Crystal Chemistry of Three-Component White Dwarfs and Neutron Star Crusts: Phase Stability, Phase Stratification, and Physical Properties

Abstract

A systematic search for multicomponent crystal structures is carried out for five different ternary systems of nuclei in a polarizable background of electrons, representative of accreted neutron star crusts and some white dwarfs. Candidate structures are "bred" by a genetic algorithm, and optimized at constant pressure under the assumption of linear response (Thomas-Fermi) charge screening. Subsequent phase equilibria calculations reveal eight distinct crystal structures in the T = 0 bulk phase diagrams, five of which are complicated multinary structures not before predicted in the context of compact object astrophysics. Frequent instances of geometrically similar but compositionally distinct phases give insight into structural preferences of systems with pairwise Yukawa interactions, including and extending to the regime of low density colloidal suspensions made in a laboratory. As an application of these main results, we self-consistently couple the phase stability problem to the equations for a self-gravitating, hydrostatically stable white dwarf, with fixed overall composition. To our knowledge, this is the first attempt to incorporate complex multinary phases into the equilibrium phase layering diagram and mass-radius-composition dependence, both of which are reported for He-C-O and C-O-Ne white dwarfs. Finite thickness interfacial phases ("interphases") show up at the boundaries between single-component bcc crystalline regions, some of which have lower lattice symmetry than cubic. A second application -quasi-static settling of heavy nuclei in white dwarfs -builds on our equilibrium phase layering method. Tests of this nonequilibrium method reveal extra phases which play the role of transient host phases for the settling species. Subject headings: dense matter -methods: numerical -plasmas -stars: neutron -white dwarfs arXiv:1508.05088v1 [astro-ph.HE]

Anisotropic crystal structure of magnetized neutron star crust

“…Previous studies of Coulomb solids have thus focused on a handful of lattices characterized by near minimum energies and with a possible presence of a uniform magnetic field (e.g., Cohen & Keffer 1955;Carr 1961;Nagai & Fukuyama 1982, 1983Baldereschi, Senatore & Oriani 1992;Baiko, Potekhin & Yakovlev 2001;Kozhberov & Baiko 2015;Chamel & Fantina 2016). Formation of the bcc crystal in neutron star envelopes has been confirmed in recent molecular dynamics simulations (Engstrom, Yoder & Crespi 2016). Linear elasticity theory based on the bcc lattice has been used to study neutron star crust oscillations (e.g., Piro 2005;Sotani 2016).…”

Electrostatic properties and stability of Coulomb crystals

“…21 it was suggested that ions in the inner crust of a neutron star form one-component perovskite crystals but this statement was not proofed in Refs. 11,12 and seems to us is incorrect.…”

“…Crystal structures of He-C-O, C-O-Ne, and C-O-Fe mixtures were examined in Ref. 11 where it was shown that multicomponent crystals can form between layers of one-component bcc crystals. A similar result was obtained in Ref.…”

“…13 In this paper, we follow the basic ideas of Refs. 11,12 We use the analytical expression evaluated in Refs. 14,15 to calculate the electrostatic energies of various lattices, including those that were never studied before.…”

Colloquium: Astromaterial science and nuclear pasta

“…The liquid-solid phase diagram of the three component C, O, Ne system has now been determined by direct MD simulation (Hughto et al, 2012). The optimal structures of three component He-C-O and C-O-Ne white dwarfs were determined using a genetic optimization technique (Engstrom et al, 2016). This technique was originally developed to optimize the structure of conventional materials.…”