Quark Stars

By Anil Ananthaswamy
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.

Protons and 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.

The 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 conditions.


AR Interesting: a neutron star with a gravitationally polished iron surface played a big role in my 1996 sci-fi novel LIFEBALL.