Free Astronomy Magazine July-August 2014

STELLAR EVOLUTION which magnetars are the result of the evolution of a binary system com- posed of high- mass stars. The evolution of such a system involves transferring and loosing matter that may facilitate the formation of a magnetar rather than a black hole. To confirm this hy- pothesis was nec- essary to demon- strate that the progenitor of at least a ma- gnetar had actually had a companion. To look for it in an orbit around whichever magnetar would be pointless because the explosions of supernovae break the gravi- tational bonds of that type, speeding up companion stars at an unusually high ve- locity and disperse them in space (the so- called “runaway stars”). Thus, what had to be primarily looked for is a star with a fast proper motion, in wha- tever direction around a magnetar, hoping to find it at a distance not too far away so as to reduce the search time and make it easier to associate the two separated stars. Wd1 has the ideal environment for con- ducting this kind of search as there is only one known magnetar and it can only have formed less than 10,000 years ago. More- over, given the dimensions of the clus- ter and speeds typical of runaway stars (usually below 100 km/s), the former com- panion of the magnetar should still be in- side the cluster or, in the worst of cases, just outside it. That star must therefore still be out there so- mewhere, and to find it, Clark and collea- gues measured the ra- dial velocity of every possible candidate and analyzed its light with the FLAMES in- strument (Fibre Large Array Multi Element Spectrograph) in use at the Kueyen unit of the Very Large Tele- scope (VLT/ESO, Chile). Among the handful of candidates select- ed during the search, the most interesting proved to be Wd1-5, a blue star of about 20 solar masses, whose O n the side, a video that makes us ideally fly through the open cluster Wd1, taking us right close to the mag- netar J1647-45. [ESO/L. Calçada] Below, a photo- graph dating back to when Westerlund 1 was discovered, por- traying Professor Bengt Westerlund (second from left) in company of the famous as- tronomer and science author Åke Wallenquist (extreme left), Mrs. Wallenquist and some young researchers. [Photo: Bertil Pet- tersson]