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Watch Out for that Flash !

During the autumn of 2006, European,1 Japanese, and Chinese astrophysicists observed the explosion of a star, in many aspects quite exceptional.2 Firstly because of the high magnitude of the explosion, suspected as soon as the first information was picked up by the La Palma Observatory ( Spain), and subsequently confirmed by the three months of observation and analysis that followed. Supernova SN2006jc–thus named according to the order in which it was discovered in 2006–probably attained a maximum luminosity of over a billion times greater than that of the Sun.

Another unusual aspect was the nature of the chemical elements observed during the explosion. Usually, very little hydrogen and helium is detected when small compact stars, (white dwarfs, for example), explode, a phenomenon also known as type 1 explosions. These two chemical elements predominate in type II explosions, which correspond to more massive stars. But in the case of SN2006jc, neither of these scenarios was observed, and no trace of hydrogen or helium was seen in the light emitted during the explosion. It was only ten days later that traces of helium appeared, as measured by analyzing the star's spectrum–i.e., the distribution of light according to its energy, with each chemical element corresponding to characteristic spectral lines. Events of this kind were only recently discovered. They are characteristic of huge stars, initially 60 to 100 solar masses, which would have likely lost their envelope of helium and hydrogen and been reduced to a core of carbon and oxygen of 15 to 25 solar masses. It is probably this central core that exploded, which explains why the helium in the envelope only appeared late in the explosion. Last but not least, this explosion was preceded two years earlier by a flash of light observed at the time by amateur astronomers. Just as for earthquakes, scientists have no advance warning that a star is going to explode. If correlation between the flash of light and the explosion observed two years later is confirmed, astronomers would for the first time have a predictive technique for the explosion of massive stars.


Isabelle Tratner



Notes :

1. For France, Institut d'astrophysique de Paris (CNRS / Université Paris Pierre et Marie Curie), Observatoire de haute provence (CNRS) and CEA Astrophysics department / Dapnia.
2. A. Pastorello, “A giant outburst two years before the core-collapse of a massive star.” Nature. 447(7146): 829-32. 2007.

Contacts :

Michel Dennefeld
IAP, Paris.


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