Low‐Mass Neutron Stars and the Equation of State of Dense Matter

Neutron-star radii provide useful information on the equation of state ofneutron rich matter. Particularly interesting is the density dependence of theequation of state (EOS). For example, the softening of the EOS at high density,where the pressure rises slower than anticipated, could signal a transition toan exotic phase. However, extracting the density dependence of the EOS requiresmeasuring the radii of neutron stars for a broad range of masses. A ``normal''1.4 solar mass neutron star has a central density of a few times nuclear-mattersaturation density. In contrast, low mass (of the order of 0.5 solar masses)neutron stars have central densities near nuclear-matter saturation density soits radius provides information on the EOS at low density. Unfortunately,low-mass stars are rare because they may be hard to form. Instead, a precisionmeasurement of nuclear radii on atomic nuclei may contain similar information.Indeed, we find a strong correlation between the neutron radius of 208Pb andthe radius of a 0.5 solar-mass neutron star. Thus, the radius of such a neutronstar can be inferred from a measurement of the the neutron radius of 208Pb.Comparing this value to the measured radius of a 1.4 solar-mass neutron starshould provide the strongest constraint to date on the density dependence ofthe equation of state.Comment: 9 pages and 5 eps. figures (included