Neutron Star Crusts
Neutron stars are unique natural laboratories to study quantum chromodynamics at finite density and low temperature. Our understanding of neutron stars has greatly expanded in recent years, thanks to X-ray observatories such as Chandra, by experimental probes of the nuclear symmetry energy, and by advances in first-principles theoretical calculations of low-density bulk nuclear matter. The outermost kilometer of a neutron star is at sub-nuclear density; this layer, known as the crust, comprises electrons, free neutrons, and nuclei arranged in a rigid lattice. A wide variety of observational phenomena originate in the crust, from oscillations in strongly magnetized neutron stars, to cooling after periods of accretion from a companion star, to energetic explosions in the light-element atmosphere. Studying these phenomena requires coordination between nuclear physicists, both experimental and theoretical, observational astronomers, and theoretical astrophysicists. To facilitate these interactions, the JINA-CEE working group on neutron star crusts was convened. Among this group's topics are rigorous tests of crust cooling; evolution of the composition with depth, including neutron-transfer reactions and phase separation; realistic models of heat transport at low temperatures; and tests for exotic matter in the core and crust via thermal relaxation of the crust.