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3.4 Impurities and defects

There are many reasons why the real crust of neutron stars can be imperfect. In particular, apart from a dominating (A, Z) nuclide at a given density ρ, it can contain an admixture of different nuclei (“impurities”). The initial temperature at birth exceeds 1010 K. At such a high T, thermodynamic equilibrium is characterized by a statistical distribution of A and Z. With decreasing T, the A, Z peak becomes narrower [5563]. After crystallization at Tm, the composition is basically frozen. Therefore, the composition at T < Tm reflects the situation at T ∼ Tm, which can differ from that in the absolute ground state at T = 0. For example, between the neighboring shells, with nuclides (A1,Z1 ) and (A ,Z ) 2 2, respectively, one might expect a transition layer composed of a binary mixture of the two nuclides [108109]. Another way of forming impurities is via thermal fluctuations of Z and Ncell, which, according to Jones [224225], might be quite significant at ρ ≳ 1012 g cm −3 and T ≳ 109 K.

The real composition of neutron star crusts can also differ from the ground state due to the fallback of material from the envelope ejected during the supernova explosion and due to the accretion of matter. In particular, an accreted crust is a site of X-ray bursts. The ashes of unstable thermonuclear burning at accretion rates 10− 8M ⊙ y− 1 ≳ M˙ ≳ 10− 9M ⊙ y− 1 could be a mixture of A ≃ 60 –100 nuclei and could therefore be relatively “impure” (heterogeneous and possibly amorphous) [365364Jump To The Next Citation Point]. If the initial ashes are a mixture of many nuclides, further compression under the weight of accreted matter can keep the heterogeneity. If the crust is weakly impure but rather amorphous, its thermal and electrical conductivities in the solid phase would be orders of magnitude lower than in the perfect crystal as discussed in Section 9. This would have dramatic consequences as far as the rate of the thermal relaxation of the crust is concerned (Section 12.7.3).


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