Star is capable of forming the universe’s most powerful magnet – 9/6/2023 – Science

Magnetars are the objects that have the strongest magnetic fields known in the Universe – on the order of 1,013 to 1,015 gauss. For comparison, the magnetic field at the Earth’s surface ranges from 0.25 to 0.65 gauss.

One hypothesis of formation is that the magnetar is a neutron star whose precursor star already had a magnetic field expressive enough that it would have been enormously intensified during the supernova explosion and subsequent gravitational collapse that originated the neutron star.

An observational study carried out now could bring important clarifications for understanding the phenomenon, as it identified a precursor star, HD 45166, with conditions to generate a magnetar.

It is the first time that a star with these conditions has been observed: its mass is great enough to explode in a supernova and, subsequently, 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 Shenar, from the University of Amsterdam, in the Netherlands. And it had the important participation of the Brazilian Alexandre Soares de Oliveira, from the University of Vale do Paraíba (Univap). An article about it was published in Science magazine.

“The star we identified, HD 45166, has a magnetic field of 43 kilogauss [43 X 103 G]. And it should produce a magnetar with a magnetic field of the order of 100 trillion gauss. The physical explanation for this astounding growth is that gravitational collapse causes the star to shrink dramatically. And, as its surface is greatly reduced, the flux density of the magnetic field grows proportionally”, explains Oliveira to Agência Fapesp.

The flux density is given by the number of magnetic field lines crossing a unit area. And, to have an idea of ​​what the researcher is saying, it is necessary to remember that, in neutron stars, masses of the order of 1.1 to 2.1 solar masses are compressed into spheres of only 20 kilometers in radius, approximately. The surface of the neutron star is extremely small. And this makes it possible to understand why the magnetic field intensifies so much.

Oliveira recalls some predictions of the standard model of stellar evolution. “Stars with masses up to eight times greater than the mass of the Sun evolve as white dwarfs. After they eject much of their material, what is left is the hot and dense core, with a size approximately equal to that of the Earth. However, when the mass is greater than eight solar masses, the star explodes as a supernova upon completion of its cycle. And the remaining material collapses due to gravitational effect, forming a neutron star. When the mass is even much greater than that, the gravitational collapse after the explosion in supernova originates a black hole.”

HD 45166 is the most magnetic evolved massive star found to date. The study in question showed that it has a magnetic field of 43 kilogauss.

“Our calculations suggest that when it explodes as a type Ib or IIb supernova and undergoes gravitational collapse, a few million years from now its magnetic field will concentrate due to the collapse and it will likely become a neutron star with a magnetic field of order of 100 trillion gauss”, informs the researcher.

At that moment, HD 45166 will have originated a magnetar, the most powerful type of magnet known in the Universe – more than 100 million times stronger than the strongest magnets produced by mankind. About 30 magnetars are currently known. HD 45166 lies about 3,200 light-years from Earth, in the constellation of Monoceros.

Researcher provides details. “HD 45166 is a binary system, formed by a star of type qWR [quasi–Wolf-Rayet], which is an evolved helium star, massive and extremely hot, and by a main sequence star of spectral type B, therefore, a blue star in its adult phase, but not so evolved. They are separated by about 10.5 astronomical units, that is, by 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 barely smaller than the Sun, yet ten times hotter, while its companion star is two and a half times the volume of the Sun and twice its temperature.”

Historic

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 to study HD 45166 in his doctoral research, which was carried out from 1998 to 2003, initially at the Pico dos Dias Observatory, of the National Astrophysics Laboratory (LNA), located between the municipalities of Brazópolis and Piranguçu, in Minas Gerais. , and then at the La Silla Observatory, part of the European Southern Observatory (ESO) collaboration, located in the Atacama Desert in Chile. And Tomer Shenar and his team aggregated information obtained from various installations around the world, most notably the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii.

“The spectropolarimetry data produced by Shenar and collaborators at the CFHT were fundamental”, emphasizes Oliveira. In astronomy and astrophysics, spectropolarimetry is a technique that analyzes the spectrum of polarized light emitted by objects to determine some of their properties, particularly the magnetic field. “The characteristics of circular polarization observed 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”, says the researcher.

The most active component of the HD 45166 binary system is, of course, the qWR. These Wolf-Rayet-type stars, named after the French astronomers Charles Wolf and Georges Rayet, who discovered them in 1867, are massive objects with broad and intense emission lines characteristic of helium and other heavier chemical elements (carbon, nitrogen and oxygen), which attest to their maturity, that is, the fact that they are in an advanced phase of the stellar evolution cycle.

“Our star of interest is basically the exposed helium core of a star that has lost its outer hydrogen layers. We propose that it formed by the merger 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 strong enough in a magnetic field to generate a magnetar”, concludes Oliveira.

Part of this work was funded by Fapesp through a scholarship abroad granted to Oliveira.

The article A massive helium star with a sufficiently strong magnetic field to form a magnetar can be accessed here.