243 research outputs found
Magnetic field evolution and equilibrium configurations in neutron star cores: the effect of ambipolar diffusion
As another step towards understanding the long-term evolution of the magnetic
field in neutron stars, we provide the first simulations of ambipolar diffusion
in a spherical star. Restricting ourselves to axial symmetry, we consider a
charged-particle fluid of protons and electrons carrying the magnetic flux
through a motionless, uniform background of neutrons that exerts a collisional
drag force on the former. We also ignore the possible impact of beta decays,
proton superconductivity, and neutron superfluidity. All initial magnetic field
configurations considered are found to evolve on the analytically expected
time-scales towards "barotropic equilibria" satisfying the "Grad-Shafranov
equation", in which the magnetic force is balanced by the degeneracy pressure
gradient, so ambipolar diffusion is choked. These equilibria are so-called
"twisted torus" configurations, which include poloidal and toroidal components,
the latter restricted to the toroidal volumes in which the poloidal field lines
close inside the star. In axial symmetry, they appear to be stable, although
they are likely to undergo non-axially symmetric instabilities.Comment: MNRAS, accepte
Internal Heating of Old Neutron Stars: Contrasting Different Mechanisms
Context: The standard cooling models of neutron stars predict temperatures
yr. However, the likely thermal emission
detected from the millisecond pulsar J0437-4715, of spin-down age yr, implies a temperature K. Thus, a heating
mechanism needs to be added to the cooling models in order to obtain agreement
between theory and observation. Aims: Several internal heating mechanisms could
be operating in neutron stars, such as magnetic field decay, dark matter
accretion, crust cracking, superfluid vortex creep, and non-equilibrium
reactions ("rotochemical heating"). We study these mechanisms in order to
establish which could be the dominant source of thermal emission from old
pulsars. Methods: We show by simple estimates that magnetic field decay, dark
matter accretion, and crust cracking mechanism are unlikely to have a
significant effect on old neutron stars. The thermal evolution for the other
mechanisms is computed using the code of Fern\'andez and Reisenegger. Given the
dependence of the heating mechanisms on the spin-down parameters, we study the
thermal evolution for two types of pulsars: young, slowly rotating "classical"
pulsars and old, fast rotating millisecond pulsars. Results: We find that
magnetic field decay, dark matter accretion, and crust cracking do not produce
detectable heating of old pulsars. Rotochemical heating and vortex creep can be
important both for classical pulsars and millisecond pulsars. More restrictive
upper limits on the surface temperatures of classical pulsars could rule out
vortex creep as the main source of thermal emission. Rotochemical heating in
classical pulsars is driven by the chemical imbalance built up during their
early spin-down, and therefore strongly sensitive to their initial rotation
period.Comment: 7 pages, 5 figures, accepted version to be published in A&
Search for Stable Magnetohydrodynamic Equilibria in Barotropic Stars
It is now believed that magnetohydrodynamic equilibria can exist in stably
stratified stars due to the seminal works of Braithwaite & Spruit (2004) and
Braithwaite & Nordlund (2006). What is still not known is whether
magnetohydrodynamic equilibria can exist in a barotropic star, in which stable
stratification is not present. It has been conjectured by Reisenegger (2009)
that there will likely not exist any magnetohydrodynamical equilibria in
barotropic stars. We aim to test this claim by presenting preliminary MHD
simulations of barotropic stars using the three dimensional stagger code of
Nordlund & Galsgaard (1995).Comment: 4 pages, 2 figures, to appear in the proceedings of IAUS 302:
"Magnetic Fields Throughout Stellar Evolution
Neutrino emission rates in highly magnetized neutron stars revisited
Magnetars are a subclass of neutron stars whose intense soft-gamma-ray bursts
and quiescent X-ray emission are believed to be powered by the decay of a
strong internal magnetic field. We reanalyze neutrino emission in such stars in
the plausibly relevant regime in which the Landau band spacing of both protons
and electrons is much larger than kT (where k is the Boltzmann constant and T
is the temperature), but still much smaller than the Fermi energies. Focusing
on the direct Urca process, we find that the emissivity oscillates as a
function of density or magnetic field, peaking when the Fermi level of the
protons or electrons lies about 3kT above the bottom of any of their Landau
bands. The oscillation amplitude is comparable to the average emissivity when
the Landau band spacing mentioned above is roughly the geometric mean of kT and
the Fermi energy (excluding mass), i. e., at fields much weaker than required
to confine all particles to the lowest Landau band. Since the density and
magnetic field strength vary continuously inside the neutron star, there will
be alternating surfaces of high and low emissivity. Globally, these
oscillations tend to average out, making it unclear whether there will be any
observable effects.Comment: 7 pages, 2 figures; accepted for publication in Astronomy &
Astrophysic
Magnetic Field Evolution in Neutron Stars: One-Dimensional Multi-Fluid Model
This paper is the first in a series aimed at understanding the long-term
evolution of neutron star magnetic fields. We model the stellar matter as an
electrically neutral and lightly ionized plasma composed of three moving
particle species: neutrons, protons, and electrons, which can be converted into
each other by weak interactions (beta decays), suffer binary collisions, and be
affected by each other's macroscopic electromagnetic fields. Since the
evolution of the magnetic field occurs over thousands of years or more,
compared to dynamical time scales (sound and Alfv\'en) of milliseconds to
seconds, we use a slow-motion approximation in which we neglect the inertial
terms in the equations of motion for the particles. We restrict ourselves to a
one-dimensional geometry in which the magnetic field points in one Cartesian
direction but varies only along an orthogonal direction. We study the evolution
of the system in three different ways: (i) estimating time scales directly from
the equations, guided by physical intuition; (ii) a normal-mode analysis in the
limit of a nearly uniform system; and (iii) a finite-difference numerical
integration of the equations of motion. We find good agreement between our
analytical normal-mode solutions and the numerical simulations. We show that
the magnetic field and the particles evolve through successive
quasi-equilibrium states, on time scales that can be understood by physical
arguments. Depending of the parameter values the magnetic field can evolve by
ohmic diffusion or by ambipolar diffusion, the latter being limited either by
interparticle collisions or by relaxation to chemical equilibrium through beta
decays. The numerical simulations are further validated by verifying that they
satisfy the known conservation laws also in highly non-linear situations.Comment: Paper Accepted in Astronomy & Astrophysics: 24 April 2008, Paper
Reference Number: AA/2008/09466. Paper contains 8 Figures. In this version
the section: Summary and Conclusions has been expande
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