Low-mass neutron stars and the equation of state of dense matter

Neutron star radii provide useful information about the equation of state of neutron-rich matter. Particularly interesting is the density dependence of the equation of state (EOS). For example, the softening of the EOS at high density, where the pressure rises slower than anticipated, could signal a transition to an exotic phase. However, extracting the density dependence of the EOS requires measuring the radii of neutron stars for a broad range of masses. A "normal" 1.4 M_⊙ (M_⊙ = solar mass) neutron star has a central density of a few times the nuclear-matter saturation density (ρ₀). In contrast, low-mass (≃0.5 M _⊙) neutron stars have central densities near ρ₀, so their radii provide information about the EOS at low density. Unfortunately, low-mass neutron stars are rare because they may be hard to form. Instead, a precision measurement of nuclear radii of atomic nuclei may contain similar information. Indeed, we find a strong correlation between the neutron radius of ²⁰⁸Pb and the radius of a 0.5 M_⊙ neutron star. Thus, the radius of a 0.5 M_⊙ neutron star can be inferred from a measurement of the neutron radius of ²⁰⁸Pb. Comparing this value to the measured radius of a ≃ 1.4 M_⊙ neutron star should provide the strongest constraint to date on the density dependence of the EOS.