Session 2WE: Workshop: Nucleons, Nuclei and Neutron Stars in the Era of Gravitational Waves II

The Equation of State of neutron matter and neutron stars

Recent advances in experiments of the symmetry energy of nuclear matter and in neutron star observations yield important new insights on the equation of state of neutron matter at nuclear densities. In this regime the equation of state of neutron matter plays a critical role in determining the mass-radius relationship for neutron stars. We show how microscopic calculations of neutron matter, based on realistic two- and three-nucleon forces that reproduce very accurately properties of light nuclei, make clear predictions for the relation between the isospin-asymmetry energy of nuclear matter and its density dependence, and the mass and radius for a neutron star. At densities higher than nuclear density the situation is more complicated, as there is still no clear evidence on the composition of matter. We will discuss a model of high density matter based on the speed of sound, and discuss the effect of various constraints to the maximum mass of neutron stars.   

Chiral effective field theory for the nuclear equation of state

Neutron stars are astrophysical objects of extremes. They contain the largest reservoirs of degenerate fermions, reaching the highest densities we can observe in the cosmos, and probe matter under conditions that cannot be recreated in terrestrial experiments. In August 2017, the first neutron-star merger has been observed, which provided compelling evidence that these events are an important site for the production of elements heavier than iron in the universe. Furthermore, the gravitational-wave signal of such events might shed light upon the nature of strongly interacting matter in the core of neutron stars. To understand these remarkable events, reliable nuclear physics input is essential. In this talk, I will explain how to use Chiral effective field theory and advanced many-body methods to provide a consistent and systematic approach to strongly interacting systems from nuclei to neutron stars and allow precision studies with controlled theoretical uncertainties. I will present recent results for the equation of state relevant for the nuclear astrophysics of neutron stars and neutron-star mergers, and will discuss future directions and opportunities.   

The significance of a neutron star's interior composition

The discovery of 2 solar mass neutron stars, detection of gravitational waves with the concomitant detection of electromagnetic radiation from the binary neutron star merger GW170787, recent reports of additional mergers involving neutron stars, and accumulating data on the cooling of neutron star crusts have all given much impetus to ongoing theoretical investigations of the dense matter equation of state. In this talk, I will highlight recent work on the role played by nuclei, nucleons and nonnucleonic degrees of freedom (hyperons, quarks, etc.) in understanding the many observable facets of a neutron star. Special emphasis will be placed on the composition of a neutron star at all layers from the surface to the core. The time is ripe now to achieve consistency between the global properties such as masses and radii with dynamical observables that include tidal deformations, rotational periods and their time derivatives, surface temperatures of isolated neutron stars and of those that undergo periodic accretion. While several puzzles await solutions, the need for updates of LIGO detectors to detect gravitational radiation from the remnants of post merger events will be emphasized.