Exotic Superfluid Found in Ultra-Dense Stellar Corpse

The ultra-dense remains of the galaxy’s youngest supernova are full of bizarre quantum matter.

Two new studies show for the first time that the core of the neutron star Cassiopeia A, is a superfluid, a friction-free state of matter that normally only exists in ultra-cold laboratory settings.

“The interior of neutron stars is one of the best kept secrets of the universe,” said astrophysicist Dany Page of the National Autonomous University in Mexico, lead author of a paper in the Feb. 25 Physical Review Letters describing the state of the star. “It looks like we broke one of them.”

Cassiopeia A (Cas A) was a massive star 11,000 light-years away whose explosion was observed from Earth about 330 years ago. The supernova left behind a tiny, compact body called a neutron star, in which matter is so densely packed that electrons and protons are forced to fuse into neutrons. Neutron star material is some of the most extreme matter in the universe. Just a teaspoonful of neutron star stuff weighs about 6 billion tons.

The neutron star in Cas A was first spotted in 1999, shortly after the Chandra X-Ray Observatory began scanning the sky for objects that emit X-rays.

Last year, astronomers Craig Heinke of the University of Alberta and Wynn Ho of the University of Southampton noticed something odd: The neutron star was cooling down at an alarmingly fast rate. In just 10 years, the star had cooled from 2.12 million degrees to 2.04 million degrees, a drop of 4 percent.

Theoretical models predicted that neutron stars should cool slowly as the neutrons inside decayed into electrons, protons and nearly-massless particles called neutrinos that flee the star quickly, taking heat with them.

But ordinary neutron decay is too slow. Two competing groups of physicists, one led by Page and one including Heinke and Ho, saw that something else must be going on in Cas A.

Almost simultaneously, both teams came to the same solution: The matter inside the neutron star is converting to a superfluid as astronomers watch. Heinke and Ho’s paper will appear in the Monthly Notices of the Royal Astronomical Society.

Here’s how it works: Normally, the laws of quantum mechanics dictate that a collection of neutrons can get only so cold, but no colder. But at extremely cold temperatures in the lab, or the extremely high pressures inside a neutron star, pairs of neutrons can link up. Together, the neutron pairs relax into the lowest energy state quantum physics allows, and convert to a superfluid.

“A superfluid is essentially a macroscopic quantum liquid, in which if you take any given particle in the fluid, it’s moving in essentially the same way as the particles around it,” said Bennett Link of the University of Montana, who was not involved in the new studies. “The whole system behaves as a quantum system even though it’s large in size.”

Superfluids flow without friction. On Earth, they can climb walls and escape from airtight containers. When the particles in a superfluid are charged, the fluid is a superconductor, which carries electricity with no resistance.

As the neutrons and protons in the neutron star link up to form superfluids, they release massive amounts of neutrinos. The mass exodus of neutrinos fleeing Cas A explains the rapid cooling, the physicists conclude.

The idea that neutron stars should contain superfluids had been around since the 1950s. Page and colleagues had even predicted theoretically that the core of Cas A in particular should be a superfluid.

“We knew that it was there, our models had it all included before, but we did not have the data to actually hang our coats on,” said Madappa Prakash of Ohio University, a coauthor on Page’s paper.

Page didn’t expect that superfluidity would actually show itself in Cas A. When he learned that Heinke and Ho had seen the star’s temperature drop precipitously, “I jumped and my head hit the ceiling,” he said.

Both teams knew the other group was working on the same idea, and raced in friendly competition to publish their theory first. Page’s team ended up winning the race by one day. Heinke and Ho were waiting for one more observation from Chandra, taken in November 2010, before submitting their paper for publication.

The papers differ only in the details. The two teams made different assumptions about how hot the neutrons were to begin with, so their calculations for the temperature at which the superfluid state is possible are different.

Both teams predict that Cas A will continue to cool down over the next 10 years.

“That allows people to test it against alternative hypotheses, such as, it’s some kind of episodic thing,” Link said. “If it’s still cooling at the same rate, that would give evidence for their hypothesis, that we are actually seeing a superfluid form.”

X-ray Image: NASA/CXC/UNAM/Ioffe/D.Page,P.Shternin et al; Optical Image: NASA/STScI; Illustration: NASA/CXC/M.Weiss