Do magnets ever lose their magnetism

How the strongest magnets in the universe are made

Heidelberg - About a tenth of the known neutron stars belong to a very special subgroup: Their magnetic field is about a thousand times stronger than that of "ordinary" neutron stars, which makes them the strongest magnets in the universe. In other words, it is "up to a hundred million times stronger than the strongest magnetic field ever generated by humans," says Friedrich Röpke from the Heidelberg Institute for Theoretical Studies (HITS).

But how can such an object come into being? A German-British research team believes they have found the answer with extensive computer simulations: Extremely strong magnetic fields are created when two stars merge. If such stars explode in a supernova, magnetars could arise from them. The researchers from the University of Heidelberg, the Max Planck Society, the Heidelberg Institute for Theoretical Studies and the University of Oxford have published their findings in the journal "Nature".

The context

The universe is pervaded by magnetic fields. For example, our sun has an envelope in which convective currents continuously generate magnetic fields. "Although massive stars do not have such a shell, we still observe a strong, large-scale magnetic field on the surface of around ten percent of them," says Fabian Schneider from the Center for Astronomy at Heidelberg University and first author of the study.

According to the information, such fields were discovered as early as 1947, without their origin having yet been fully clarified. More than a decade ago, scientists suspected that strong magnetic fields are generated when two stars merge. "Up to now, however, we have not been able to test this hypothesis because we did not have the necessary computer tools," adds Sebastian Ohlmann from the data center of the Max Planck Society in Garching.

Now the researchers used the so-called Arepo code, a highly dynamic simulation code on the computer clusters of the HITS, to explain the properties of the star Tau Scorpii. It is a massive magnetic star located 500 light years from Earth in the constellation Scorpio and has a radius six times larger than our sun.

In 2016, Schneider and Philipp Podsiadlowski from the University of Oxford found out that Tau Scorpii was a so-called blue straggler. These Blue Stragglers are the result of fused stars. "We assume that Tau Scorpii received its strong magnetic field during the merging process," says Podsiadlowski. With its computer simulations for Tau Scorpii, the German-British research team has now shown that such a field can form due to strong turbulence when two stars merge.

Quite a common process

According to the Heidelberg University, star mergers occur relatively frequently: Scientists assume that around ten percent of all massive stars in the Milky Way are the product of such a process. This in turn would fit very well with the frequency with which magnetic massive stars are observed, as Schneider pointed out.

Astronomers assume that precisely these stars could form magnetars when they explode in supernovae. This is also likely to happen to Tau Scorpii when the magnetic star explodes at the end of its life. (red, APA, 10.10.2019)