August 18, 2010

Massive Neutron Star Challenging Black Hole Theory

A team of European researchers have pinpointed a magnetar--an unusual type of neutron star that possesses an immensely powerful magnetic field--that was formed by a star that had at least 40 times more mass than our solar system's sun.

The discovery, they say, could force scientists to rethink current theories of star evolution, and specifically, the amount of mass needed to form a black hole.

According to the scientific website PhysOrg.com, "This proves for the first time that magnetars can evolve from stars so massive we would normally expect them to form black holes. The previous assumption was that stars with initial masses between about 10 and 25 solar masses would form neutron stars and those above 25 solar masses would produce black holes."

The star in question was observed by Dr. Ben Ritchie and colleagues from The Open University in the Westerlund 1 (Wd1) star cluster--which is located some 16,000 light years from Earth in the southern constellation of Ara--using the Wide Field Imager on the MPG/ESO 2.2-meter telescope the European Southern Observatory's (ESO) La Silla Paranal Observatory in Chile.

Dr. Ritchie and his colleagues used "spectroscopic and photometric observations of the eclipsing double-lined binary W13 to derive dynamical masses for the two components, in order to determine limits for the progenitor masses of the magnetar CXOU J164710.2-455216 and the population of evolved stars in Wd1," according to their report, which appears in the journal Astronomy and Astrophysics.

The results, the paper notes, confirm "the high progenitor mass of the magnetar CXOU J164710.2-455216 inferred from its membership in Wd1" and represent "the first dynamical constraint on the progenitor mass of any magnetar."

"The red supergiants in Wd1 must have similar progenitor masses to W13 and are therefore amongst the most massive stars to undergo a red supergiant phase, representing a challenge for population models that suggest stars in this mass range end their redwards evolution as yellow hypergiants," Dr. Ritchie, the study's lead author, wrote.

Under currently accepted theory, a star this massive should have become a black hole once went supernova and collapsed. Since it did not, however, Dr. Ritchie and other scientists are searching for an explanation. One such explanation offered is that the star lost a considerable amount of its mass to neighboring stellar bodies before collapsing.

"These stars must get rid of more than nine tenths of their mass before exploding as a supernova, or they would otherwise have created a black hole instead," co-author Ignacio Negueruela said in an August 18 press release. "Such huge mass losses before the explosion present great challenges to current theories of stellar evolution."

In an interview with BBC News, University of Birmingham professor and astrophysicist Mike Cruise called the paper "a brilliant piece of detective work"¦ what is especially attractive about this paper is the way the researchers' arguments are based on robust measurements, not just theory."


Image Caption: Artist's impression of the magnetar in the extraordinary star cluster Westerlund 1


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