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Register a new userMagneto-rotational neutron star evolution: the role of core vortex pinning
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We consider the pinning of superfluid (neutron) vortices to magnetic fluxtubes associated with a type II (proton) superconductor in neutron star cores. We demonstrate that core pinning affects the spin-down of the system significantly, and discuss implications for regular radio pulsars and magnetars. We find that magnetars are likely to be in the pinning regime, while most radio pulsars are not. This suggests that the currently inferred magnetic field for magnetars may be overestimated. We also obtain a new timescale for the magnetic field evolution which could be associated with the observed activity in magnetars, provided that the field has a strong toroidal component.
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The nature of the interaction between superfluid vortices and the neutron star crust, conjectured by Anderson and Itoh in 1975 to be at the heart vortex creep and the cause of glitches, has been a long-standing question in astrophysics. Using a qualitatively new approach, we follow the dynamics as superfluid vortices move in response to the presence of nuclei (nuclear defects in the crust). The resulting motion is perpendicular to the force, similar to the motion of a spinning top when pushed. We show that nuclei repel vortices in the neutron star crust, and characterize the force as a function of the vortex-nucleus separation.
We revisit the population synthesis of isolated radio-pulsars incorporating recent advances on the evolution of the magnetic field and the angle between the magnetic and rotational axes from new simulations of the magneto-thermal evolution and magnetosphere models, respectively. An interesting novelty in our approach is that we do not assume the existence of a death line. We discuss regions in parameter space that are more consistent with the observational data. In particular, we find that any broad distribution of birth spin periods with $P_0lesssim 0.5$ s can fit the data, and that if the alignment angle is allowed to vary consistently with the torque model, realistic magnetospheric models are favoured compared to models with classical magneto-dipolar radiation losses. Assuming that the initial magnetic field is given by a lognormal distribution, our optimal model has mean strength $langlelog B_0{rm [G]}rangle approx 13.0-13.2$ with width $sigma (log B_0) = 0.6-0.7$. However, there are strong correlations between parameters. This degeneracy in the parameter space can be broken by an independent estimate of the pulsar birth rate or by future studies correlating this information with the population in other observational bands (X-rays and $gamma$-rays).
Bearing in mind the application to core-collapse supernovae, we study nonlinear properties of the magneto-rotational instability (MRI) by means of three- dimensional simulations in the framework of a local shearing box approximation. By changing systematically the shear rates that symbolize the degree of differential rotation in nascent proto-neutron stars (PNSs), we derive a scaling relation between the turbulent stress sustained by the MRI and the shear- vorticity ratio. Our parametric survey shows a power-law scaling between the turbulent stress ($<< w_{rm tot}>>$) and the shear- vorticity ratio ($g_q$) as $<<w_{rm tot}>> propto g_q^{delta}$ with its index $delta sim 0.5$. The MRI-amplified magnetic energy has a similar scaling relative to the turbulent stress, while the Maxwell stress has slightly smaller power-law index ($sim 0.36$). By modeling the effect of viscous heating rates due to the MRI turbulence, we show that the stronger magnetic fields or the larger shear rates initially imposed lead to the higher dissipation rates. For a rapidly rotating PNS with the spin period in milliseconds and with strong magnetic fields of $10^{15}$ G, the energy dissipation rate is estimated to exceed $10^{51} {rm erg sec^{-1}}$. Our results suggest that the conventional magnetohydrodynamic (MHD) mechanism of core-collapse supernovae is likely to be affected by the MRI-driven turbulence, which we speculate, on one hand, could harm the MHD-driven explosions due to the dissipation of the shear rotational energy at the PNS surface, on the other hand the energy deposition there might be potentially favorable for the working of the neutrino-heating mechanism.
Magnetic field evolution in neutron-star crusts is driven by the Hall effect and Ohmic dissipation, for as long as the crust is sufficiently strong to absorb Maxwell stresses exerted by the field and thus make the momentum equation redundant. For the strongest neutron-star fields, however, stresses build to the point of crustal failure, at which point the standard evolution equations are no longer valid. Here, we study the evolution of the magnetic field of the crust up to and beyond crustal failure, whence the crust begins to flow plastically. We perform global axisymmetric evolutions, exploring different types of failure affecting a limited region of the crust. We find that a plastic flow does not simply suppress the Hall effect even in the regime of a low plastic viscosity, but it rather leads to non-trivial evolution -- in some cases even overreacting and enhancing the impact of the Hall effect. Its impact is more pronouced in the toroidal field, with the differences on the poloidal field being less substantial. We argue that both the nature of magnetar bursts and their spindown evolution will be affected by plastic flow, so that observations of these phenomena may help to constrain the way the crust fails.
While magnetic fields have long been considered to be important for the evolution of magnetic non-degenerate stars and compact stars, it has become clear in recent years that actually all of the stars are deeply affected. This is particularly true regarding their internal angular momentum distribution, but magnetic fields may also influence internal mixing processes and even the fate of the star. We propose a new framework for stellar evolution simulations, in which the interplay between magnetic field, rotation, mass loss, and changes in the stellar density and temperature distributions are treated self-consistently. For average large-scale stellar magnetic fields which are symmetric to the axis of rotation of the star, we derive 1D evolution equations for the toroidal and poloidal components from the mean-field MHD equation by applying Alfvens theorem, and a conservative form of the angular momentum transfer due to the Lorentz force is formulated. We implement our formalism into a numerical stellar evolution code and simulate the magneto-rotational evolution of 1.5 M$_odot$ stars. The Lorentz force aided by the $Omega$ effect imposes torsional Alfven waves propagating through the magnetized medium, leading to near-rigid rotation within the Alfven timescale. Our models with different initial spins and B-fields can reproduce the main observed properties of Ap/Bp stars. Calculations continued to the red-giant regime show a pronounced core-envelope coupling, which reproduces the core and surface rotation periods determined by asteroseismic observations.
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