Not all explosions are created equal – it's as true for film effects as it is for stars. An international team of scientists, including a University of Arizona astronomer, has identified a third type of exploding star, or supernova.
Until now, scientists had only observed two kinds of supernovae – 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 January 2005. Seeing that it had recently begun the process of exploding, scientists 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. The team, led by principal investigator Avishay Gal-Yam at the Weizmann Institute of Science in Rehovot, Israel, soon found that the new supernova did not fit either of the known patterns.
"Such events were predicted but never observed until now," said team member David Arnett, a Regents' Professor of Astronomy at the University of Arizona’s Steward Observatory. A senior theorist on the paper, Arnett performed calculations that explained the puzzling distributions of chemical elements in the supernova remnant, the "smoking gun" of the stellar explosion. The findings appeared in the May 20 issue of the journal Nature.
Working like detectives on a crime scene of cosmic proportions, the team deciphered the evidence and reconstructed the events leading to the star's demise. By analyzing the spectral pattern of light coming from the supernova, they sifted through the thermonuclear ashes to infer the conditions under which the explosion unfolded, even to the point of identifying the progenitor as a helium-rich white dwarf that detonated.
On 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," said the paper's lead author Hagai 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.
Where did the helium come from? The simulations suggest that a pair of white dwarfs 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," said Gal-Yam.
"This supernova confirms my old idea that there should be several ways that supernova explosions could happen, and that we should observe a variety of events," Arnett said. "This event was classified in a group that included vastly different explosions, including core collapse supernovae, which make a neutron star or black hole, and detonations of white dwarfs made of carbon and oxygen."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.
"Observations like this are beginning to sort out the complexity that we see out there," Arnett said.
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 the cause of 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," said Gal-Yam, "but these positrons are perhaps just as easily accounted for by the third type of supernova."
Other members of the research team include Paolo Mazzali of the Max-Planck Institute for Astrophysics in Germany; the Scuola Normale Superiore, Pisa; and the INAF/Padova Observatory in Italy. Additional researchers from the U.S., Canada, Chile and the UK also contributed.
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