An irresistible pull – when massive stars collide

Simulation, originally developed for cosmology, shows that a strong magnetic field is indeed produced in the merger of two stars

October 09, 2019

All neutron stars are magnetic, but some are more magnetic than others. The latter, so-called magnetars, are the strongest magnets in the Universe. The reason for their exceptionally large magnetic field is most probably that they formed in supernovae of already highly magnetized stars. But how do these massive stars acquire their large magnetic field? A team of astrophysicists from Germany and the UK may now have solved this more than 70-year-old conundrum of the origin of strong magnetic fields in massive stars. With the help of large computer simulations, they developed a model which shows that these can be formed in stellar mergers. The results were published (09 Oct 2019) in the scientific journal Nature.

Our Universe is threaded by magnetic fields. “We know that the Sun has a turbulent envelope in which its magnetic field is continuously generated. But more massive stars do not have such an envelope. Still, about 10 percent have a strong, large-scale surface magnetic field whose origin has eluded us since their discovery in 1947,” says Fabian Schneider from Heidelberg University in Germany, first author of the study. It is these stars that astronomers believe to form highly magnetic neutron stars when they explode in supernovae.

“Already over a decade ago, it was suggested that strong magnetic fields might be produced when two stars collide”, says Sebastian Ohlmann from the Max Planck Society in Garching, Germany. “But up until now, we had not been able to test this hypothesis, because we did not have the necessary computational tools.” This time, the team utilised the novel AREPO code, developed by Volker Springel and Rüdiger Pakmor from MPA, that was originally developed for cosmological simulations. “I am pleased that this method is of great utility in other areas of astrophysics as well.” says Volker Springel. Here it shows that a strong magnetic field is indeed produced in the merger of two stars, thanks to the strong shear and turbulent motions involved. Stellar mergers occur frequently, and it is thought that about 10 percent of all massive stars in the Milky Way are the products of stellar mergers – a good match with the occurrence rate of magnetic stars.

When stars merge, they appear younger than they really are. This phenomenon is well known, and such stars are called blue stragglers. “In 2016, we realised that the magnetic star Tau Scorpii (τ Sco) is a blue straggler and could show that, if τ Sco was a merger product, it would explain its anomalously young age,” remembers Philipp Podsiadlowski from the University of Oxford, UK. “We then suggested that this star may also have obtained its strong magnetic field in the merger process.”

At the end of its life, τ Sco will explode in a supernova when its core collapses and most probably leave behind a highly magnetized neutron star. “These magnetars are thought to have the strongest magnetic fields in the Universe – one hundred million times stronger than the strongest magnetic field ever produced by humans,“ says Friedrich Röpke from HITS. “Our simulations show that the generated magnetic field could be sufficient to explain the exceptionally strong magnetic fields inferred to exist in magnetars,” adds Fabian Schneider. “It makes our model a promising channel to explain the origin of such extremely strong magnetic fields. It is great to see that this idea now seems to work out so beautifully.”


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