New Scientist, December 2013
Edited by Andy Ross
A star that goes supernova twice in quick succession may mark the birth of a
quark star. These stars could help explain extended gamma-ray bursts and the
formation of the heaviest elements in the universe.
neutrons are made of quarks: protons of two up quarks and one down quark
(uud) and neutrons of two downs and one up (udd). Strange quark matter
includes hyperons containing strange (s) quarks and may be more stable than
nuclear matter made of protons and neutrons.
When a star many times
more massive than the sun runs out of fuel, its core implodes. The outer
layers are cast off in a supernova, to leave behind a rapidly spinning
neutron star with a crust of iron. As it spins down, the forces opposing
gravity weaken, allowing gravity to cook up hyperons. As the density
increases, the core melts to quark soup, freeing quarks from their bound
state. A runaway process converts the neutron star into a quark star.
Calgary professor Rachid Ouyed: "If quark stars exist, then the
conversion of normal, ordinary matter into a quark star will be a very
exothermic process." Simulations show that the seed of strange quark matter
spreads until it reaches the outer crust of the neutron star, then it
separates from the iron crust and collapses. The collapse halts when the
dense inner core rebounds, creating a shock wave. The crust and leftover
neutrons are ejected, and slam into the earlier supernova remnants to light
them up again in a quark nova.
Ouyed is convinced that SN 2009ip and
SN 2010mc heralded the birth of two quark stars: "We just applied our model
of the dual shock quark nova, and it was actually easy to fit."
Predictions based on the quark nova model include the creation of heavy
elements in the universe. In a supernova, heavy elements are cooked up in
milliseconds as iron nuclei absorb neutrons and decay into elements further
up in the periodic table as neutrons turn into protons. Ouyed: "But the
challenge with supernovae has always been to go to really heavy elements."
The quark nova ejects a mixture of neutrons and iron from the crust,
which interact to form the heaviest elements. Ouyed urges astronomers to
study double explosions. His team predicts that only the second blast should
show the presence of elements heavier than atomic mass 130.
conversion of a neutron star to a quark star could explain some
long-duration gamma-ray bursts. On 2011-07-09, a gamma-ray burst was
observed with two peaks spaced 11 minutes apart, with the second one
stronger. The old "collapsar" model of gamma-ray bursts does not easily
explain such an emission.
Colliders like the LHC have been smashing heavy ions to create a
quark-gluon plasma, where quarks are essentially free. The best way to study
this phase of matter is using lattice QCD. But physicists have only been
able to solve the equations of lattice QCD for the conditions created at the
LHC. Quark stars would prove that quarks can exist freely in other
AR Interesting: a neutron star with a
gravitationally polished iron surface played a big role in my 1996 sci-fi