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News · Space & Cosmology

News

Strange ring envelops highly magnetic star

Friday, 30 May 2008

by Heather Catchpole

Cosmos Online

The ring: This image shows a ghostly ring extending seven light-years across around the corpse of a massive star. The collapsed star, called a magnetar, is located at the exact center of this ring, but is not visible in the infrared wavelengths used to capture the image. The bright region near the centre of the image is a cluster of young stars.

Credit: NASA

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SYDNEY: Using NASA's Spitzer Space Telescope, astrophysicists have found a strange ring surrounding a highly magnetised star formed in a supernova. Unlike anything seen before, the discovery offers a rare glimpse into the formation of these unusual stars.

The ring around the magnetar SGR 1900+14 was discovered by astrophysicist Stefanie Wachter, from NASA's Spitzer Science Centre at the California Institute of Technology, USA.

Detailed in the U.K. journal Nature, it is thought to have appeared 10 years ago when a giant flare carved out a bubble-shaped cavity into the dust surrounding the magnetar.

Mega-magnets

When massive stars collapse and go supernova they create dense, compact neutron stars. A handful of neutron stars become magnetars; extremely magnetic stars that spin very slowly, pulsate with X-rays or gamma rays and are the most magnetic objects known in the universe.

Little is known about the evolution of these stars, whose magnetic fields are 1015 times more powerful than the Sun; so powerful that they distort matter in the space surrounding them.

The ring (which is around seven light-years long by three light-years wide) differs from other rings discovered around magnetars, which formed as debris slammed into the gas and dust surrounding them. Unlike these rings, the new structure, imaged at two narrow infrared frequencies in 2005 and 2007, doesn't appear to be expanding. Instead, it may represent a cavity carved into the dust around the star, the researchers said.

The structure is visible because light from young, massive stars nearby illuminate the surface of the cavity, causing it to appear as a ring-shaped structure centred on the magnetar.

"It's as if the magnetar became a huge flaming torch and obliterated the dust around it, creating a massive cavity. Then the stars nearby lit up a ring of fire around the dead star, marking it for eternity," said astrophysicist and co-author Chryssa Kouveliotou, of NASA's Marshall Space Flight Centre, in Huntsville, Alabama.

Death of a giant

Analysing the masses of the stars surrounding the magnetar could help to determine its original mass, the researchers say. It is not known why some massive stars form neutron stars after going supernova while others become magnetars.

Another team member, Donald Figer, from the Rochester Institute of Technology, New York State, said the ring discovery is groundbreaking because it unveils other phenomenon associated with a magnetar, which are thought to form from stars at least eight times more massive than the Sun. "When you know so little about an object, each new morsel you can gather up is very important," he said.

Figer is part of another team, led by Rolf Kudritzki of University of Hawaii, that will further probe the magnetar using the two 10-metre telescopes of the W.M. Keck Observatory.

University of Sydney astrophysicist Qinghuan Luo, whose research interests include magnetars, said the discovery was interesting. Luo, who is not one of the study authors, commented that there may be other such structures around magnetars, only a small fraction of which have been detected so far.

More information

The Spitzer Space Telescope - NASA

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