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Neutron stars with exotic cores

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The neutron star in Cassiopeia A could prove that such stars have exotic-superfluid centres.

The neutron star in Cassiopeia A could prove that such stars have exotic-superfluid centres.

Nature reports new physics seen in the youngest neutron star discovered to date:

Two teams of astrophysicists might have found the first direct evidence that the interiors of neutron stars — the husk left behind after a massive star explodes — exist in a strange, frictionless state known as a superfluid. The teams found that the temperature of a young neutron star in our Galaxy is dropping faster than can be explained by standard cooling theories, matching researchers’ expectations for a neutron star on its way to superfluidity.

Superfluids are perfect heat conductors with zero viscosity. On Earth, a superfluid’s behaviour is counterintuitive — it stays completely still even if its container is rotating, it can defy gravity by creeping up walls, or it can play Houdini by escaping from an airtight container.

At 330 years old, the neutron star of Cassiopeia A is the youngest known in our Galaxy. After the star’s initial explosion, interactions between protons and neutrons would have produced neutrinos, near-massless particles that escaped the star, allowing it to cool.

After the first few days or weeks, the protons, which make up about 10% of the star, would have reached a temperature that allowed them to become a superfluid. In this state, they would have been able to ignore the neutrons, thus stopping the neutrino-emitting process and slowing the star’s rate of cooling.

This condition has kept the star hot since its explosion. But sometime in the past century, the star has begun to slip below the temperature that allows neutrons to pair off with each other, allowing them to form a superfluid.

When they reach this critical temperature, the fledgling neutron pairs repeatedly split apart and try to pair again. They release energy each time they pair up, and when that energy is released in neutrino form, the star is cooled.

This process of coupling and breaking up has driven the rapid cooling of the star for the past few decades, and both teams project that it will continue for a few more. Then, once as many of the neutrons as possible have gone superfluid, the star will return to a slow cooling rate. If Chandra and future telescopes find evidence that bears out this prediction, astrophysicists can be fairly certain that neutron stars really are superfluid on the inside.

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Written by Arhopala Bazaloides

February 5, 2011 at 3:30 pm

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