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September 05, 2010

Neutron Rich Matter

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Arxiv - Neutron rich matter, neutron stars, and their crusts (10 age pdf)

(H/t Adam Crowl
Neutron rich matter is at the heart of many fundamental questions in Nuclear Physics and Astrophysics. What are the high density phases of QCD? Where did the chemical elements come from? What is the structure of many compact and energetic objects in the heavens, and what determines their electromagnetic, neutrino, and gravitational-wave radiations? Moreover, neutron rich matter is being studied with an extraordinary variety of new tools such as Facility for Rare Isotope Beams (FRIB) and the Laser Interferometer Gravitational Wave Observatory (LIGO). We describe the Lead Radius Experiment (PREX) that is using parity violation to measure the neutron radius in 208Pb. This has important implications for neutron stars and their crusts. Using large scale molecular dynamics, we model the formation of solids in both white dwarfs and neutron stars. We nd neutron star crust to be the strongest material known, some 10 billion times stronger than steel. It can support mountains on rotating neutron stars large enough to generate detectable gravitational waves. Finally, we describe a new equation of state for supernova and neutron star merger simulations based on the Virial expansion at low densities, and large scale relativistic mean eld calculations.

There is a very interesting relationship between the neutron radius of 208Pb, of order 6 femtometers, and the radius of a neutron star, of order 10 km. This involves a breathtaking extrapolation of 18 orders of magnitude in size, or 55 orders of magnitude in mass.



How large can a neutron star mountain be before it collapses under the extreme gravity?

This depends on the strength of the crust. We performed large scale Molecular Dynamic simulations of crust breaking, where a sample was strained by moving top and bottom layers of frozen ions in opposite directions. These simulations involve up to 12 million ions and explore the e ffects of defects, impurities, and grain boundaries on the breaking stress. For example, in Fig. 2 we show a polycrystalline sample involving 12 million ions. In the upper right panel the initial system is shown, with the diff erent colors indicating the eight original microcrystals that make up the sample. The other panels are labeled with the strain, i.e. fractional deformation, of the system. The red color indicates distortion of the body centered cubic crystal lattice. The system starts to break along grain boundaries. However the large pressure holds the microcrystals together and the system does not fail until large regions are deformed.

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