Larger and more heterogeneous neutron star crusts: A result of strong magnetic fields

Abstract

We study the effect of strong magnetic fields, of the order of 10 15 -10 17 G, on the extension of the crust of magnetized neutron stars. The dynamical instability region of neutron-proton-electron (npe) matter at subsaturation densities and the mode with the largest growth rate are determined within a relativistic mean-field model. It is shown that the effect of a strong magnetic field on the instability region is very sensitive to the density dependence of the symmetry energy, and that it is at the origin of an increase of the extension of the crust and of the charge content of clusters. DOI: 10.1103/PhysRevC.94.062801 Soft-γ -ray repeaters and some anomalous x-ray pulsars are strongly magnetized neutron stars known as magnetars [1][2][3]. These stars have strong surface magnetic fields of the order of 10 14 -10 15 G [4], and slow rotation with a period of ∼1-12 s. Recently, the time evolution of the magnetic field of isolated x-ray pulsars has been studied by Pons et al. [5]. The authors show that a fast decay of the magnetic field could explain the nonobservation of stars with periods above 12 s. The decay of the magnetic field was obtained by including a high electrical resistivity in the inner crust, attributed to the possible existence of an amorphous and heterogeneous layer at the bottom of the inner crust. The lack of isolated x-ray pulsars with a period higher than 12 s could, therefore, be a direct indication of the existence of an amorphous inner crust, possibly in the form of pasta phases [5].At low nuclear matter densities, a competition between the long-range Coulomb repulsion and short-range nuclear attraction will lead to the formation of clusterized matter, known as nuclear pasta [6], near the crust-core transition. These geometrical configurations are observed not only in nuclear matter, but also in a variety of amorphous solids, crystals, and magnetic and biological materials [7]. One of the main interests of the existence of these exotic structures in the crust of neutron stars is the effect that they might have on the neutrino transport and the subsequent cooling of the neutron star [8].Molecular dynamics simulations of the nuclear pasta have shown that topological defects in the pasta could increase electron scattering and reduce the electrical and the thermal conductivities [9]. Electron conductivity in magnetized neutron star matter was also studied in Ref. [10], and it was shown that the electron transport is strongly anisotropic, due to the presence of strong magnetic fields. The complexity introduced by the magnetic field suggests that both suppression and enhancement of the electron conduction in the presence of the pasta phases are possible, and further calculations are required.* jian-junfang@163.com † pais.lena@gmail.com ‡ sidney.avancini@ufsc.br § cp@teor.fis.uc.pt Stellar matter contains, besides neutrons and protons, also electrons, which neutralize the proton charge. The transition clusterized-homogeneous matter has been estimated by using different methods. In particular, ...

Effect of strong magnetic fields on the crust-core transition and inner crust of neutron stars

“…The structure of the spinodal section obtained for the strongest field considered, B * = 10 4 , clearly shows the effect of the Landau quantization, as already shown in Ref. [34]: there are instability regions that extend to much larger densities than the B = 0 spinodal section, while there are also stable regions that at B = 0 would be unstable. This is due to the fact that the energy density becomes softer, just after the opening of a new Landau level, and harder when the Landau level is most filled.…”

“…The present work aims to study the effect of the magnetic field on the crust-core transition and completes the one in Ref. [34], where this study was first introduced. We present the formalism that was not introduced in Ref.…”

“…[34], where the dynamical spinodals are calculated within the Vlasov formalism, as previously discussed in Refs. [20,27,41].…”

“…[34], the summation in ν in the above expressions terminates at ν i max (i = p,e), which is the largest value of ν for which the square of the Fermi momenta of the particle is still positive and which corresponds to the closest integer from below, which is defined by the ratio At zero temperature, the ground state of the system is characterized by the Fermi momenta P i F (i = p,n,e) and is described by the equilibrium distribution function…”

“…We present the formalism that was not introduced in Ref. [34], and we discuss the importance of including the anomalous magnetic field. A relativistic mean-field (RMF) model, that satisfies several accepted laboratory and astrophysical constraints [35,36], is considered.…”