M. Guedel/ESA

Strange attraction.
Astronomers say the star known as AB Aurigae (orange, at right) is emitting x-rays produced by a magnetic field that it could not have generated on its own.

A Magnetic Surprise

Astronomers have detected a strong magnetic field emanating from a star that computer models say shouldn't be able to produce one. The field is coming from a star called AB Aurigae--located about 460 light-years away--and its existence lends further credence to one of the more bizarre astronomical discoveries in recent years.

Powerful magnetic fields aren't easy for stars to generate. Until now, astronomers assumed that they only arose inside the fierce internal furnaces of hot, young stars much larger than the sun, or from the violent interaction between two closely orbiting stars in a binary system. Neither situation is true for AB Aurigae. The star is only about 2.7 times the mass of our sun, which should make it too small and cool to produce a strong magnetic field. But when a team led by Manuel Guedel of the Paul Scherrer Institute in Villigen, Switzerland, used the European Space Agency's orbiting XMM-Newton x-ray observatory to scan the young star, they picked up an x-ray signal, evidence of strong magnetism. What's more, AB Aurigae's x-ray, optical, and ultraviolet radiation all vary in intensity during the same 42-hour cycle, meaning the star itself is the only possible x-ray source.

Further observations and analysis revealed that AB Aurigae's x-rays are emanating from high above its surface, says Guedel. That's a clear sign that the solar winds flowing from both of the star's hemispheres are being pulled back and slammed together by magnetic lines of force. The resulting collisions are generating x-rays, but what is creating the magnetism?

Guedel says his team might have found the answer in an unlikely source: phenomena called fossil magnetic fields. As his group will report in an upcoming issue of Astronomy and Astrophysics, other researchers have predicted that, rather than being generated solely by stars themselves, such fields can also develop within stellar-forming clouds. When new stars are born nearby, they can grab onto parts of the preexisting fields and continue to generate them. "We cannot prove that this is what happened [with AB Aurigae]," Guedel says, but no other explanation from stellar dynamic theory makes sense. He adds that now astronomers can take a close look at other similar stars--called Herbig stars--to see whether their x-rays are being generated by fossil magnetic fields as well.

If confirmed, the findings would be an important illustration of how a young star can interact with the surrounding material in a stellar nursery, in terms of latching onto a fossil magnetic field, says astrophysicist Rachel Osten of NASA's Goddard Space Flight Center in Greenbelt, Maryland. No one knows whether the other Herbig stars are generating x-rays from interactions with as-yet-hidden binary companions, Osten says, but AB Aurigae does not appear to have a companion. And whatever the source, she adds, "we know these stars can't generate magnetic fields on their own."

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