Artist’s impression of HD 45166, the star that might become a magnetar

A possible future magnetar turns being a new astronomical object

Astronomers studied the mysterious HD 45166 star and found it to be a new type of astronomical object. Have they found how magnetar is born?

A magnetar is a young neutron star, which means it has a high density despite its tiny dimensions, with a huge magnetic field and emitting x-rays and gamma-rays. Its formation mechanism is unknown; however, astronomers seem to have found a star that is a good candidate to become a magnetar and this led to the discovery of a new type of astronomical object: massive magnetic helium stars.

Artist’s impression of HD 45166, the star that might become a magnetar
Artist’s impression of HD 45166. Credit: ESO


The star

The star under the magnifying glass is the HD 45166. It is part of a binary system, rich in helium, and has a smaller mass than our Sun. That’s all that was known about it up to now. It was a mysterious star.

Tomer Shenar, the lead author of the study, thought that to solve the case, the hypothesis of a magnetic field could be the answer. It is known it can influence the behavior of the stars, explaining why no models were able to well describe the star.

Observations of HD 45166’s spectrum were made at the Canada-France-Hawaii Telescope (CFHT), at the Pico dos Dias (Laboratorio Nacional de Astrofisica, LNA), at the ESO’s La Silla Observatory and at the Mercator Telescope. All the results lead to confirmation of Shenar’s hypothesis: the star has a magnetic field of 43 thousand gauss (the gauss is the unit of measurement of the magnetic flux density, for the Earth it is around 0.45 gauss). This is an important result since it’s the first helium magnetar to be found, a new type of astronomical object.

Three planets dance over La Silla
Three planets dance over La Silla, where some of the observation of HD 45166’s spectrum were done. Credit: ESO


What will be its future?

Shenar’s team expects that HD 45166 will evolve until it collapses into a magnetar, as shown by the models elaborated for the system. This will happen with an increase in the magnetic field (that will reach 100 trillion gauss) as the core collapses to reach a diameter of 12 km. Just after the collapse, moreover, the magnetar could reach a spin period shorter than 40 ms, like the pulsar located inside the Crab Nebula, but it won’t be enough to generate a superluminous supernova (called superluminous since it’s 10 times brighter than a standard supernova).

How did HD 45166 evolve?

Astronomers, then, tried to explain how the star in question formed. First, they excluded the possibility of being the descendant of a massive star. Models, indeed, do not predict the possibility for those kinds of stars to strip themselves, while the HD 45166’s companion is too far to be able to strip it.
They also considered the idea of the HD 45166 being the result of the merging of two white dwarfs rich in carbon and oxygen. This merge, however, would probably explode immediately into a supernova.

The winning hypothesis resulted in the merging of two small stars rich in helium. This means that, at the beginning, the system was in a triple configuration with two stars close and one further. At a certain point, the biggest component of the sub-binary system expanded, stripping its companion. Later, the second star started expanding causing an unstable exchange of mass and leading to the formation of a gaseous envelope around the two. This phase ends with both the helium core merging to form a star that models show to be similar to HD 45166, confirming this as the most plausible hypothesis.


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Matteo Ferrarini

Matteo Ferrarini

B.Sc in aerospace engineering, now studying the world of renewable energy. Always looking at the stars, but sometimes you can find me underwater.

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