CAREER: The Composition of Dense Matter and Observations of Neutron Stars

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Stars with masses greater than eight times the mass of the sun end their lives in an explosion called a core-collapse supernova. If the original star is between eight and twenty times the mass of the sun, the supernova leaves behind a neutron star. Neutron stars are ultra-dense objects with the mass of the sun and a diameter of about 15 miles. Normal matter is made of atoms consisting of electrons and nuclei, nuclei are made of neutrons and protons, and neutrons and protons, in turn, are made of up and down quarks. The gravitational force in the center of a neutron star, however, is sufficient to compress neutrons and protons together so tightly that neutrons and protons may be forced to convert into exotic particles containing strange quarks. The question of whether or not strange quarks are present in neutron stars, and the more general question of the nature of extremely dense matter, are some of the principal questions facing the nuclear physics and astrophysics communities. This project aims to answer these questions and to determine the nature of matter in the center of neutron stars where gravity compresses matter to a thousand trillion times the density of water. This project involves training opportunities for students and postdoctoral fellow. Community outreach includes the education of students through the continued development of a Studio Physics course, involvement in research, and a web-based visualization of neutron stars.Nuclear experiments and observations of neutron stars have successfully generated novel constraints on the equation of state of matter at densities near and above the central densities in atomic nuclei. However, our current understanding of high-densities is limited: we do not yet know the composition of the ground state. This project uses a combination of nuclear theory matched to nuclear data and neutron star observations to determine the composition of neutron star cores. Modern models of strongly-interacting matter will be combined with new nuclear structure calculations of neutron-rich nuclei in order to construct state of the art models of the neutron star crust and core. These models, and their concomitant uncertainties, will be calibrated with nuclear data and compared with neutron star cooling, mass, and radius observations. For example, neutron star cooling data has already given indications of fast cooling arising from a large proton-to-neutron ratio. This project will study the posterior probability distribution for the proton-to-neutron ratio and several other observables that will give insights into neutron star composition. This proposal is co-funded by the Theoretical Nuclear Physics Program in the Physics Division and the Division of Astronomical Sciences at NSF.