842 research outputs found
Seed Magnetic Fields Generated by Primordial Supernova Explosions
The origin of the magnetic field in galaxies is an open question in
astrophysics. Several mechanisms have been proposed related, in general, with
the generation of small seed fields amplified by a dynamo mechanism. In
general, these mechanisms have difficulty in satisfying both the requirements
of a sufficiently high strength for the magnetic field and the necessary large
coherent scales. We show that the formation of dense and turbulent shells of
matter, in the multiple explosion scenario of Miranda and Opher (1996, 1997)
for the formation of the large-scale structures of the Universe, can naturally
act as a seed for the generation of a magnetic field. During the collapse and
explosion of Population III objects, a temperature gradient not parallel to a
density gradient can naturally be established, producing a seed magnetic field
through the Biermann battery mechanism. We show that seed magnetic fields can be produced in this multiple explosion scenario on
scales of the order of clusters of galaxies (with coherence length ) and up to on scales of galaxies ().Comment: Accepted for publication in MNRAS, 5 pages (MN plain TeX macros v1.6
file). Also available at http://www.iagusp.usp.br/~oswaldo (click "OPTIONS"
and then "ARTICLES"
Dependence of the MHD shock thickness on the finite electrical conductivity
The results of MHD plane shock waves with infinite electrical conductivity
are generalized for a plasma with a finite conductivity. We derive the
adiabatic curves that describe the evolution of the shocked gas as well as the
change in the entropy density. For a parallel shock (i.e., in which the
magnetic field is parallel to the normal to the shock front) we find an
expression for the shock thickness which is a function of the ambient magnetic
field and of the finite electrical conductivity of the plasma. We give
numerical estimates of the physical parameters for which the shock thickness is
of the order of, or greater than, the mean free path of the plasma particles in
a strongly magnetized plasma.Comment: 8 pages, uses standard revtex, to appear in Journal of Plasma Physic
The Vector Direction of the Interstellar Magnetic Field Outside the Heliosphere
We propose that magnetic reconnection at the heliopause only occurs where the
interstellar magnetic field points nearly anti-parallel to the heliospheric
field. By using large-scale magnetohydrodynamic (MHD) simulations of the
heliosphere to provide the initial conditions for kinetic simulations of
heliopause (HP) reconnection we show that the energetic pickup ions downstream
from the solar wind termination shock induce large diamagnetic drifts in the
reconnecting plasma and stabilize non-anti-parallel reconnection. With this
constraint the MHD simulations can show where HP reconnection most likely
occurs. We also suggest that reconnection triggers the 2-3 kHz radio bursts
that emanate from near the HP. Requiring the burst locations to coincide with
the loci of anti-parallel reconnection allows us to determine, for the first
time, the vector direction of the local interstellar magnetic field. We find it
to be oriented towards the southern solar magnetic pole.Comment: Submitted to ApJ; incorporates minor referee-suggested revision
A new inflaton model beginning near the Planck epoch
The Starobinsky model predicts a primordial inflation period without the
presence of an inflaton field. The modified version of this model predicts a
simple time dependence for the Hubble parameter , which decreases slowly
between the Planck epoch and the end of the inflation, , where is a dimensionless constant to be adjusted from
observations. We investigate an inflaton model which has the same time
dependence for . A reverse engineered inflaton potential for the time
dependence of is derived. Normalization of the derived inflaton potential
is determined by the condition that the observed density fluctuations,
, are created at -folds before the
end of inflation. The derived potential indicates an energy (mass) scale,
, at the end of inflation. Using the slow
roll parameters, which are obtained from this potential, we calculate the
spectral index for the scalar modes and the relative amplitude of the
tensor to scalar modes . A tensor contribution, , and an
approximately Harrison-Zeldovich density perturbation spectrum, , are predicted.Comment: 7 pages, minor changes, improved discussion. To appear in Braz.J.Phy
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