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Supernova may be in a new class

Oddball stellar explosion doesn’t match known outbursts

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4:52pm, June 17, 2009
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Just in time for July 4, astronomers say they have found a new type of stellar firecracker.

Stars that die an explosive death generally fall into two categories: young, massive stars that collapse under their own weight and hurl their outer layers into space, and older, sunlike stars that undergo a thermonuclear explosion. But the stellar explosion recorded in January 2005 and known as SN 2005E doesn’t fit either class, according to a new analysis reported online June 11 at arXiv.org.

The explosion ejected only a small amount of material — the equivalent of 0.3 solar masses — and erupted in the halo of an isolated galaxy, a region devoid of any star formation. These findings suggest that the explosion, or supernova, did not arise from the collapse of a massive star, report study coauthors Hagai-Binyamin Perets and Avishay Gal-Yam of the Weizmann Institute of Science in Rehovot, Israel, and their colleagues. A massive star would have cast off much more material and would have erupted in a star-forming region. Since stellar heavyweights are so short-lived, they can’t move far from their birth site.

On the other hand, the researchers note, the explosion’s dimness and the abundance of elements forged in the eruption indicate it was not a typical thermonuclear explosion. Spectra show that the debris from the outburst contains five to 10 times more calcium than observed in any other known stellar explosion and probably contains a high abundance of radioactive titanium-44.

“In my experience, there’s lots of strange supernovas out there … but it really does look like this one might be something different,” comments theorist Andrew MacFadyen of New York University.

The authors of the paper declined to be interviewed because they had submitted the report to Nature. In their article, they report that the erupting oddball matches a model in which a compact star called a white dwarf nabs a thick layer of helium from a companion star. The star would then undergo a thermonuclear explosion that would destroy the helium but leave the rest of the white dwarf intact. By contrast, in a common type of supernova known as a type 1a supernova, a white dwarf made up mostly of carbon and oxygen blows itself to smithereens after stealing matter from a companion.

Perets, Gal-Yam and their collaborators report that SN 2005E resembles a few other peculiar supernova, notably an explosion found last year and known as SN 2008ha.

“Both of these objects have very low luminosity, low velocity [of debris] and strong calcium lines,” says Rober Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Kirshner, along with some of the collaborators on the SN 2005e study, is a coauthor of a study on SN 2008ha set to appear in an upcoming issue of The Astronomical Journal

The conclusions of both papers suggest a weak thermonuclear explosion, although the study of SN 2005E is more far-reaching, Kirshner says. “My guess is that the same interpretation would probably work for both,” he says.

Because both SN 2005E and SN 2008ha are so faint, telescopes may have failed to detect other similar explosions, comments MacFadyen. Supernovas are known to seed galaxies with an assortment of heavy elements. If the number of explosions in the new class is large enough, they may be an important contributor to this process. It’s a well-known story how supernovas produce these elements, “but there’s always room for adding new players to the team,” says MacFadyen.

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