Supernova to superfluid

Neutron stars may be cooled by frictionless liquid

Neutron stars, second only to black holes and pints of Guinness as the densest objects in the Universe, may have liquid in their cores, observations of a dead star shrouded in the debris of a distant supernova suggest. Two separate teams of scientists say that a frictionless state of matter called a superfluid is the only reasonable explanation for temperature changes recently observed in the youngest known neutron star.

LIQUID CENTER The neutron star (illustration, inset) at the center of the remains of the Cassiopeia A supernova may have a core filled with a strange frictionless fluid, new research suggests. X-ray: NASA/CXC/Southampton/W. Ho et al/Nature 2009; Illustration: NASA/CXC/M.Weiss

“This the first direct evidence for superfluidity in neutron stars,” says Wynn Ho, an astrophysicist at the University of Southampton in England and coauthor of a paper that will appear in the Monthly Notices of the Royal Astronomical Society describing one team’s findings. The other team’s results will appear in an upcoming Physical Review Letters.

Located 11,000 light-years away in the constellation Cassiopeia, the neutron star formed when a larger star collapsed in a brilliant explosion that was visible from Earth about 300 years ago. The orbiting Chandra X-Ray Observatory first spotted the neutron star created by this supernova in 1999.

Subsequent measurements revealed that its 2-million-degree surface had cooled by 4 percent over the decade since its discovery. “This was the first time anyone found a young neutron star clearly changing temperature,” says Craig Heinke, an astrophysicist at the University of Alberta in Edmonton, Canada, who reported the observations last year in Astrophysical Journal Letters.

Theorists had long speculated that a young neutron star should cool for the first 100 years after its creation. Neutrons can break down into protons, ejecting nearly massless particles called neutrinos that carry energy away from the star. But this energy-sapping Urca process (named for a money-sapping casino in Brazil) couldn’t account for the steep temperature drop seen by Chandra hundreds of years after the Cassiopeia supernova.

Superfluids inside neutron stars, proposed as early as 1959, offered an alternative to the Urca process. Created in laboratories by chilling liquids to ultracold temperatures, superfluids have mind-boggling abilities to climb walls and leak through solid glass containers. And they make great coolants: Superfluid helium keeps the superconducting magnets in the world’s most powerful particle collider chilled to just 1.9 degrees above absolute zero.

Superfluids could also cool off stars, according to computer simulations developed independently by astrophysicists Dmitry Yakovlev of Ioffe Physical-Technical Institute in St. Petersburg, Russia, and Dany Page of the National Autonomous University of Mexico in Mexico City. Yakovlev and Page worked with two separate teams to compare these simulations to the Chandra data. “These two groups have the best calculations for neutron star cooling in the world,” says astrophysicist Oleg Gnedin of the University of Michigan in Ann Arbor, who was not involved in the research.

Both groups agree that after an initial drop in temperature and centuries of slow, gradual cooling, the star reached a critical temperature sometime around the turn of the 20th century. At 500 million or maybe 700 million to 900 million degrees, depending on some assumptions, neutrons began to pair up and split apart like teens at a high school dance, releasing large splashes of neutrinos. As more neutrons entered this superfluid state during the next couple of decades, they kicked off a rapid temperature drop — aided perhaps by a second superconducting superfluid made of protons.

The new work has also generated a testable prediction that will keep the two groups watching and waiting. “The star should keep cooling off at the same rate for a few more decades,” says Page. Confirming this superfluidity should help scientists understand the behavior of neutrons both in distant dead stars and in the nuclei of atoms here on Earth.

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