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Quark stars could answer cosmic questions

A trio of the most luminous supernovae ever seen may have been caused by the conversion of neutron stars to never-before-observed quark stars, astronomers say.
An artist's conception shows a stellar explosion on the scale seen in the supernova known as SN 2006gy.
An artist's conception shows a stellar explosion on the scale seen in the supernova known as SN 2006gy.M. Weiss / CXC / NASA
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Quark stars, exotic objects that have yet to be directly observed, are part of a new theory to explain some of the brightest stellar explosions recorded in the universe.

Super-luminous supernovae, which produce more than 100 times more light energy than normal supernovae and occur in about one out of every 1,000 supernovae explosions, have long baffled astrophysicists. The problem has been finding a source for all of that extra energy.

University of Calgary astrophysicists Denis Leahy and Rachid Ouyed think they have a possible source — the explosive conversion of a neutron star into a quark star.

A neutron star is a compact stellar corpse with a mass equal to about 1.5 suns packed into a space no more than 16 miles (26 kilometers) across. Though still just theoretical, as no direct evidence yet exists, a quark star is thought to be even denser, packing a similar mass into an object just 12 miles (19 kilometers) across.

Leahy and Ouyed's computer models suggest a quark-nova explosion would account for the extra energy observed in super-luminous supernovae. The properties they found in their simulations matched up with those of three of the most luminous supernovae to date: SN2006gy, SN2005gj and SN2005ap.

"In theory, when a neutron star converts into a quark star it releases a lot of energy, and it produces something that looks like a supernova explosion in terms of energetics," Leahy said during a presentation of the results Tuesday here at a meeting of the American Astronomical Society.

The low-down
Here's how the scenario could work: The explosive collapse of a massive star generates a neutron star. If that neutron star is massive enough, the neutron star will convert into a quark star, which is packed with quarks.

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"If you make a neutron star massive enough, gravity compresses it so you get a higher and higher density in the center," Leahy said. "If you compress matter to a high enough density you'll get quark matter."

Since quarks are a lower energy state than neutrons, the conversion should release loads of energy, enough to power a second explosion called a quark-nova.

In a typical supernova explosion, most of the released energy is used to push off the cloud of gas as the star collapses. This so-called envelope of gas expands outward. Just a fraction of a percent of the energy goes into the spectacular light shows of supernovae.

Leahy said that if a second explosion, the quark-nova, were to occur 10 to 20 days after the supernova, the energy wouldn't have to go into expanding the gas envelope. Instead, most of the energy would be in the form of light radiation. That radiation could explain the brightest supernova recorded, he said.

Odd matter
The results are of special interest for two reasons: Astronomers previously did not have a satisfactory explanation for super-luminous supernovae; and the model provides indirect evidence for the existence of quark stars, Leahy said. 

"No one has given a satisfactory explanation for these super-luminous supernovae," Leahy told "Until somebody does with a normal mechanism, I think it [this theory] does provide some evidence, because you need to get that energy."

Quarks are considered to be the tiniest elementary particles that form the building blocks for protons and neutrons, which in turn form atoms. While protons and neutrons are thought to be made of three quarks each, a short-lived particle called a pion is made up of just two quarks and eventually decays into photons, electrons and neutrinos.

So the finding, albeit theoretical, brings astronomers a step closer to understanding and possibly finding more evidence for the existence of quarks.

Other explanations for the bright supernovae are possible, the researchers say, so further research is needed to confirm the new quark-nova model.

This work was supported by the Natural Sciences and Engineering Research Council of Canada.