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

Text: José Tadeo Arantes | FAPESP Agency

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

An observational study conducted now can provide important clarifications for understanding this phenomenon, as it has identified an earlier star, HD 45166, with magnetar-generating conditions. This is the first time a star has been observed with these conditions: its mass is large enough to explode in a supernova, and then collapse into a neutron star; And its magnetic field is strong enough to produce a magnetar during collapse.

The work was carried out by an international team led by the Israeli Tomer Schnarfrom the University of Amsterdam in the Netherlands. And she had an important participation from the Brazilian Alexandre Soares de Oliveirafrom the University of Valle do Paraíba (Univap). Article on this topic was published in the journal Sciences.

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

The flux density is determined by the number of magnetic field lines that cross a unit area. To get an idea of ​​what the researcher is saying, it is necessary to remember that in neutron stars, masses of 1.1 to 2.1 solar masses are compressed into spheres with a radius of only 20 kilometers, approximately. The surface of a neutron star is very small. This makes it possible to understand why the magnetic field has intensified so much.

Oliveira recalls some predictions made by the Standard Model of stellar evolution. “Stars up to eight times the mass of the Sun develop into white dwarfs. After they have discarded most of the material, what is left is a hot, dense mass about the size of Earth. However, when it is more than eight times the mass of the Sun, the star explodes as a supernova When its cycle is complete, the remaining material is gravitationally collapsed to form a neutron star.When the mass is much greater, the gravitational collapse after the supernova explosion leads to the formation of a black hole.

HD 45166 is the most massive magnetically evolved star yet discovered. The study in question showed it to have a magnetic field of 43 kilojoules. “Our calculations indicate that when it explodes as a type Ib or IIb supernova and undergoes gravitational collapse, its magnetic field will concentrate after a few million years due to the collapse and will likely become a neutron star with a magnetic field of magnitude 100 trillion gauss,” says the researcher.

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

The researcher provides details. HD 45166 is a binary system consisting of a star of type qWR [quasi–Wolf-Rayet]It is a massive, hot, massive, helium-evolving star with a main sequence star of spectral type B and, therefore, a blue star in its adult stage, but it has not evolved much. They are separated by about 10.5 astronomical units, or 10.5 times the average distance between the Earth and the Sun, and they orbit each other with a period of 22.5 years. The qWR is currently only slightly smaller than the Sun, yet 10 times hotter, while its companion star is two and a half times the size of the Sun and twice the temperature.

Historical

These and many other pieces of information raised by the study are the result of work that, in addition, spanned more than 20 years. Oliveira began studying HD 45166 in his doctoral research that he conducted from 1998 to 2003, initially in Pico dos Dias Observatoryfrom the National Laboratory for Astrophysics (LNA), located between the municipalities of Brazopolis and Piranguco, in the state of Minas Gerais, and subsequently in La Silla Observatory, from the European Southern Observatory (ESO) collaboration, located in the Atacama Desert, Chile. Tomer Schnar and his team collected information obtained from various facilities around the world, most notably the Canada-France-Hawaii Telescope (cft) in Mauna Kea, Hawaii.

“The spectrometry data produced by Shinar and his collaborators at CFHT was key,” Oliveira asserts. In astronomy and astrophysics, spectroscopy is a technique that analyzes the spectrum of polarized light emitted by objects to determine some of their properties, particularly the magnetic field. The researcher says: “The observed circular polarization properties in HD 45166, as well as the Zeeman effect, that is, the splitting of spectral rays, detected in some rays, confirm the presence of a strong magnetic field.”

The most active component of the HD 45166 binary is, of course, qWR. These Wolf-Rayet stars, named after the French astronomers Charles Wolfe and Georges Wright, who discovered them in 1867, are massive objects with characteristic broad and intense emission lines for helium and other heavier chemical elements (carbon, nitrogen and oxygen), testifying to their maturity, i.e. They are at an advanced stage in the stellar evolution cycle.

“Our star of interest is essentially the exposed helium core of a star that has lost its outer layers of hydrogen. Our suggestion is that it was formed by the merger of two lower-mass helium stars. At the present stage, it is massive enough to explode in a supernova and produce a neutron star.” , and strong enough in the magnetic field to generate a magnetar.

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

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