Betelgeuse, a boiling and magnetic supergiant star

Jun 18, 2010
The dome of Telescope Bernard Lyot, at Pic du Midi Observatory (credits : Pascal Petit)

(PhysOrg.com) -- An international research team, lead by French astrophysicists from the Laboratoire d'Astrophysique de Toulouse-Tarbes, has detected a magnetic field at the surface of the supergiant star Betelgeuse. This observational result, published in the journal Astronomy & Astrophysics, demonstrates that, in spite of the theoretical framework usually proposed to account for the magnetism of astrophysical bodies like the Earth or the Sun, the rotation of cosmic objects is not a necessary ingredient to trigger the efficient generation of a magnetic field.

According to a scenario elaborated more than half a century ago, the rotation of stars like the Sun produces huge flows of ionized material in their internal layers. These large-scale flows trigger a dynamo mechanism causing the continuous generation of their magnetic field. This process, called a "large-scale dynamo", is generally invoked to describe the solar magnetic cycle, which is particularly spectacular during solar eruptive phases. Yet, even when the Sun is having a temporary respite in its magnetic firework, for instance during the last, unusually long activity minimum that recently ended, our star still hosts a surface magnetic field. The origin of this residual magnetism, which seems to be unaffected by the solar cycle, is still a disputed question among astronomers.

The key of this enigma may be hidden in supergiant stars, a class of objects of which Betelgeuse is one of the most famous members. With about 15 times the solar mass, 1,000 solar radii and a luminosity 100,000 times higher than the Sun's, Betelgeuse is a star reaching the end of its life while burning the last remaining nuclear fuel at its disposal before exploding as a supernova. In addition, another physical parameter of Betelgeuse is differing from the solar case : its rotation is extremely slow. It takes probably several years for Betelgeuse to complete a full rotation, against barely one month for the Sun. This situation seems inadequate to allow for the onset of a large-scale dynamo.

A model of the convective envelope of Betelgeuse (credits : MPA/GRAAL/LESIA)

However, observations collected with the NARVAL instrument at Telescope Bernard Lyot (Pic du Midi Observatory, France) reveal a weak polarization level in the light emitted by Betelgeuse : an observational clue unveiling the presence of a weak magnetic field at the surface of the star. This observation is therefore demonstrating that a fast rotation is not a necessary ingredient for the efficient production of a magnetic field. Supergiant stars may use another trick : vigorous convective motions, similar to a continuous boiling, are evacuating the huge amount of energy released in the stellar core by nuclear reactions. Observations obtained at Pic du Midi suggest that this continuous agitation is able, in itself, to generate the stellar magnetic field, through "small-scale" dynamo processes operating on the same scale as the convective cells. Since the Sun itself is exhibiting turbulent motions in its outer layers, it could very well be able to host a similar type of small-scale dynamo, that could be (at least partly) responsible for its residual magnetism during activity minima.

Furthermore, the detection of a magnetic field on Betelgeuse is precious for several reasons. Massive stars reaching the end of their evolution, like Betelgeuse, contribute to spread heavy chemical species in the Galaxy, thanks to a strong wind constituted of ionized particles. Current theoretical models have trouble explaining why the wind ejection is so efficient in supergiants. Here again, the solution is maybe linked to the presence of the , due to its known ability to accelerate charged particles.

Boiling and magnetic, supergiant therefore seem to constitute perfect cosmic laboratories to test the recent theories developed to explain the generation of magnetic fields in the Universe.

Explore further: Mixing in star-forming clouds explains why sibling stars look alike

More information: "The magnetic field of Betelgeuse: a local dynamo from giant convection cells?", by M. Auriere et al. Published in Astronomy & Astrophysics, 2010, vol. 516, L2 www.aanda.org/10.1051/0004-6361/201014925

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Caliban
3 / 5 (2) Jun 18, 2010
While I don't doubt the presence of something at least analogous to a "convective cell" below the surface of B's atmosphere- why are they necessary to explain the magnetism of B? Why not just the inherent differential in speed between various layers of material in the star due to its rotation? Would not this alone be sufficient for the creation of a "super dynamo"? Or am I missing something? Somebody help!
Jigga
1 / 5 (2) Jun 18, 2010
The foamy structure of Betelgeuse is pretty similar to giant elementary particle or atom nuclei.
eachus
not rated yet Jun 18, 2010
More to the point, Betelgeuse expanded to become a red giant. Conservation of momentum implies that its rotation was much faster when it was smaller. So a dynamo may have existed in the past. If so, what would make it stop when the star expanded? Could happen, but I suspect that any damping mechanism hasn't had time to work. The red giant phase of large stars doesn't seem to last long. (Hmmm. I should offer evidence for that, but it doesn't seem to need a research project. Count the number of stars within a certain distance over say 5 solar masses. Now check how many are red giants? I doubt if it would be over 1%.)
omatumr
1 / 5 (3) Jun 18, 2010
Neutron repulsion - recorded as mass in nuclear rest mass data of all known nuclei - is the key to understanding the source of magnetism and energy in stars.

With kind regards,
Oliver K. Manuel

barakn
not rated yet Jun 19, 2010
...Why not just the inherent differential in speed between various layers of material in the star due to its rotation? Would not this alone be sufficient for the creation of a "super dynamo"? Or am I missing something? Somebody help!

Thomas G. Cowling proved that any self-sustaining dynamo cannot be axially symmetric.
in7x
not rated yet Jun 21, 2010
eachus, remember, Betelgeuse is shrinking/contracting:

http://www.physor...639.html