An article (link to the file in PDF format) published in the journal “Science” reports a study that identifies one of the stars in the binary system HD 45166 as the likely precursor of a magnetar. A team of researchers led by Tomer Shenar of the University of Amsterdam used various telescopes to study a so-called helium star, a massive star that is nearing the end of its life and has already ejected its outer layers and is left with a core formed by helium on its surface. This star revealed an extremely powerful magnetic field, estimated to be about 100,000 times the Earth’s.
Massive stars can end their lives leaving a neutron star or a black hole, depending on their mass. Models of neutron star formation are quite accurate but it’s unclear under what conditions a neutron star could generate an extremely strong magnetic field and become a magnetar. This new study ties magnetars to helium stars.
The binary system HD 45166 is formed by a star that is much more massive than the Sun which is still in the prime of its life and by another massive star which is now in the final stage of its life. The agony of a massive star is violent, with very intense solar winds that eject the outer layers of hydrogen and can leave behind a shell of helium that gives helium stars their names.
So far, helium stars were considered interesting in the study of the last stages of the life of massive stars and for the possibility that they also form from the merger of two white dwarfs. Tomer Shenar stated that he has a bit of an obsession with the helium star of the HD 45166 pair, a star known for over a century but whose nature has remained difficult to fully explain, at least so far.
Tomer Shenar’s team conducted examinations of the helium star using the Canada-France-Hawaii telescope’s ESPaDONS spectropolarimeter and three spectrographs mounted on three different telescopes. This made it possible to estimate its magnetic field at around 43,000 gauss, about 100,000 times more powerful than the Earth’s magnetic field. It’s the first helium star for which a magnetic field has been discovered, the strongest known so far in a massive star.
Actually, the new estimates of the masses of the two stars of the HD 45166 system indicate that the helium star has a mass that is “only” about twice the Sun’s, even if the estimate is imprecise. That’s half of the previous estimates but still higher than the Chandrasekhar limit of about 1.4 solar masses which indicates the limit beyond which a stable white dwarf cannot form. In those cases, the core that remains at the end of the star’s life collapses into a neutron star or a black hole. The analysis also indicates that it formed by the merger of two smaller stars.
In the case of the helium star in the HD 45166 system, models indicate that it will collapse into a neutron star. Most interestingly, the discovery of its very strong magnetic field suggests that the neutron star will be of the magnetar class. In fact, the collapse will lead to its mass being compacted, enormously increasing the already very powerful magnetic field, which could reach 100 trillion gauss.
The combined use of various sophisticated instruments offered a new view of the helium star of the HD 45166 system. Tomer Shenar’s intuition that led to investigating its magnetism and the set of information gathered led to a possible explanation for the formation of magnetars. This will make it possible to study other helium stars to search for other potential magnetar progenitors and obtain new information on the most powerful magnets in the universe.
