Il Chandra X-ray Observatory della NASA ha rilevato la prima evidenza diretta di un bizzarro superfluido, privo di attrito di stato della materia, al centro di una stella di neutroni.
Le stelle di neutroni contengono la materia nota più densa direttamente osservabile. Un cucchiaino di materiale di una stella di neutroni pesa sei miliardi di tonnellate. La pressione nel nucleo della stella è talmente elevata che la maggior parte delle particelle cariche, elettroni e protoni si fondono con la conseguenza che le stelle sono prevalentemente composte da particelle chiamate neutroni senza carica.
Due gruppi di ricerca indipendenti hanno studiato il resto della supernova Cassiopea A, o Cas A. Si tratta dei resti di una stella massiccia posta a 11 mila anni luce di distanza, che sarebbe esplosa circa 330 anni fa, come fu osservato dalla Terra.
I dati di Chandra hanno rilevato un rapido calo della temperatura della stella di neutroni ultra-densa che è rimasta dopo la supernova, dimostrando che si era raffreddata di circa il quattro per cento in soli 10 anni.
"Questo calo della temperatura, anche se piccolo, è quite dramatic and surprising to see " said Dany Page of the National Autonomous University of Mexico, the principal scientist of the team that published the document in the journal Physical Review Letters 25 February 2011.
" This means that something unusual is happening within this neutron star.
The superfluid containing charged particles are superconducting in the sense that they act as perfect electric conductors and never lose energy. The new results suggest, that the residual protons in the nucleus of the star, are in a superfluid, and as leading a charge, are also a superconductor.
"The rapid cooling of the neutron star, Cas A, seen with Chandra is the first direct evidence that the nuclei of these neutron stars are, in fact, made from superconductors and superfluids" said Peter Shternin Ioffe Institute in St Petersburg, Russia, head of the team that published a paper in the Bulletin Monthly Notices of the Royal Astronomical Society.
Both teams have shown that rapid cooling can be explained by the formation of a superfluid of neutrons in the core of the star. Rapid cooling is expected to continue for some decade and then it should cease. The onset of superfluidity
materials on Earth occurs at extremely low temperatures near absolute zero, but in neutron stars, can occur at temperatures close to a billion degrees Celsius. Until now there has been considerable uncertainty in the estimates of this critical temperature. This new research limits the critical temperature of between half a billion and just under a billion degrees.
Cas A will allow researchers to test models of how the strong nuclear force that binds subatomic particles behave in ultra-dense material. These results are important for understanding a range of behaviors of neutron stars, including the "defects" as their precession and pulsation, the magnetar explosion and the evolution of the magnetic fields of neutron star.
Small sudden changes in speed of rotation of the rotating neutron stars, called "glitch", have already shown their respect to the superfluid density of the star.
Cas A reveals new insights into the inner region with high density of neutron stars.
"Previously we had no idea how superconductivity had been extended by the protons in a neutron star" , said co-author Shternin Dmitry Yakovlev, who was also at the Ioffe Institute.
The cooling of Cas A was discovered by co-author Craig Heinke, University of Alberta, Canada, and Wynn Ho, at the University of Southampton, UK, in 2010. It was the first time astronomers measured the cooling rate of a young neutron star.
Photo above:
This composite image shows a beautiful sight to the X-ray and optical Cassiopeia A (Cas A), a supernova remnant located in our galaxy about 11,000 light years distance. They are the remains of a massive star that exploded about 330 years ago.
I raggi X di Chandra sono mostrati in rosso, verde e blu assieme ai dati ottici di Hubble in oro. Al centro dell'immagine vi è una stella di neutroni ultra-densa creatasi dall'esplosione della supernova.
L'inserto artistico mostra un'impressione della stella di neutroni al centro del Cas A. I diversi strati di colore della regione nel ritaglio mostrano la crosta (arancione) e il nucleo (rosso), dove la densità è molto più alta e la parte del nucleo ritenuta di essere in uno stato superfluido (sfera rossa interna).
I raggi blu emanati dal centro della stella, rappresentano gli abbondanti neutrini, particelle debolmente interagenti quasi senza mass, which are created when the internal temperature drops below a critical level and form a superfluid in a process that began about 100 years.
These neutrinos escaping from the star, bringing with them the energy causing the rapid cooling of the star. (Credit: Credit: X-ray: NASA / CXC / xx, Optical: NASA / STScI; Illustration: NASA / CXC / M. Weiss)
Translation by Arthur McPaul
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