A team of European, Japanese and Chinese astronomers, led by A. Pastorello from the Astrophyical Research Centre at Belfast University and including French scientists from the Paris Institute for Astrophysics (IAP) and the Service d'Astrophysique (SAp) at the French Atomic Commision (CEA), have uncovered one of the most remarkable and peculiar stellar explosions. This event occured on 9 Octobre 2006 in a remote galaxy, about 80 millions light-years away and was watched for more than three months by several telescopes spread over Europe, China and Japan. The observations revealed that the star that exploded was massive and mainly made of carbon and oxygen. But more surprisingly, this very uncommon explosion was predated nearly exactly two years before by a bright outburst. This unexpected precursory signal offers for the first time to astronomers a chance to predict the desintegration of a star and to watch it just before its very last moments. These results are reported today in the last issue of the science journal Nature. (Watch the video).
The story begins on Octobre 14th, 2004, when with its small 60cm diameter telescope, the Japanese veteran supernova hunter, Koichi Itagaki, detects a faint object in the galaxy UGC 4904 in the Lynx constellation. At first, he thought to have discovered a "supernova", a luminous phenomenon which comes with the destruction of a star at the end of its life. But instead of increasing the luminosity and remaining visible for a few months, the object disapeared after only a few days. The event was therefore thought to be a transitory optical flash that sometimes occured when rare extremely massive and blue stars, called Luminous Blue Variables (LBVs), shed part of their envelope in a violent spasm.
Image of the galaxy UGC4904 at three different epochs. In Octobre 2004, a luminous object appears in the outskirts of the galaxy during a few days and disapears. In Septembre 2006, it is still absent. The 9th October 2006, it becomes as bright as the center of the galaxy, reaching a luminosity equivalent to one billion stars. This supernova, labelled SN2006jc, reaches a maximum apparent magnitude of mv=14 before slowly declining. On October 29th, it is still at a magnitude of mv=15.65. It is an unique case of a supernova, a star explosion, predates by a giant outburst. Click to enlarge the images.
The amateur Koichi Itagaki did not interrupt his careful monitoring while, by contrast, the 2004 even felt into oblivion among the professional community. Nearly exactly two years later, on October 9th, 2006 at 18h03 Universal Time, he reports again a source at the same position. But this time, the object is more than 40 times more luminous and lasts longer. The European-Asian collaboration led by A. Pastorello is rapidly informed and a large set of telescopes is activated .
"At the beginning, we thought that it was a classical supernova but very rapidly we realised that we were facing a extremely unusual explosion" tells A. Pastorello. The first observations collected at the La Palma Observatory (Spain) revealed in the light emitted in the explosion, a total lack of hydrogen and helium, the two most abundant elements in normal stars. In the first days, the spectrum i.e. the repartition of light with energy, only shows intermediate-mass elements such as oxygen, magnesium, calcium, in form of very broad lines as a result of the large velocities reached in the explosion. In addition, indication of helium appeared solely in prominent narrow lines which could only be produced in the helium-rich circumstellar materiel lost in outbursts similar to the one observed in 2004.
The supernova labelled SN2006jc (according to its rank, the 263th among the supernovae discovered in 2006 ) reached a maximum brightness typical of the strongest explosions, reaching more than one billion times times the solar luminosity. Astronomers usually classified supernovae in two different categories, the thermonuclear supernovae and the core collapse supernovae which covered two very different mechanisms. The thermonuclear explosions are produced when a very compact star, a white dwarf, becomes unstable after accumulating too much mass from a nearby companion and desintegrates. The core-collapse events, on the contrary, signals the death and collapse of a massive star. In the first case, virtually no hydrogen and helium is produced, while in the second these two elements largely dominate. SN2006jc falls between the two but it is in fact the collapse of a stellar core.
The SN2006jc supernova spectrum. The light distribution shows the total lack of hydrogen. Only a few "narrow" lines of Helium became visible after ten days (red points). All other lines are broad as a result of the high velocity in the explosion and are coming from evolved elements (oxygen, magnesium,...) originating from the core of the star.
According to A. Pastorello "SN2006jc is definitely not a standard supernova. It is only similar to very few others observed before such as SN1999cq or SN2002ao. The rarity of such events could be due to the high progenitor mass. As the computations show, we are probably dealing with a star of 60 to 100 solar mass that lost most of its mass before. It is only the remaining 15-25 solar mass carbon-oxygen core that exploded". In these conditions, most of the elements seen in high-velocity spectral lines come from the central C-O core while helium responsible for the narrow lines is present only in the surrounding material lost by the precursor star which is caught up by the supernova ejecta. This scenario is also supported by independent X-ray observations from the Swift and Chandra satellites which suggest that the supernova’s blast wave lighted up X-rays when it reached a shell of material ejected by the progenitor before its explosion,
Astronomers still wonder what may be the results of such an explosion. A massive core is expected to collapse into a black hole, a very dense object, dense enough to retain the light. However in this case, smaller amount of radioactive material is expelled resulting in a fainter and under-energetic explosion whereas SN2006jc is a high luminosity supernova. To solve this contradiction, the authors have still to wait later observations at different wavelengths with the hope to fully resolve this aenigmatic supernova.
The major enigma is however the warning signal received two years before. This was never observed before. As for the earthquakes, the scientists have only very few forerunners that can tell them about the imminence of a star explosion. In the last years, it was proved that some of the most massive stars emit a brief and violent burst of gamma rays just a few seconds before the launch of the supernova expansion. These gamma-ray bursts are today extensively searched but they leave very little time to the astronomers.
In the case of SN2006jc, the first optical outburst occurs two years before and looks alike that is seen in the Blue Luminous Variables. These stars are however far from exploding and still retain an hydrogen and helium-rich normal atmosphere. Did the precursory signal witness the last mass loss episode from an LBV from its atmosphere ? No stellar theory can presently explain such short timescale evolution. Even if the outburst originates from the stellar core itself, a novel mechanism has also to be invoked as no such core has ever been seen to produce such a bright outburst
The situation is so puzzling that the scientists have also to consider a possible alternative. The star could be a massive binary system in which one of the component will be an LBV star responsible for the outburst while the companion will be an evolved star that, by coincidence, collapses two years later to give SN2006jc. The hope is now that the Hubble Space Telescope may be able to find the remaining star if it exists.
Both the supernovae and the LBV eruptions are very rare astronomical spectacles and it is very exciting that these two phenomena can be linked for the first time.
The unique SN2006jc supernova also opens totally new horizons as it gives tracks to search for the next exploding stars. One of the most famous LBV star close to us is Eta-Carina, famous for its outburst in 1843 that turned it into the second brightest star in the sky. Now it is again invisible to the naked eyes. But the next warning could well be the last before the very final show.
|see||Press release CEA - France (14 June 2007 )|
|Press release CNRS - France (14 June 2007 )|
|Press release IAP - France (14 June 2007 )|
|Press release INAF - Italy (14 June 2007 )|
|Press release U.K (14 June 2007 )|
|Press release NAOC - China (14 June 2007 )|
|Press release Kyushu - Japan (14 June 2007 )|
|- Explosion asymmétrique des supernovae (1 décembre 2006)|
|- Explosion radioactive dans Cassiopée (9 octobre 2006)|
 Supernova name. Several hundreds supernovae are detected each year. The name given to a supernova corresponds to the year of discovery with letters indicating its rank. "SN2007a" is the first supernova discovered in 2007, "SN2007aa" the 27th.
 Observations : Observations have been collected at the Asiago Observatory (Telescope Copernico 1,82m Italy), Haute-Provence Observatory (Telescope 1.93m, CNRS, France), National Astronomical Observatory (Telescope 2.16m, Xinglong Observatory, Beijing, China) and La Palma Observatory (Telescopio Nationale Galileo 3.58m, Nordic Optical Telescope 2.56m, Liverpool Telescope 2.0m and William Herchell Telescope 4.2m, Canarias, Spain).
Rédaction: J.M. Bonnet-Bidaud