Constraining URCA cooling of neutron stars from the neutron radius of 208Pb

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.