Betelgeuse, a boiling
and magnetic supergiant star !
An international
research team[1], lead by 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.
The dome of Telescope Bernard Lyot, at Pic du
Midi Observatory (credits : Pascal Petit)
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 magnetic field, due to its known ability to accelerate
charged particles.
Boiling and
magnetic, supergiant stars therefore seem to constitute perfect cosmic
laboratories to test the recent theories developed to
explain the
generation of magnetic fields in the Universe !
Contact :
Dr Pascal Petit, LATT-OMP, Tel: +33 5 61 33 28 28, e-mail: petit@ast.obs-mip.fr
Dr Renada
Konstantinova-Antova, Bulgarian Academy of Science, e-mail : antovi@astro.bas.bg
Dr Guy Perrin, LESIA,
e-mail : guy.perrin@obspm.fr