Magnetar Formation May Require a Double Star
Posted on May 14, 2014
A double star may be needed for the formation of a magnetar, a rare form of neutron star with a powerful magnetic field. The European Southern Observatory (ESO) describes magnetars as the "bizarre super-dense remnants of supernova explosions." They are also the strongest magnets known in the Universe.
European astronomers have been studying the magnetar CXOU J164710.2-455216 in the Westerlund 1 star cluster. They have been trying to determine why a magnetar formed in the cluster instead of an expected black hole. Stars as massive as the one believed to produce the magnetar should collapse to form black holes after their deaths. The researchers theorized that the magnetar must have formed from the interaction of two massive stars, but no star was detected near the Westerlund 1 magnetar.
The astronomers then decided to hunt for runaway stars and they located one. Using the ESO's Very Large Telescope (VLT) the astronomers found a star, Westerlund 1-5, escaping the star cluster at a high velocity.
Ben Ritchie (Open University), a co-author on the new paper to be published in Astronomy and Astrophysics, says in a release, "Not only does this star have the high velocity expected if it is recoiling from a supernova explosion, but the combination of its low mass, high luminosity and carbon-rich composition appear impossible to replicate in a single star - a smoking gun that shows it must have originally formed with a binary companion."
The astronomers used the runaway star to reconstruct the stellar life story that permitted a magnetar to form instead of a black hole. In the first stage, the more massive star of the pair starts running out of fuel and begins transferring its outer layers to its less massive companion (which becomes the magnetar). The causes the less massive star to rotate faster and faster. The astronomers say this rapid rotation is also likely to be the essential ingredient in the formation of the magnetar's ultra-strong magnetic field. In the second stage, as a result of this mass transfer, the companion itself becomes so massive that it in turn sheds a large amount of its recently gained mass. Most of the mass is lost, but some is passed back to the original star, runaway star Westerlund 1-5.