A dynamo mechanism could explain the incredibly strong magnetic fields in
white dwarf stars according to an international team of scientists, including
a University of Warwick astronomer.
One of the most striking phenomena in astrophysics is the presence of
magnetic fields. Like the Earth, stars and stellar remnants such as white
dwarfs have one. It is known that the magnetic fields of white dwarfs can be
a million times stronger than that of the Earth. However, their origin has
been a mystery since the discovery of the first magnetic white dwarf in the
1970s. Several theories have been proposed, but none of them has been able
to explain the different occurrence rates of magnetic white dwarfs, both as
individual stars and in different binary star environments.
This uncertainty may be resolved thanks to research by an international team
of astrophysicists, including Professor Boris Gänsicke from the University
of Warwick and led by Professor Dr. Matthias Schreiber from Núcleo Milenio
de Formación Planetaria at Universidad Santa María in Chile. The team showed
that a dynamo mechanism similar to the one that generates magnetic fields on
Earth and other planets can work in white dwarfs, and produce much stronger
fields. This research, part-funded by the Science and Technology Facilities
Council (STFC) and the Leverhulme Trust, has been published in the
prestigious scientific journal Nature Astronomy.
Professor Boris Gänsicke of the Department of Physics at the University of
Warwick said: "We have known for a long time that there was something
missing in our understanding of magnetic fields in white dwarfs, as the
statistics derived from the observations simply did not make sense. The idea
that, at least in some of these stars, the field is generated by a dynamo
can solve this paradox. Some of you may remember dynamos on bicycles:
turning a magnet produces electric current. Here, it works the other way
around, the motion of material leads to electric currents, which in turn
generate the magnetic field."
According to the proposed dynamo mechanism, the magnetic field is generated by
electric currents caused by convective motion in the core of the white dwarf.
These convective currents are caused by heat escaping from the solidifying
core.
"The main ingredient of the dynamo is a solid core surrounded by a
convective mantle—in the case of the Earth, it is a solid iron core
surrounded by convective liquid iron. A similar situation occurs in white
dwarfs when they have cooled sufficiently," explains Matthias Schreiber.
The astrophysicist explains that at the beginning, after the star has
ejected its envelope, the white dwarf is very hot and composed of liquid
carbon and oxygen. However, when it has sufficiently cooled, it begins to
crystallize in the center and the configuration becomes similar to that of
the Earth: a solid core surrounded by a convective liquid. "As the
velocities in the liquid can become much higher in white dwarfs than on
Earth, the generated fields are potentially much stronger. This dynamo
mechanism can explain the occurrence rates of strongly magnetic white dwarfs
in many different contexts, and especially those of white dwarfs in binary
stars" he says.
Thus, this research could solve a decades-old problem. "The beauty of our
idea is that the mechanism of magnetic field generation is the same as in
planets. This research explains how magnetic fields are generated in white
dwarfs and why these magnetic fields are much stronger than those on Earth.
I think it is a good example of how an interdisciplinary team can solve
problems that specialists in only one area would have had difficulty with,"
Schreiber adds.
The next steps in this research, says the astrophysicist, are to perform a
more detailed model of the dynamo mechanism and to test observationally the
additional predictions of this model.
Reference:
Matthias R. Schreiber et al. The origin and evolution of magnetic white dwarfs
in close binary stars, Nature Astronomy (2021). DOI:
10.1038/s41550-021-01346-8
No comments:
Post a Comment