Properties of Low-Density Neutron-Star Matter

A nuclear Thomas-Fermi model is used to determine the ground state of matter at subnuclear densities allowing for inhomogeneities on a nuclear scale ("clusters"). The "neutron drip" phase transition occurs at 3.7 × ${10}^{11}$ ${\mathrm{g}/\mathrm{c}\mathrm{m}}^3$. As a function of the average density the proton number $Z$ of the clusters first increases from $Z\simeq 29$ at 1.4 × ${10}^8$ ${\mathrm{g}/\mathrm{c}\mathrm{m}}^3$ to a maximum of $Z\simeq 35$ at ∼3.0 × ${10}^{12}$ ${\mathrm{g}/\mathrm{c}\mathrm{m}}^3$ and then decreases until the clusters gradually evanesce just above ${10}^{14}$ ${\mathrm{g}/\mathrm{c}\mathrm{m}}^3$; this transition is smooth.