241 research outputs found
Hyperbolic character of the angular moment equations of radiative transfer and numerical methods
We study the mathematical character of the angular moment equations of
radiative transfer in spherical symmetry and conclude that the system is
hyperbolic for general forms of the closure relation found in the literature.
Hyperbolicity and causality preservation lead to mathematical conditions
allowing to establish a useful characterization of the closure relations. We
apply numerical methods specifically designed to solve hyperbolic systems of
conservation laws (the so-called Godunov-type methods), to calculate numerical
solutions of the radiation transport equations in a static background. The
feasibility of the method in any kind of regime, from diffusion to
free-streaming, is demonstrated by a number of numerical tests and the effect
of the choice of the closure relation on the results is discussed.Comment: 37 pags, 12 figures, accepted for publication in MNRA
The relevance of ambipolar diffusion for neutron star evolution
We study ambipolar diffusion in strongly magnetised neutron stars, with
special focus on the effects of neutrino reaction rates and the impact of a
superfluid/superconducting transition in the neutron star core. For
axisymmetric magnetic field configurations, we determine the deviation from
equilibrium induced by the magnetic force and calculate the velocity of
the slow, quasi-stationary, ambipolar drift. We study the temperature
dependence of the velocity pattern and clearly identify the transition to a
predominantly solenoidal flow. For stars without superconducting/superfluid
constituents and with a mixed poloidal-toroidal magnetic field of typical
magnetar strength, we find that ambipolar diffusion proceeds fast enough to
have a significant impact on the magnetic field evolution only at low core
temperatures, K. The ambipolar diffusion timescale
becomes appreciably shorter when fast neutrino reactions are present, because
the possibility to balance part of the magnetic force with pressure gradients
is reduced. We also find short ambipolar diffusion timescales in the case of
superconducting cores for K, due to the reduced interaction
between protons and neutrons. In the most favourable scenario, with fast
neutrino reactions and superconducting cores, ambipolar diffusion results in
advection velocities of several km/kyr. This velocity can substantially
reorganize magnetic fields in magnetar cores, in a way that can only be
confirmed by dynamical simulations.Comment: 14 pages, 11 figures, version accepted for publication in MNRA
Population synthesis of isolated Neutron Stars with magneto--rotational evolution
We revisit the population synthesis of isolated radio-pulsars incorporating
recent advances on the evolution of the magnetic field and the angle between
the magnetic and rotational axes from new simulations of the magneto-thermal
evolution and magnetosphere models, respectively. An interesting novelty in our
approach is that we do not assume the existence of a death line. We discuss
regions in parameter space that are more consistent with the observational
data. In particular, we find that any broad distribution of birth spin periods
with s can fit the data, and that if the alignment angle is
allowed to vary consistently with the torque model, realistic magnetospheric
models are favoured compared to models with classical magneto-dipolar radiation
losses. Assuming that the initial magnetic field is given by a lognormal
distribution, our optimal model has mean strength with width .
However, there are strong correlations between parameters. This degeneracy in
the parameter space can be broken by an independent estimate of the pulsar
birth rate or by future studies correlating this information with the
population in other observational bands (X-rays and -rays).Comment: 10 pages, 9 figures, submitted and accepted to MNRAS, comments
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Relativitat i Astrofísica
Lección Inaugural Curso Académico 2017-2018Lliçó Inaugural Curs Acadèmic 2017-201
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