Magnetic Stars Puzzle Solved

The problem has been solved by scientists of the Max Planck Institute for Astrophysics in
Garching. With 3-dimensional numerical simulations they have found the
magnetic field configurations that underly the strong magnetic fields
observed on the surface of the so-called magnetic A-stars and magnetic
White Dwarfs, and how these fields can survive for the life time of
these stars (Nature, 14 October 2004).

The results confirm the 'fossil
field' hypothesis, which proposes that these magnetic fields are
remnants of the magnetic field in the gas clouds from which stars are
born.

Fig.: Shape
of the magnetic field lines in a magnetic star, computed by numerical
simulations (stereo images upper left).They form a ring of field lines
twisted around each other (blue). Field lines protruding through the
surface of the star (red) are held together and stabilized by the
twisted ring inside the star. This is illustrated by the schematic
sketch (the lower right) and the cut through the star (upper right).
This magnetic field configuration drifts slowly outward (over a period
of hundreds of millions of years) under the influence of the finite
electrical resistivity of the star, then distorts into the shape of the
seam on a tennis ball (lower left), after which it disappears from the
star. Image: Max Planck Institute for Astrophysics. Click on the image for a larger version.

This discovery is important for three classes of stars in which strong
magnetic fields are observed. The most well-known are the so-called
`magnetic A-stars', otherwise known as normal stars (about 2 to 10
times heavier than the Sun) which have a magnetic field like a bar
magnet. An example is Alioth (Epsilon Ursae Majoris, the fifth star in
the Big Dipper).

Among the White Dwarf stars there are stars with
magnetic fields 100,000 stronger than this, and finally there are 'magnetars': neutron stars with fields 100 billion times stronger than
that of a commercial bar magnet. The magnetic field of all these stars
is smooth and static, in contrast with the field of the Sun and similar
stars, which is weaker, consists of small patches, and changes
continually.

Since
the discovery of magnetic stars over 50 years ago, there have been two
competing hypotheses for these magnetic fields. In one theory, the
field is assumed to be generated by convective motions in the core of
the star, in same way as the Earth's magnetic field.

The other is the 'fossil field hypothesis': the idea that it is just a remnant of the
magnetic fields trapped in the gas clouds from which stars are born.
Some evidence points to this possibility, for example the fact that the
observed fields do not change over time. The main problem with the
latter theory has been that no magnetic field configuration was known
that can survive for the life of a star. All configurations studied so
far turned out to be unstable, and would decay within a few years.

Since
the discovery of magnetic stars over 50 years ago, there have been two
competing hypotheses for these magnetic fields.This
demonstrated that two criteria would need to be fulfilled for the
fossil theory to be viable. First, stable field configurations have to
exist in stars, not self-evident in view of the slippery, fluid nature
of the stellar material. Secondly, there has to be a path towards such
a field configuration from the magnetic field with which the star is
born. This configuration has now been found by the Max Planck
researchers with numerical simulations in which the evolution of an
arbitrary unstable initial field configuration was followed to a stable
final state.

The stable state always turned out to have the same
shape - a ring (torus) of twisted field lines, similar to the
configurations used in modern controlled fusion experiments. It looks a
bit like a car tire in which broken steel wires of the wire mesh stick
out through the surface. At the surface of the star, these field lines
make up an approximate dipole field, as seen in the observations.

With
these results the Max Planck researchers have provided a solid basis
for the theory of magnetic stars: as remnants of the magnetic fields
threading the gas clouds from which stars are formed, and it explains
why the fields can survive for hundreds of millions of years. At the
same time, the results make it likely that the magnetic fields in White
Dwarfs and neutron stars have the same structure and stability.

Original work:
J. Braithwaite and H.C. Spruit
A fossil origin for the magnetic field in A-stars and white dwarfs
Nature, 431, 819-821 (14 October 2004)

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