179 research outputs found
Neutrino transport and hydrodynamic stability of rotating proto-neutron stars
We consider stability of differentially rotating non-magnetic proto-neutron
stars. When neutrino transport is efficient, the star can be subject to a
diffusive instability that can occur even in the convectively stable region.
The instability arises on the time-scale comparable to the time-scale of
thermal diffusion. Hydrodynamic motions driven by the instability can lead to
anisotropy in the neutrino flux since the instability is suppressed near the
equator and rotation axis.Comment: 6 pages. Accepted for publication in A&
Magnetorotational instability in proto-neutron stars
Magnetorotational instability (MRI) has been suggested to lead a rapid growth
of the magnetic field in core collapse supernovae and produce departures from
spherical syymmetry that can be important in determining the explosion
mechanism. We address the problem of stability in differentially rotating
magnetized proto-neutron stars at the beginning of their evolution. Criteria
for MRI in proto-neutron stars are derived without simplying assumptions about
a weak magnetic field and are substantially different from the standard
condition. If the magnetic field is strong, MRI can occur only in the
neighbourhood of the region where the spherical radial component of the
magnetic field vanishes. The growth rate of MRI is relatively low except for
perturbations with very small scales which usually are not detected in
numerical simulations. We find that MRI in proto-neutron stars grows more
slowly than than the double diffusive instability analogous the
Goldreich-Schubert-Fricke instability in ordinary stars.Comment: 6 pages, 2 figures, accepted for publication in Astronomy and
Astrophysic
Force-free pulsar magnetosphere: instability and generation of MHD waves
Magnetohydrodynamic (MHD) instabilities can play an important role in the
structure and dynamics of the pulsar magnetosphere. We consider the
instabilitycaused by differential rotation that is suggested by many
theoretical models. Stability is considered by means of a linear analysis
within the frane of the force-free MHD. We argue that differentially rotating
magnetospheres are unstable for any particular geometry of the magnetic field
and rotation law. The characteristic growth time of instability is of the order
of the rotation period. The instability can lead to fluctuations of the
emission and enhancement of diffusion in the magnetosphere.Comment: 5 pages; to appear in Astronomy and Astrophysic
Magnetic and spin evolution of neutron stars in close binaries
The evolution of neutron stars in close binary systems with a low-mass
companion is considered assuming the magnetic field to be confined within the
solid crust. We adopt the standard scenario of the evolution in a close binary
system in accordance with which the neutron star passes throughout four
evolutionary phases ("isolated pulsar" -- "propeller" -- accretion from the
wind of a companion -- accretion due to Roche-lobe overflow). Calculations have
been performed for a great variety of parameters characterizing the properties
both of the neutron star and low-mass companion. We find that neutron stars
with more or less standard magnetic field and spin period being processed in
low-mass binaries can evolve to low-field rapidly rotating pulsars. Even if the
main-sequence life of a companion is as long as yr, the neutron star
can maintain a relatively strong magnetic field to the end of the accretion
phase. The considered model can well account for the origin of millisecond
pulsars.Comment: 18 pages + 10 figures, uses epsf.sty. Accepted by MNRA
Evolution of Crustal Magnetic Fields in Isolated Neutron Stars : Combined Effects of Cooling and Curvature of Space-time
The ohmic decay of magnetic fields confined within the crust of neutron stars
is considered by incorporating both the effect of neutron star cooling and the
effect of space-time curvature produced by the intense gravitational field of
the star. For this purpose a stationary and static gravitational field has been
considered with the standard as well as the accelerated cooling models of
neutron stars. It is shown that general relativistic effect reduces the
magnetic field decay rate substantially. At the late stage of evolution when
the field decay is mainly determined by the impurity-electron scattering, the
effect of space-time curvature suppresses the role of the impurity content
significantly and reduces the decay rate by more than an order of magnitude.
Even with a high impurity content the decay rate is too low to be of
observational interest if the accelerated cooling model along with the effect
of space-time curvature is taken into account. It is, therefore, pointed out
that if a decrease in the magnetic field strength by more than two orders of
magnitude from its initial value is detected by observation then the existence
of quark in the core of the neutron star would possibly be ruled out.Comment: 15 pages, AAS LATEX macros v4.0, 5 postscript figures, Accepted for
publication in the Astrophysical Journal (Part I
The neutron star in Cassiopeia A: equation of state, superfluidity, and Joule heating
The thermomagnetic evolution of the young neutron star in Cassiopea A is
studied by considering fast neutrino emission processes. In particular, we
consider neutron star models obtained from the equation of state computed in
the framework of the Brueckner-Bethe-Goldstone many-body theory and variational
methods, and models obtained with the Akmal-Pandharipande-Ravenhall equation of
state. It is shown that it is possible to explain a fast cooling regime as the
one observed in the neutron star in Cassiopea A if the Joule heating produced
by dissipation of the small-scale magnetic field in the crust is taken into
account. We thus argue that it is difficult to put severe constraints on the
superfluid gap if the Joule heating is considered.Comment: 4 pages, 2 figures, to appear on A&A Letter
Mixing zones in magnetized differentially rotating stars
We study the secular instability of magnetized differentially rotating
radiative zones taking account of viscosity and magnetic and thermal
diffusivities. The considered instability generalizes the well-known
Goldreich-Schubert-Fricke instability for the case of a sufficiently strong
magnetic field. In magnetized stars, instability can lead to a formation of
non-spherical unstable zones where weak turbulence mixes the material between
the surface and interiors. Such unstable zones can manifest themselves by a
non-spherical distribution of abundance anormalies on the stellar surface.Comment: 8 pages, accepted by Astronomy and Astrophysic
Generation of the magnetic field in jets
We consider dynamo action under the combined influence of turbulence and
large-scale shear in sheared jets. Shear can stretch turbulent magnetic field
lines in such a way that even turbulent motions showing mirror symmetry become
suitable for generation of a large-scale magnetic field. We derive the integral
induction equation governing the behaviour of the mean field in jets. The main
result is that sheared jets may generate a large-scale magnetic field if shear
is sufficiently strong. The generated mean field is mainly concentrated in a
magnetic sheath surrounding the central region of a jet, and it exhibits sign
reversals in the direction of the jet axis. Typically, the magnetic field in a
sheath is dominated by the component along the jet that can reach equipartition
with the kinetic energy of particles, The field in the central region of jets
has a more disordered structure.Comment: 7 pages, accepted for publication in A&
Magnetohydrodynamic waves in the pulsar magnetosphere
MHD waves can be responsible for plasma fluctuations and short-term
variations of the pulsar emission. We consider the properties of plane and
cylindrical waves that can exist in the force-free magnetosphere. Waves are
considered by means of a linear analysis of the force-free MHD equations. We
argue that these particular types of waves can exist in the magnetosphere of
pulsars. These waves are closely related to the Alfven waves of the standard
magnetohydrodynamics but are modified by the force-free condition and non-zero
charge density. We derive the dispersion relation for magnetospheric waves and
show that the wave periods are likely within the range \sim 10^{-2}-10^(-4} s
depending on the magnetospheric parameters.Comment: 4 pages; to appear in Astronomy and Astrophysic
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