169 research outputs found
Relativistic Outflows in Gamma-Ray Bursts
The possibility that gamma-ray bursts (GRBs) were not isotropic emissions was
devised theoretically as a way to ameliorate the huge energetic budget implied
by the standard fireball model for these powerful phenomena. However, the
mechanism by which after the quasy-isotropic release of a few erg
yields a collimated ejection of plasma could not be satisfactory explained
analytically. The reason being that the collimation of an outflow by its
progenitor system depends on a very complex and non-linear dynamics. That has
made necessary the use of numerical simulations in order to shed some light on
the viability of some likely progenitors of GRBs. In this contribution I will
review the most relevant features shown by these numerical simulations and how
they have been used to validate the collapsar model (for long GRBs) and the
model involving the merger of compact binaries (for short GRBs).Comment: 8 pages, 1 figure. Proceedings of the conference: "Circumstellar
Media and Late Stages of Massive Stellar Evolution". Ensenada (Mexico). To be
published by Revista Mexicana de Astronomia y Astrofisic
Simulations of the Magneto-rotational Instability in Core-Collapse Supernovae
We assess the importance of the magneto-rotational instability in
core-collapse supernovae by an analysis of the growth rates of unstable modes
in typical post-collapse systems and by numerical simulations of simplified
models. The interplay of differential rotation and thermal stratification
defines different instability regimes which we confirm in our simulations. We
investigate the termination of the growth of the MRI by parasitic
instabilities, establish scaling laws characterising the termination amplitude,
and study the long-term evolution of the saturated turbulent state.Comment: 6 pages, 1 figure. To appear in Proceedings of 4th International
Conference on Numerical Modeling of Space Plasma Flows (Chamonix 2009
Energetic particle acceleration and transport by Alfven/acoustic waves in tokamak-like Solar flares
Alfv´en/acoustic waves are ubiquitous in astrophysical as well as in laboratory plasmas. Their interplay with energetic ions is of crucial importance to understanding the energy
and particle exchange in astrophysical plasmas as well as to obtaining a viable energy source
in magnetically confined fusion devices. In magnetically confined fusion plasmas, an experimental phase-space characterisation of convective and diffusive energetic particle losses induced by Alfv´en/acoustic waves allows for a better understanding of the underlying physics. The relevance of these results in the problem of the anomalous heating of the solar corona is checked by MHD simulations of Tokamak-like Solar flare tubes
A method for computing synchrotron and inverse-Compton emission from hydrodynamic simulations of supernova remnants
The observational signature of supernova remnants (SNRs) is very complex, in
terms of both their geometrical shape and their spectral properties, dominated
by non-thermal synchrotron and inverse-Compton scattering. We propose a
post-processing method to analyse the broad-band emission of SNRs based on
three-dimensional hydrodynamical simulations. From the hydrodynamical data, we
estimate the distribution of non-thermal electrons accelerated at the shock
wave and follow the subsequent evolution as they lose or gain energy by
adiabatic expansion or compression and emit energy by radiation. As a first
test case, we use a simulation of a bipolar supernova expanding into a cloudy
medium. We find that our method qualitatively reproduces the main observational
features of typical SNRs and produces fluxes that agree with observations to
within a factor of a few. allowing for further use in more extended sets of
models.Comment: 15 pages, 3 figures; accepted, HEDLA 2014 special issue of High
Energy Density Physic
Local simulations of the magnetized Kelvin-Helmholtz instability in neutron-star mergers
Context. Global MHD simulations show Kelvin-Helmholtz (KH) instabilities at
the contact surface of two merging neutron stars. That region has been
identified as the site of efficient amplification of magnetic fields. However,
these global simulations, due to numerical limitations, were unable to
determine the saturation level of the field strength, and thus the possible
back-reaction of the magnetic field onto the flow. Aims. We investigate the
amplification of initially weak fields in KH unstable shear flows, and the
back-reaction of the field onto the flow. Methods. We use a high-resolution
ideal MHD code to perform 2D and 3D local simulations of shear flows. Results.
In 2D, the magnetic field is amplified in less than 0.01ms until it reaches
locally equipartition with the kinetic energy. Subsequently, it saturates due
to resistive instabilities that disrupt the KH vortex and decelerate the shear
flow on a secular time scale. We determine scaling laws of the field
amplification with the initial field strength and the grid resolution. In 3D,
this hydromagnetic mechanism may be dominated by purely hydrodynamic
instabilities limiting the amplification. We find maximum magnetic fields of
10^16 G locally, and r.m.s. maxima within the box of 10^15 G. However, such
strong fields exist only for a short period. In the saturated state, the
magnetic field is mainly oriented parallel to the shear flow for strong initial
fields, while weaker initial fields tend to lead to a more balanced
distribution of the field energy. In all models the flow shows small-scale
features. The magnetic field is at most in equipartition with the decaying
shear flow. (abridged)Comment: 26 pages, 22 figures (figure quality reduced); accepted for
publication in Astronomy & Astrophysic
Estimation of the mechanical properties of the eye through the study of its vibrational modes
Measuring the eye's mechanical properties in vivo and with minimally invasive
techniques can be the key for individualized solutions to a number of eye
pathologies. The development of such techniques largely relies on a
computational modelling of the eyeball and, it optimally requires the synergic
interplay between experimentation and numerical simulation. In Astrophysics and
Geophysics the remote measurement of structural properties of the systems of
their realm is performed on the basis of (helio-)seismic techniques. As a
biomechanical system, the eyeball possesses normal vibrational modes
encompassing rich information about its structure and mechanical properties.
However, the integral analysis of the eyeball vibrational modes has not been
performed yet. Here we develop a new finite difference method to compute both
the spheroidal and, specially, the toroidal eigenfrequencies of the human eye.
Using this numerical model, we show that the vibrational eigenfrequencies of
the human eye fall in the interval 100 Hz - 10 MHz. We find that compressible
vibrational modes may release a trace on high frequency changes of the
intraocular pressure, while incompressible normal modes could be registered
analyzing the scattering pattern that the motions of the vitreous humour leave
on the retina. Existing contact lenses with embebed devices operating at high
sampling frequency could be used to register the microfluctuations of the
eyeball shape we obtain. We advance that an inverse problem to obtain the
mechanical properties of a given eye (e.g., Young's modulus, Poisson ratio)
measuring its normal frequencies is doable. These measurements can be done
using non-invasive techniques, opening very interesting perspectives to
estimate the mechanical properties of eyes in vivo. Future research might
relate various ocular pathologies with anomalies in measured vibrational
frequencies of the eye.Comment: Published in PLoS ONE as Open Access Research Article. 17 pages, 5
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