The irresistible pull – when massive stars collide

All neutron stars are magnetic, but some are more magnetic than others. The latter, so-called magnetars, are the strongest magnets in the Universe. How do these massive stars acquire their large magnetic field?

A team of astrophysicists from Germany and the UK may now have solved the more than 70-year-old conundrum of the origin of strong magnetic fields in massive stars. Published today in Nature, they have shown how strong magnetic fields can be formed in stellar mergers by developing a model with large computer simulations.

See video: https://youtu.be/F8ybsth83VY

First author, Fabian Schneider, currently at Heidelberg University in Germany and previously a Hintze Fellow in Oxford’s Physics Department, said: ‘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.’

It is these stars that astronomers believe to form highly magnetic neutron stars when they explode in supernovae.

Sebastian Ohlmann from the Max Planck Society in Garching, Germany, said: ‘Over a decade ago, it was suggested that strong magnetic fields might be produced when two stars collide, but up until now, we had not been able to test this hypothesis, because we did not have the necessary computational tools.’

In the study published today, the team utilised the novel AREPO code and ran it on computing clusters of the Heidelberg Institute for Theoretical Studies (HITS). They showed that a strong magnetic field is indeed produced thanks to the strong shear and the large turbulence present in the merger of two stars.

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.

Philipp Podsiadlowski from the University of Oxford’s Department of Physics, said: ‘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. We then suggested that this star may also have obtained its strong magnetic field in the merger process and our new simulations demonstrate exactly this.’
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.

Friedrich Röpke from HITS, said: ‘These magnetars are thought to have the strongest magnetic fields in the Universe – up to one hundred million times stronger than the strongest magnetic field ever produced by humans. Our simulations show that the generated magnetic field could be sufficient to explain the exceptionally strong magnetic fields inferred to exist in magnetars. 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.’

The DOI number will be 10.1038/s41586-019-1621-5. Once published online, the paper will be available in Nature here: https://www.nature.com/articles/s41586-019-1621-5

Image caption:
“The birth of a magnetic star”
The simulation shown here marks the birth of a magnetic star such as Tau Scorpii. The image is a cut through the orbital plane where the colouring indicates the strength of the magnetic field and the hatching represents its field lines. (Pictures: Ohlmann/Schneider/Röpke)

Video:
‘Stellar magnetism: When stars collide’
Magnetic field strength in orbital plane: https://youtu.be/F8ybsth83VY
Image and video credits: Ohlmann/Schneider/Röpke

For more information:
Ruth Abrahams
University of Oxford
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