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Massive neutron star rules out exotic matter

28-06-2006 11:18pm

#1

slade_x

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Exotic states of matter such as free quarks do not arise inside neutron stars, according to a new analysis of one of the super-dense stellar corpses. The result contradicts previous theories and offers an unprecedented view into the behaviour of matter under extreme pressure.
Neutron stars are the dead cores of massive stars that exploded as supernovae. They are made of neutrons packed together so tightly that a teaspoonful of the material would weigh billions of tonnes.
Physicists have speculated that in the dense interiors of especially massive neutron stars, matter might be transformed into exotic states never seen elsewhere.

Some researchers believe the enormous pressure could cause the neutrons to break down, freeing the individual quarks of which they are made. Quarks are never found alone in nature. One group of astronomers reported tentative evidence of such a quark star in 2002 (see Exotic star is made entirely of quarks). Another theory says the pressure might lead to a form of matter called a Bose-Einstein condensate (BEC). In this weird quantum state, the neutrons do not dissociate into quarks, but their individual identities blur and they behave as a single particle.

Stretched light

In principle, it is possible to distinguish between the regular and exotic states of matter, because both free quarks and BECs would be more easily compressed than neutrons. So a star of a particular mass would have a smaller radius if it were made of squishy exotic matter.
But the exotic matter theories have received a blow from the study by Feryal Ozel of the University of Arizona, in Tucson, US. Using a new technique to analyse the mass and radius of a neutron star called EXO 0748-676, she finds that the star is probably made of ordinary neutrons.
The mass and size were determined from measurements of several other key properties of the star, taken by Europe's XMM-Newton and NASA's Rossi X-ray Timing Explorer space telescopes.

One of these properties was the.........

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