18,137 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
A Hamiltonian functional for the linearized Einstein vacuum field equations
By considering the Einstein vacuum field equations linearized about the
Minkowski metric, the evolution equations for the gauge-invariant quantities
characterizing the gravitational field are written in a Hamiltonian form by
using a conserved functional as Hamiltonian; this Hamiltonian is not the analog
of the energy of the field. A Poisson bracket between functionals of the field,
compatible with the constraints satisfied by the field variables, is obtained.
The generator of spatial translations associated with such bracket is also
obtained.Comment: 5 pages, accepted in J. Phys.: Conf. Serie
Local continuity laws on the phase space of Einstein equations with sources
Local continuity equations involving background fields and variantions of the
fields, are obtained for a restricted class of solutions of the
Einstein-Maxwell and Einstein-Weyl theories using a new approach based on the
concept of the adjoint of a differential operator. Such covariant conservation
laws are generated by means of decoupled equations and their adjoints in such a
way that the corresponding covariantly conserved currents possess some
gauge-invariant properties and are expressed in terms of Debye potentials.
These continuity laws lead to both a covariant description of bilinear forms on
the phase space and the existence of conserved quantities. Differences and
similarities with other approaches and extensions of our results are discussed.Comment: LaTeX, 13 page
OH+ in astrophysical media: state-to-state formation rates, Einstein coefficients and inelastic collision rates with He
The rate constants required to model the OH observations in different
regions of the interstellar medium have been determined using state of the art
quantum methods.
First, state-to-state rate constants for the H+ O()
H + OH reaction have been obtained using
a quantum wave packet method. The calculations have been compared with
time-independent results to asses the accuracy of reaction probabilities at
collision energies of about 1 meV. The good agreement between the simulations
and the existing experimental cross sections in the 1 eV energy range
shows the quality of the results.
The calculated state-to-state rate constants have been fitted to an
analytical form. Second, the Einstein coefficients of OH have been obtained
for all astronomically significant ro-vibrational bands involving the
and/or electronic states.
For this purpose the potential energy curves and electric dipole transition
moments for seven electronic states of OH are calculated with {\it ab
initio} methods at the highest level and including spin-orbit terms, and the
rovibrational levels have been calculated including the empirical spin-rotation
and spin-spin terms. Third, the state-to-state rate constants for inelastic
collisions between He and OH have been calculated using a
time-independent close coupling method on a new potential energy surface. All
these rates have been implemented in detailed chemical and radiative transfer
models. Applications of these models to various astronomical sources show that
inelastic collisions dominate the excitation of the rotational levels of
OH. In the models considered the excitation resulting from the chemical
formation of OH increases the line fluxes by about 10 % or less depending
on the density of the gas
Truncation effects in superdiffusive front propagation with L\'evy flights
A numerical and analytical study of the role of exponentially truncated
L\'evy flights in the superdiffusive propagation of fronts in
reaction-diffusion systems is presented. The study is based on a variation of
the Fisher-Kolmogorov equation where the diffusion operator is replaced by a
-truncated fractional derivative of order where
is the characteristic truncation length scale. For there is no
truncation and fronts exhibit exponential acceleration and algebraic decaying
tails. It is shown that for this phenomenology prevails in the
intermediate asymptotic regime where
is the diffusion constant. Outside the intermediate asymptotic regime,
i.e. for , the tail of the front exhibits the tempered decay
, the acceleration is transient, and
the front velocity, , approaches the terminal speed as , where it is assumed that
with denoting the growth rate of the
reaction kinetics. However, the convergence of this process is algebraic, , which is very slow compared to the exponential
convergence observed in the diffusive (Gaussian) case. An over-truncated regime
in which the characteristic truncation length scale is shorter than the length
scale of the decay of the initial condition, , is also identified. In
this extreme regime, fronts exhibit exponential tails, ,
and move at the constant velocity, .Comment: Accepted for publication in Phys. Rev. E (Feb. 2009
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