ST. LOUIS — 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 km) 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 km) 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 today,
here at a meeting of the American Astronomical Society (AAS).
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.
"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 SPACE.com. "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.
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