Magnetar Formation: It's A Game Of Pass The Parcel

ESO/L. Calçada. Artist's impression of a magnetar in Westerlund 1.

Using the world’s most advanced optical instrument, the ESO’s Very Large Telescope (VLT), astronomers have made a discovery which could help shed light on something that has puzzled scientists for decades: how magnetars form.

When the life of a massive star comes to an end and it explodes dramatically as a supernova, either a neutron star or black hole is formed. A magnetar is a rare and extremely unusual type of fast-spinning neutron star. They’re tiny yet incredibly dense; a mere teaspoon of magnetar material would have a mass of around a billion tonnes. They’re also the most powerful magnets known to exist in the entire universe, but the mechanism behind their formation has been shrouded in mystery.

In an attempt to find out more about these enigmatic objects, astronomer Simon Clark of the Open University, U.K., and colleagues turned to a star cluster known as Westerlund 1. This young cluster, which is found 16,000 light-years away from us within a constellation called Ara, contains one of the few known magnetars in the Milky Way called CXOU J164710.2-455216.

According to Clark, this magnetar was puzzling since they had previously demonstrated that it formed from the explosion of a star around 40 times as massive as our Sun. This left the team scratching their heads since the explosion of a star this massive should result in a black hole.

The team hypothesized that this magnetar could have formed through interactions with another massive star, but prior to this study no companion star was discovered that fitted the bill. However, the astronomers managed to identify a blue supergiant star that they postulate may have once orbited the star that was destined to become the magnetar. This highly luminous star, named Westerlund 1-5, appeared to be fleeing Westerlund 1 at a high velocity, as would be expected if it had been flung out of orbit by the supernova explosion.

In a report published in Astronomy & Astrophysics, the scientists describe the likely series of events that led up to the formation of the magnetar. As Westerlund 1-5 started to run out of fuel, it began to unload a substantial amount of its gas onto its smaller companion star, causing it to spin faster. According to the scientists, this dramatic increase in rotation was responsible for the increase in the star’s magnetic field.

As the partner star grew in size it reached a stage where it became so massive that it then started to cast off a substantial amount of its mass, some of which was passed back to Westerlund 1-5. This loss of mass was critical since if the star exploded when it was extremely massive it would have collapsed into a black hole.

In support of this second mass transfer, Westerlund 1-5 was found to have an unusual chemical composition. In particular, they found that was rich in carbon when it shouldn’t be. The team believe that shortly before its demise, the pre-magnetar star burned helium into carbon and shed this carbon onto Westerlund 1-5.

“It is this process of swapping material that has imparted the unique chemical signature to Westerlund 1-5 and allowed the mass of its companion to shrink to low enough levels that a magnetar was born instead of a black hole- a game of stellar pass-the-parcel with cosmic consequences!” said Francisco Najarro, one of the authors of the study.

Although this work is not definitive, it could serve as a reasonable theory to explain how at least some of these perplexing super magnets come into existence. 


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