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An explosive pair

The discovery of a new type of supernova may shed light on some universal mysteries

Weizmann Institute of Science

Not all explosions are created equal: It's as true for film effects as it is for the stars. Yet, until now, scientists had only observed two basic kinds of exploding stars, known as supernovae. Now, scientists at the Weizmann Institute of science, in collaboration with others around the world, have identified a third type of supernova. Their findings appeared this week in Nature.

The first two types of supernova are either hot, young giants that go out in a violent display as they collapse under their own weight, or old, dense white dwarves that blow up in a thermonuclear explosion. The new supernova appeared in telescope images in early January, 2005 and scientists, seeing that it had recently begun the process of exploding, started collecting and combining data from different telescope sites around the world, measuring both the amount of material thrown off in the explosion and its chemical makeup. But Dr. Avishay Gal-Yam, Hagai Perets, (now at the Harvard-Smithsonian Center for Astrophysics), Iair Arcavi and Michael Kiewe of the Weizmann Institute's Faculty of Physics, together with Paolo Mazzali of the Max-Planck Institute for Astrophysics, Germany, and the Scuola Normale Superiore, Pisa, and INAF/Padova Observatory in Italy, Prof. David Arnett from the University of Arizona, and researchers from across the USA, Canada, Chile and the UK, soon found that the new supernova did not fit either of the known patterns.

On the one hand, the amount of material hurled out from the supernova was too small for it to have come from an exploding giant. In addition, its location, distant from the busy hubs where new stars form, implied it was an older star that had had time to wander off from its birthplace. On the other hand, its chemical makeup didn't match that commonly seen in the second type. 'It was clear,' says the paper's lead author Perets, 'that we were seeing a new type of supernova.' The scientists turned to computer simulations to see what kind of process could have produced such a result.

The common type of exploding white dwarf (a type Ia supernova) is mainly made up of carbon and oxygen, and the chemical composition of the ejected material reflects this. The newly-discovered supernova had unusually high levels of the elements calcium and titanium; these are the products of a nuclear reaction involving helium, rather than carbon and oxygen. 'We've never before seen a spectrum like this one,' says Mazzali. 'It was clear that the unique chemical composition of this explosion held an important key to understanding it.' Where did the helium come from? The simulations suggest that a pair of white dwarves are involved; one of them stealing helium from the other. When the thief star's helium load rises past a certain point, the explosion occurs. 'The donor star is probably completely destroyed in the process, but we're not quite sure about the fate of the thief star,' says Gal-Yam.

The scientists believe that several other previously observed supernovae may fit this pattern. In fact, these relatively dim explosions might not be all that rare; if so, their occurrence could explain some puzzling phenomena in the universe. For example, almost all the elements heavier than hydrogen and helium have been created in, and dispersed by supernovae; the new type could help explain the prevalence of calcium in both the universe and in our bodies. It might also account for observed concentrations of particles called positrons in the center of our galaxy. Positrons are identical to electrons, but with an opposite charge, and some have hypothesized that the decay of yet unseen 'dark matter' particles may be responsible for their presence. But one of the products of the new supernova is a radioactive form of titanium that, as it decays, emits positrons. 'Dark matter may or may not exist,' says Gal-Yam, 'but these positrons are perhaps just as easily accounted for by the third type of supernova.'

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Dr. Avishay Gal-Yam's research is supported by the Nella and Leon Benoziyo Center for Astrophysics; the Yeda-Sela Center for Basic Research; the Peter and Patricia Gruber Awards; the Legacy Heritage Fund Program of the Israel Science Foundation; the Minerva Foundation with funding from the Federal German Ministry for Education and Research; and Miel de Botton Aynsley, UK.

The Weizmann Institute of Science in Rehovot, Israel, is one of the world's top-ranking multidisciplinary research institutions. Noted for its wide-ranging exploration of the natural and exact sciences, the Institute is home to 2,600 scientists, students, technicians and supporting staff. Institute research efforts include the search for new ways of fighting disease and hunger, examining leading questions in mathematics and computer science, probing the physics of matter and the universe, creating novel materials and developing new strategies for protecting the environment.

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