Superfluid in Neutron Star's core
Neutron stars are extremely dense objects that form when massive stars run out of nuclear fuel and collapse in on themselves. One teaspoon of neutron star material weighs six billion tons. The pressure in the star's core is so high that most of the charged particles, electrons and protons, merge resulting in a star composed mostly of uncharged particles called neutrons.
Neutron stars should exhibit both superfluidity and superconductivity, according to two independent groups of scientists. The researchers studied the neutron star in the supernova remnant known as Cassiopeia A, and found that its core should exist in a superfluid state at up to around a billion degrees kelvin, in contrast to the near absolute-zero temperatures required for superfluidity on Earth.
A 10 years' analysis of X-ray data from NASA's Chandra satellite has found that the Cassiopeia A neutron star's surface temperature has dropped more quickly than expected – by about 4% between 2000 and 2009. This drop in temperature, although it sounds small, was really dramatic and surprising to see, This means that something unusual was happening within this neutron star." "The rapid cooling in Cas A's neutron staris the first direct evidence that the cores of these neutron stars are, in fact, made of superfluid and superconducting material,
Cooper pairs and neutrinos In the current work, groups led by Dany Page of the National Autonomous University in Mexico and Peter Shternin of the Ioffe Institute in St Petersburg, Russia, say that this rapid cooling can be partly explained by invoking the zero-viscosity state of matter known as superfluidity. They argue that when the temperature of a neutron star falls below a certain critical value it becomes energetically favourable for neutrons inside the star to form Cooper pairs – the basic unit of the superfluid state – and that the energy released as a result could be easily removed from the star in the form of neutrinos.
But the two groups have found that this mechanism cannot account for all of the cooling, and they independently conclude that superconductivity must also play a role. They say that shortly after the creation of the neutron star, protons would combine to form Cooper pairs, so creating a superconducting state by virtue of their charge. Bound up in this way, the protons would not be able to take part in various neutrino-emitting reactions that occur in non-superfluid matter, reducing cooling early on in the life of the star and leading to a sharper drop in temperature later on.
However, there are alternatives to the superfluid/superconductor hypothesis, such as the idea that the rapid cooling was simply the natural consequence of the temporary heating created by an asteroid impact. Excluding this and other ideas will require further data from Chandra – with a subsequent rise in temperature suggesting that the star could in fact be experiencing asteroid impacts, while a continuation of the cooling would support the theory of the Page and Shternin groups.
Extremely high temperature superconductors If this new model is correct then, as Wynn Ho, who is also a member of Shternin's group, points out, neutron stars would probably contain the hottest superfluids and superconductors in the universe. Indeed, Shternin's team has calculated that the Cassiopeia A neutron star should exhibit superfluidity when its temperature drops below about 800 million kelvin and that proton superconductivity could take place at up to 2–3 billion kelvin. Page and colleagues, meanwhile, calculate the superfluid transition temperature to be around 500 million kelvin.
These figures are in stark contrast to the 130 kelvin that is the highest temperature at which any material on Earth has been found to superconduct. But Ho cautions that we can't draw any practical tips from neutron stars, pointing out that the huge densities of these objects mean that particles are extremely closely packed and so act via the strong nuclear force, whereas on Earth superfluidity and superconductivity are mediated by the fundamentally different, and much weaker, electromagnetic force. Ho does, however, believe that the latest work could lead to a better understanding of the strong force itself.
Neutron stars should exhibit both superfluidity and superconductivity, according to two independent groups of scientists. The researchers studied the neutron star in the supernova remnant known as Cassiopeia A, and found that its core should exist in a superfluid state at up to around a billion degrees kelvin, in contrast to the near absolute-zero temperatures required for superfluidity on Earth.
A 10 years' analysis of X-ray data from NASA's Chandra satellite has found that the Cassiopeia A neutron star's surface temperature has dropped more quickly than expected – by about 4% between 2000 and 2009. This drop in temperature, although it sounds small, was really dramatic and surprising to see, This means that something unusual was happening within this neutron star." "The rapid cooling in Cas A's neutron staris the first direct evidence that the cores of these neutron stars are, in fact, made of superfluid and superconducting material,
Cooper pairs and neutrinos In the current work, groups led by Dany Page of the National Autonomous University in Mexico and Peter Shternin of the Ioffe Institute in St Petersburg, Russia, say that this rapid cooling can be partly explained by invoking the zero-viscosity state of matter known as superfluidity. They argue that when the temperature of a neutron star falls below a certain critical value it becomes energetically favourable for neutrons inside the star to form Cooper pairs – the basic unit of the superfluid state – and that the energy released as a result could be easily removed from the star in the form of neutrinos.
But the two groups have found that this mechanism cannot account for all of the cooling, and they independently conclude that superconductivity must also play a role. They say that shortly after the creation of the neutron star, protons would combine to form Cooper pairs, so creating a superconducting state by virtue of their charge. Bound up in this way, the protons would not be able to take part in various neutrino-emitting reactions that occur in non-superfluid matter, reducing cooling early on in the life of the star and leading to a sharper drop in temperature later on.
However, there are alternatives to the superfluid/superconductor hypothesis, such as the idea that the rapid cooling was simply the natural consequence of the temporary heating created by an asteroid impact. Excluding this and other ideas will require further data from Chandra – with a subsequent rise in temperature suggesting that the star could in fact be experiencing asteroid impacts, while a continuation of the cooling would support the theory of the Page and Shternin groups.
Extremely high temperature superconductors If this new model is correct then, as Wynn Ho, who is also a member of Shternin's group, points out, neutron stars would probably contain the hottest superfluids and superconductors in the universe. Indeed, Shternin's team has calculated that the Cassiopeia A neutron star should exhibit superfluidity when its temperature drops below about 800 million kelvin and that proton superconductivity could take place at up to 2–3 billion kelvin. Page and colleagues, meanwhile, calculate the superfluid transition temperature to be around 500 million kelvin.
These figures are in stark contrast to the 130 kelvin that is the highest temperature at which any material on Earth has been found to superconduct. But Ho cautions that we can't draw any practical tips from neutron stars, pointing out that the huge densities of these objects mean that particles are extremely closely packed and so act via the strong nuclear force, whereas on Earth superfluidity and superconductivity are mediated by the fundamentally different, and much weaker, electromagnetic force. Ho does, however, believe that the latest work could lead to a better understanding of the strong force itself.