The neutron stars, the compact remnants of some supernova explosions are some of the most mysterious objects in the universe.
These peculiar objects have a mass between one and two times the Sun, but are packaged in a space of only 20 km in diameter (70, 000 times smaller than the diameter of the sun). The average density of a neutron star can therefore exceed a few hundreds of billions of grams per cubic centimeter, a density several times found within the heavier atomic nuclei. The extreme conditions that prevail in the interior of neutron stars are so far from those found in laboratory experiments that the properties of their nuclei remain largely unknown, and the theoretical description of matter the neutron star is currently one of the most challenging issues of nuclear and particle physics. In an article appearing in the journal Physical Review Letters, Dany Page of the National Autonomous University of Mexico and his colleagues in the U.S., argue that there is strong evidence that the neutrons in the nuclei of neutron stars form a superfluid. This conclusion, which was reached independently by another group led by Dima Yakovlev Technical Physics Institute in St. Petersburg, Russia, is based on recent observations of X-ray thermal emission from the young neutron star in Cassiopeia A which is the remnant of a supernova (see Fig).
From the chance discovery of pulsars radio Jocelyn Bell Burnell and Anthony Hewish in 1967, nearly two thousand neutron stars have been detected. Many more probably exist in our galaxy. As was predicted by William Baade and Fritz Zwicky in 1933, neutron stars are born of the catastrophic gravitational collapse of the iron core of massive stars at the end point of its evolution. During the first ten seconds after the explosion of the supernova, the newly formed protoneutrón star has a radius of 50 km and remains very hot, with temperatures internal order of a few billion degrees. About a minute later, the star becomes transparent protoneutrón the nearly massless particles called neutrinos produced abundantly in the interior. This allows easy and escaping neutrinos carry energy, so the protoneutrón star cools rapidly and shrinks in an ordinary neutron star. When the temperature falls below a billion degrees, the outer layers of the star crystallizes into a solid crust. At this point, the nucleus is much colder than the crust due to cooling energy produced by the neutrinos escape. After several decades, the interior of the star reaches a uniform temperature of hundreds of millions of degrees (except for a thin outer shell covering of heat). The last stage of cooling is carried out after a few hundred thousand years, when heat radiates from the interior to the surface and is dissipated as thermal electromagnetic radiation [more here and here ].
Cassiopeia A in the constellation of Cassiopeia is the youngest neutron star conocida.Basados \u200b\u200bin the cooling rate observed the star, astrophysicists believe that the neutrons in their nuclei are in a superfluid state. The upper part shows an image of Cassiopeia A in Xray at the bottom is an illustration showing the different layers of a neutron star. Credit: (Top) NASA / Chandra X-ray Observatory, (Bottom) redrawn with permission from Fridolin Weber, San Diego State University.
The metal surface, which is composed mainly of iron, often is hampered by a thin atmosphere. A few meters below the surface, the material is so compressed that atomic nuclei, which are arranged in a regular lattice Coulomb, are fully ionized and therefore live with a quantum gas electrones.En depths of the star, the nuclei become increasingly neutron-rich until the neutrons start to come out of the nucleus, forming an underground ocean of neutrons. Whereas the composition of the outer bark is almost completely determined by the experimental atomic masses, the inner bark, where neutrons are not bound, has no equivalent on Earth and therefore can only be studied theoretically. The crust is dissolved in a uniform liquid of neutrons, protons and electrons when the density reaches about half of which is inside the heavy nuclei. In the crust-core transition kernel can take very unusual ways, such as rods or plates that are called "pasta" nuclear and could be responsible for half the mass of the crust. The composition and properties of dense matter in the inner core of a neutron star are still poorly understood
In particular, it was suggested in 1959, before the actual observations of the first pulsars, the interior of neutron stars could contain a neutron superfluid-a frictionless liquid with very unusual properties. Superfluidity is one of the more macroscopic showy quantum mechanics. The nucleons are fermions , and due to Pauli exclusion principle generally tend to avoid themselves. This individualistic behavior of the nucleons, along with strong repulsive nucleon-nucleon interaction at short range, provided the pressure needed to counteract the enormous gravitational pull of a neutron star, which prevents collapse. However, at sufficiently low temperatures, the nucleons can be paired. These couples are bosons can behave consistently in a very large scale and the nucleon condensate can flow without viscosity, analogous to the superfluidity of helium-3. (Interestingly, while helium-3 becomes a superfluid only below a few mK, superfluidity is sustainable, even at a temperature of millions of degrees in a neutron star, due to the enormous density involved .) Although nuclear pairing has been theoretically studied for several decades [see here ], the core region of a neutron star where this phenomenon might occur is still uncertain. As shown in the two groups of astrophysical observations of young cooling neutron star in Cassiopeia A might shed light on the longstanding problem.
Cassiopeia A, which takes its name from its location in the constellation Cassiopeia , is the remnant of a star that exploded 330 years ago at a distance of some 11 000 light years away. The central compact object has been recently identified as a neutron star with a carbon atmosphere and a surface temperature of about two million degrees [see here ]. The neutron star in Cassiopeia A is not only the youngest known, emitting heat in isolation in our galaxy, but also is the first neutron star that has been directly observed cooling. Ten year follow-up of this object, revealed that his temperature has dropped by about 4% since its discovery in 1999 by X-ray Observatory Chandra [see here ]. This cooling rate is much faster than expected from standard theories of neutron star cooling. According to the two teams of scientists analyzed data from the Chandra X-ray to determine the cooling rate, these observations provide strong evidence for superfluidity in the cores neutron stars. In fact, the onset of superfluidity of neutrons opens a new channel for the continuous emission of neutrinos of fractionation and formation of pairs of neutrons. This process is most effective at temperatures slightly below the critical temperature of superfluid transition, improving the cooling of the star for several decades. Based on observations of Cassiopeia A, Dany Page and his colleagues determined the critical temperature of superfluid neutron half a billion degrees and argue that the protons in the cores of neutron stars are superconductors. Yakovlev's group reached similar conclusions, but its critical temperature inferred for the neutron superfluid is a few hundreds of millions of degrees higher, since they assume different microscopic entries. This rapid cooling is expected to continue for a few decades at the same speed. If Page's interpretation is confirmed by future observations, the results put stringent constraints on microscopic theories of nuclear matter density.
read the study HERE
source of information:
http://physics.aps.org/articles/v4/14
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