A star capable of forming a magnetar, the strongest magnet in the universe, has been identified

Text: José Tadeo Arantes | FAPESP Agency

Magnetars are objects that have the strongest magnetic fields known in the universe, with an average of 10¹³ to 10¹⁵ Gauss. For comparison, the magnetic field at the Earth’s surface ranges from 0.25 to 0.65 gauss. One hypothesis for the formation is that a magnetar is a neutron star whose former star already had a sufficiently expressive magnetic field which could have been greatly intensified during the supernova explosion and subsequent gravitational collapse that gave rise to the neutron star.

An observational study now could provide important clarifications for understanding this phenomenon, as it has identified a protostar, HD 45166, capable of generating a magnetar. This is the first time a star has been observed under these conditions: its mass is large enough to explode in a supernova and then collapse into a neutron star; Its magnetic field is strong enough that during collapse it produces a magnetar.

The work was carried out by an international team led by the Israeli Tomer Shenar, from the University of Amsterdam in the Netherlands. It had an important participation from the Brazilian Alexander Soares de OliveiraFrom the University of Vale do Paraíba (Univap). Article on this topic was published In the magazine Sciences.

“The star we identified, HD 45166, has a magnetic field of 43 kilogauss [43 X 10³ G]. It should produce a magnetar with a magnetic field of up to 100 trillion gauss. The physical explanation for this amazing growth is that gravitational collapse causes the star to shrink dramatically. As its surface becomes much lower, the magnetic field flux density grows proportionally FAPESP Agency.

The flux density is determined by the number of magnetic field lines crossing a unit area. To get an idea of ​​what the researcher is saying, it is necessary to remember that in neutron stars, masses ranging from 1.1 to 2.1 solar masses are compressed into spheres with a radius of about 20 kilometers. The surface of a neutron star is extremely small. This allows us to understand why the magnetic field is so strong.

Oliveira recalls some predictions from the Standard Model of stellar evolution. “Stars with a mass of up to eight times the mass of the Sun evolve into white dwarfs. After they have ejected most of their matter, what is left is the hot, dense mass, about the size of the Earth. However, when its mass is more than eight times the mass of the Sun, it explodes The star as a supernova when its cycle is completed. The remaining material collapses due to the influence of gravity to form a neutron star. When the mass is much greater, the gravitational collapse after the supernova explosion leads to the appearance of a black hole.

HD 45166 is the most magnetically evolved massive star yet discovered. The study in question showed that it has a magnetic field of 43 kilowatts. “Our calculations indicate that when it explodes as a type Ib or IIb supernova and enters gravitational collapse, within a few million years, its magnetic field will be concentrated due to the collapse and it will likely become a neutron star with a magnetic field of the order of 100 trillion gauss,” the researcher tells.

At that moment, HD 45166 will have created a magnetar, the strongest type of magnet known in the universe — more than 100 million times stronger than the strongest magnet ever produced by humanity. About 30 magnetars are currently known. HD 45166 is located about 3,200 light-years from Earth, in the direction of the constellation Monoceros.

The researcher provides the details. “HD 45166 is a binary system consisting of a qWR star [quasi–Wolf-Rayet]It is a massive, very hot, evolving helium star, a main sequence star of spectral type B, and therefore a blue star in adulthood, but not as evolved. They are separated by about 10.5 astronomical units, i.e. 10.5 times the average distance between Earth and the Sun, and they orbit each other with a period of 22.5 years. qWR is currently slightly smaller than the Sun, despite being ten times hotter, while its companion star is two and a half times the size of the Sun and twice as hot.

Historical

This information and much more gathered by the study is the result of more than 20 years of work together. Oliveira began studying HD 45166 for his doctoral research from 1998 to 2003, initially in Pico dos Dias Observatoryfrom the National Astrophysical Laboratory (LNA), located between the municipalities of Brazopolis and Piranguco, in the state of Minas Gerais, and later in the state of Minas Gerais. La Silla Observatory, from the collaboration of the European Southern Observatory (ESO), located in the Atacama Desert, Chile. Tomer Shenar and his team collected information obtained from various facilities around the world, especially the Canadian-French Hawaiian Telescope (I stopped), on Mauna Kea, Hawaii.

“The spectroscopic data produced by Shinar and collaborators at CFHT were fundamental,” Oliveira highlights. In astronomy and astrophysics, spectrophotometry is a technique that analyzes the spectrum of polarized light emitted by objects to determine some of their properties, especially the magnetic field. “The circular polarization properties observed in HD 45166, as well as the Zeeman effect, that is, the splitting of spectral lines, detected in some lines, confirm the presence of a strong magnetic field,” the researcher says.

The most active element in the binary system HD 45166 is of course qWR. These Wolf-Rayet stars, named after the French astronomers Charles Wolfe and George Wright, who discovered them in 1867, are massive objects with the characteristic broad, intense emission lines of helium and other heavier chemical elements (carbon, nitrogen, and oxygen), attesting to this. To its maturity, that is, it is in an advanced stage of the stellar evolution cycle.

“Our star of interest is essentially the exposed helium core of a star that has lost its outer hydrogen layers. The proposal we have made is that it was formed by the fusion of two lower-mass helium stars. At its current stage, it is massive enough to explode in a supernova and produce a neutron star, and is powerful enough to In the magnetic field to generate a magnetar.

Part of this work was funded by FAPESP through hand bag Abroad given to Oliveira.

Article A massive helium star with a magnetic field strong enough to form a magnetar Can be reached at: www.science.org/doi/10.1126/science.ade3293.