Neutron stars are compact objects composed mainly of neutrons. They have masses of 1.5 to 2 times the solar mass. One the the main ingredients in their theoretical understanding is the equation of state (EOS) of pure neutron matter and the related symmetry energy. We were the first to point out in the context of the Skyrme energy-density functionals (EDFs) that the neutron skin of heavy nuclei such as 208Pb (the difference in the rms radii for neutrons and protons) is highly correlated with the stiffness of the neutron equation of state. [1] More recently we have shown that the experimental binding energies and radii of magic nuclei determine the value of the neutron EOS at a density of 0.10 neutrons/fm3 [2].

In collaboration with Achim Schwenk, we have shown that ab-initio results for the low-density neutron EOS based on N3LO type interactions with two and three body terms can be used to put much tighter cons taints on the form of the Skyrme EDFs. [3] With this method we are able to make a prediction of 0.182(12) for the neutron skin of 208Pb.

The neutron EOS obtained with a simultaneous fit to the ab initio results for low-density and nuclear data for magic nuclei are shown in the bottom panel of the figure. The upper panel shows the symmetric-matter EOS (for equal numbers of protons and neutrons) obtained from the fits to nuclear data. The middle panel shows the symmetry energy that is given by the difference between the neutron EOS (bottom) and symmetric matter EOS (top). The different lines show the results for thirteen Skyrme EDFs based on reasonable range of effective masses and incompressibilties for symmetric nuclear matter. The spread of the lines towards higher density is an indication of the remaining uncertainty.


[1] Neutron Radii in Nuclei and the Neutron Equation of State, B. A. Brown, Phys. Rev. Lett. 85, 5296 (2000). link to paper

[2] Constraints on the Skyrme Equations of State from Properties of Doubly Magic Nuclei, B. A. Brown, Phys. Rev. Lett. 111, 232502 (2013). link to paper

[3] Constraints on the Skyrme Equations of State from Properties of Doubly Magic Nuclei and ab initio Calculations of Low-Density Neutron Matter, B. A. Brown and Achim Schwenk, Phys. Rev. C 89 011307(R) (2014). link to paper