Physics – Nuclear Physics – Nuclear Theory
Scientific paper
2002-07-23
Phys.Rev. C66 (2002) 055803
Physics
Nuclear Physics
Nuclear Theory
8 pages with 3 (eps) figures
Scientific paper
10.1103/PhysRevC.66.055803
Recent observations by the Chandra observatory suggest that some neutron stars may cool rapidly, perhaps by the direct URCA process which requires a high proton fraction. The proton fraction is determined by the nuclear symmetry energy whose density dependence may be constrained by measuring the neutron radius of a heavy nucleus, such as 208Pb. Such a measurement is necessary for a reliable extrapolation of the proton fraction to the higher densities present in the neutron star. A large neutron radius in 208Pb implies a stiff symmetry energy that grows rapidly with density, thereby favoring a high proton fraction and allowing direct URCA cooling. Predictions for the neutron radius in 208Pb are correlated to the proton fraction in dense matter by using a variety of relativistic effective field-theory models. Models that predict a neutron (Rn) minus proton (Rp) root-mean-square radius in 208Pb to be Rn-Rp<0.20 fm have proton fractions too small to allow the direct URCA cooling of 1.4 solar-mass neutron stars. Conversely, if Rn-Rp>0.25 fm, the direct URCA process is allowed (by all models) to cool down a 1.4 solar-mass neutron star. The Parity Radius Experiment at Jefferson Laboratory aims to measure the neutron radius in 208Pb accurately and model independently via parity-violating electron scattering. Such a measurement would greatly enhance our ability to either confirm or dismiss the direct URCA cooling of neutron stars.
Horowitz Charles J.
Piekarewicz Jorge
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