145 research outputs found
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
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&
Strange stars in low-mass binary pulsar systems
Based on observational facts and a variety of theoretical arguments we
discuss in this work the possibility that pulsars in Low-Mass Binary Pulsar
systems could be strange stars rather than neutron stars. It is shown that,
although subject to reasonable uncertainties, the consideration of the physics
of the SQM core and thin normal crusts leads to the prediction of several
observed features of the magnetic field history of these systems whitin this
working hypothesis.Comment: 6 pages, no figures, PlainTex file submitted to IJMP
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