Artist’s representation of HD 45166 (credit: L. Calçada/ ESO)
Published on 11/13/2023
By José Tadeu Arantes | Agência FAPESP – Magnetars are neutron stars with the strongest magnetic fields of any known celestial object in the Universe, on the order of 1013-1015 gauss. For comparison, the magnetic field of Earth’s surface ranges from 0.25 to 0.65 gauss. The mechanism that gives rise to magnetars is unknown, but a widely accepted theory is that it begins when a massive star with an already intense magnetic field dies in a supernova explosion and collapses gravitationally into a neutron star, which is a superdense stellar remnant made up entirely of neutrons.
A recent observational study offers what may be an important contribution to a deeper understanding of the phenomenon, identifying a binary system (HD 45166) that contains a potential magnetar precursor. This is the first time scientists have observed a star in these conditions: it has enough mass to go supernova and then collapse into a neutron star, and its magnetic field is strong enough to produce a magnetar when it does so.
An article on the study is published in the journal Science. The study was conducted by an international collaboration. The first author of the article and its corresponding author is Tomer Shenar, an Israeli-born astronomer currently at the University of Amsterdam in the Netherlands. One of its co-authors and an important participant is a Brazilian, Alexandre Soares de Oliveira, a professor at the University of Paraiba Valley (UNIVAP).
“HD 45166, the binary system we identified, contains a star with a magnetic field of 43 kilogauss [43 x 103 G] and will produce a magnetar with a magnetic field on the order of 100 trillion gauss. The physical explanation for this astounding growth is that gravitational collapse will make it shrink drastically, and the flux density of the magnetic field will expand in proportion to the significant reduction in surface area,” Oliveira told Agência FAPESP. Magnetic field flux density is the number of magnetic field lines that pass through a unit area perpendicular to the magnetic field.
Here it is useful to note that astronomers typically consider a neutron star to have 1.1-2.1 times the mass of the Sun, a radius of about 10 km, and a very small surface. This may help understand why the intensity of the magnetic field increases so dramatically.
Oliveira recalled some of the predictions of the standard model of stellar evolution. “Stars with up to eight times the mass of the Sun evolve to white dwarves. After they eject most of their matter, what remains is a hot dense core with roughly the same size as Earth. However, if a star has more than eight solar masses, it explodes as a supernova on completing its cycle. The remaining matter eventually collapses under the influence of its own gravity, forming a neutron star. If the mass is even greater than this, gravitational collapse after the star goes supernova originates a black hole,” he said.
HD 45166 is the most magnetic-evolved massive star found to date. The study 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, in several million years its magnetic field will become concentrated owing to the collapse and it will probably become a neutron star with a magnetic field on the order of 100 trillion gauss,” he said.
At that point, HD 45166 will have originated a magnetar, the most powerful type of magnet known to exist in the Universe – more than 100 million times stronger than the strongest magnets produced by humans. About 30 magnetars have been detected so far. HD 45166 is around 3,200 light-years from Earth in the direction of the constellation Monoceros.
Oliveira supplied details. “HD 45166 is a binary system comprising a qWR [quasi Wolf-Rayet] star, which is a massive, extremely hot evolved helium star, and a main sequence star classified as spectral type B7 V – a blue star in adulthood but not very evolved. They’re separated by about 10.5 astronomical units [AU], or 10.5 times the mean distance between Earth and the Sun, and they orbit each other with a period of 22.5 years. The qWR is currently a little smaller than the Sun, but ten times hotter, while its companion has two and a half times the Sun’s volume and is twice as hot,” he said.
These findings are the outcome of more than 20 years of research. Oliveira began studying HD 45166 as part of his PhD research, which he conducted from 1998 to 2003, initially at the Pico dos Dias Observatory, which is owned and operated by the National Astrophysical Laboratory (LNA) and located in between Brazópolis and Piranguçu in Minas Gerais state, and later at the La Silla Observatory, located in the Atacama Desert region of Chile and operated by the European Southern Observatory (ESO).
The group led by Shenar added information obtained from several facilities around the world, especially the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii, USA.
“Shenar and the CFHT collaborators provided crucial spectropolarimetry data,” Oliveira said. Spectropolarimetry is used in astronomy and astrophysics to analyze the spectrum of polarized light emitted by celestial objects, as a means to determine some of their properties, especially magnetic field. “The circular polarization characteristics observed in HD 45166, as well as the Zeeman effect or spectral splitting detected in some lines confirmed the presence of a strong magnetic field.”
The more active component of the binary system is qWR. Wolf-Rayet (WR) stars are named after French astronomers Charles Wolf and Georges Rayet, who discovered them in 1867. They are massive stars with broad, intense emission lines characteristic of helium and other heavier chemical elements (carbon, nitrogen and oxygen) that attest to their maturity, i.e. the fact that they have reached an advanced stage of stellar evolution.
“Our star of interest is basically the exposed helium core of a star that has lost its outer layers of hydrogen. The theory we put forward is that it was formed by the fusion of two helium stars with less mass. In its current stage, it is massive enough to go supernova and produce a neutron star, and has a strong enough magnetic field to become a magnetar,” Oliveira said.
Part of the research was funded by FAPESP via a scholarship abroad awarded to Oliveira.
The article “A massive helium star with a sufficiently strong magnetic field to form a magnetar” is at: www.science.org/doi/10.1126/science.ade3293.