 000 (left), 50 000 (centre) and 100 000 (right) yr, for a simulation without an interstellar magnetic

Density of the circumstellar medium in g/cmafter 20 000 (left), 50 000 (centre) and 100 000 (right) yr, for a simulation without an interstellar magnetic field (right side of each panel) and a 3 μG interstellar magnetic field (left side of each panel). In the presence of the interstellar magnetic field the instabilities shrink as they move past the star. Images courtesy of the researchers.

The results of the simulation were unambiguous. The magnitude of stellar bow shock instabilities increased over time without the presence of external magnetic field. In the opposite case, the instabilities emerged, but their growth was stopped and even reversed over a timescale of several thousand years. The magnitude of distortions also decreased under the influence of stronger magnetic fields. However, the authors note that even a relatively weak magnetic field (even lower than estimated by the previous observations) could be sufficient to accomplish the same. The team also points that the size of the instabilities could be reduced further by a warm interstellar medium (~ 8000 K).

The simulation also showed that the presence of the interstellar magnetic field does not influence the overall size or shape of the bow shock. The authors of the study are going to perform a similar simulation of bow shock of Betelgeuse in a three-dimensional (3D) space in the nearest future.

By Alius Noreika, Source:

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