Dynamical response of the nuclear “pasta” in neutron star crusts

Abstract

The nuclear pasta-a novel state of matter having nucleons arranged in a variety of complex shapes-is expected to be found in the crust of neutron stars and in core-collapse supernovae at subnuclear densities of about 10 14 g/cm 3 . Owing to frustration, a phenomenon that emerges from the competition between short-range nuclear attraction and long-range Coulomb repulsion, the nuclear pasta displays a preponderance of unique low-energy excitations. These excitations could have a strong impact on many transport properties, such as neutrino propagation through stellar environments. The excitation spectrum of the nuclear pasta is computed via a molecular-dynamics simulation involving up to 100,000 nucleons. The dynamic response of the pasta displays a classical plasma oscillation in the 1-to 2-MeV region. In addition, substantial strength is found at low energies. Yet this low-energy strength is missing from a simple ion model containing a single-representative heavy nucleus. The low-energy strength observed in the dynamic response of the pasta is likely to be a density wave involving the internal degrees of freedom of the clusters. PACS number(s): 26.60.+c, 24.10. Lx, 25.30.Pt Baryonic matter is organized as a result of short-range nuclear attraction and long-range Coulomb repulsion. Often the corresponding nuclear and atomic length scales are well separated, so nucleons bind into atomic nuclei that are themselves segregated into a crystal lattice. However, at the enormous densities present in astrophysical objects-densities that exceed that of ordinary matter by 14 orders of magnitudethese length scales become comparable and complex new phenomena emerge. Complexity arises because it is impossible for the constituents to be simultaneously correlated from nuclear attraction and anticorrelated from Coulomb repulsion. Competition among these interactions plays a fundamental role in the organization of matter and results in Coulomb frustration. Frustration-a ubiquitous behavior in complex systems ranging from magnetism to protein folding to neural networks-develops from the inability of a system to simultaneously satisfy all of its elementary interactions. For example, the Ising antiferromagnet on a triangular lattice is frustrated because not all of the nearest neighbor spins can be antiparallel to each other. Frustrated systems have unusual dynamics owing to the preponderance of low-energy excitations [1].At subnuclear densities of about 10 14 g/cm 3 (normal nuclear matter saturation density is 2.5 × 10 14 g/cm 3 ) Coulomb frustration is expected to promote the development of complex shapes. These shapes follow from the competition between surface tension and Coulomb energies. Whereas surface tension favors spherical shapes, Coulomb interactions often * favor nonspherical configurations. Therefore, a variety of complex structures with a diversity of shapes-such as spheres, cylinders, and plates-have been predicted. The many phases of nuclear matter displaying this variety of shapes are known collectively as nuclear ...

Vlasov formalism for extended relativistic mean field models: The crust-core transition and the stellar matter equation of state

“…The inner crust of neutron stars may contain exotic geometrical structures termed nuclear pasta [20] at its upper boarder, just before the crust-core transition. Several methods have been used to compute these pasta phases: quantum [21], semiclassical [22,23], or classical [24] molecular dynamics calculations, three-dimensional (3D) Hartree-Fock calculations [25,26], and Thomas-Fermi (TF) calculations [13,14,[27][28][29]. The authors of Ref.…”

Light clusters, pasta phases, and phase transitions in core-collapse supernova matter

“…At low densities, the frustrated system called the "pasta phase," which is caused by the competition between the Coulomb interaction and the strong force, appears. It is constituted by different configurations whose geometry changes as the density increases [1][2][3][4][5][6][7][8][9].…”

Nuclear “pasta” formation

“…This could be important for the damping of collective r-mode oscillations of rapidly rotating neutron stars. The excitation spectrum is important for the pasta heat capacity, see also [27]. Meanwhile, the shear modulus of the pasta is important for the speed of shear waves and crustal oscillation frequencies of neutron stars.…”