Free Astronomy Magazine July-August 2014

STELLAR EVOLUTION ferred to its out- side by the convec- tive motions of the plasma. Especially abun- dant among those “metals” are the lighter ones such as carbon (origi- nated from the fu- sion of helium), part of which ends up settling on the photosphere of Wd 1-5, while a great deal of other mat- ter (including other carbon) is disper- sed in every direc- tion in space. After several un- stable pulsations making it shed tens of solar mas- ses, the future mag- netar has now ex- hausted also its heavier fuels allow- ed in the stellar nucleosynthesis, while its core collapses to set off the supernova phase. Its final mass is still however suffi- ciently low to not turn into a black hole and the powerful magnetic field inherited from the progenitor concentrates around the newly formed neutron star, “trans- forming” into a magnetar. The mighty ex- plosion breaks the gravitational bond of the binary system and the only thing re- maining where the event took place (or nearby) is the collapsed star, while the sur- viving one, Wd1-5, is flung away without the shock wave being able to strip off the metals gained in the meantime – in fact, the explosion of its companion further en- riches it. This is the scenario that most convincing- ly explains both the characteristics of Wd1-5 and those of J1647-45, and at the same time solves the mystery of the for- mation of magnetars. To confirm it will be sufficient to find another pair, or more pairs, of the same type. C lose up of the spectrograph FLAMES, used by the team led by Simon Clark to identify and study Wd1-5, the massive star that thousands of years ago was linked to the pro- genitor of the magnetar J1647- 45. [ESO] n sphere of the expanding star (through the intersection of the two Roche lobes), up to the point of increasing its mass by about 20 solar masses. The transfer of matter has a double consequence: the initially more massive star finds itself slimmed down and this postpones its explosive end (not yet occurred to date); while for the star that gained matter, growing to about 55 solar masses, it is as if a rapid aging process had started. As the latter gains matter, it also acquires its angular momentum, causing it to rotate on its axis much faster than be- fore and to intensify its own magnetic field, thus laying the foundations of the super strong magnetic field that will cha- racterize it after the gravitational collapse. Events then quickly escalate, with the fu- ture magnetar that after burning up all the hydrogen, and a good part of the he- lium, starts to go through an instability phase during which it ejects into space layers of surface material enriched with elements synthesized in the core and trans-