142 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
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
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
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
Axisymmetric simulations of magnetorotational core collapse: approximate inclusion of general relativistic effects
We continue our investigations of the magnetorotational collapse of stellar cores by discussing simulations performed with a modified Newtonian gravitational potential that mimics general relativistic effects. The approximate TOV gravitational potential used in our simulations captures several basic features of fully relativistic simulations quite well. In particular, it is able to correctly reproduce the behavior of models that show a qualitative change both of the dynamics and the gravitational wave signal when switching from Newtonian to fully relativistic simulations. For models where the dynamics and gravitational wave signals are already captured qualitatively correctly by a Newtonian potential, the results of the Newtonian and the approximate TOV models differ quantitatively. The collapse proceeds to higher densities with the approximate TOV potential, allowing for a more efficient amplification of the magnetic field by differential rotation. The strength of the saturation fields (âź10^15 G at the surface of the inner core) is a factor of two to three higher than in Newtonian gravity. Due to the more efficient field amplification, the influence of magnetic fields is considerably more pronounced than in the Newtonian case for some of the models. As in the Newtonian case, sufficiently strong magnetic fields slow down the coreâs rotation and trigger a secular contraction phase to higher densities. More clearly than in Newtonian models, the collapsed cores of these models exhibit two different kinds of shock generation. Due to magnetic braking, a first shock wave created during the initial centrifugal bounce at subnuclear densities does not suffice for ejecting any mass, and the temporarily stabilized core continues to collapse to supranuclear densities. Another stronger shock wave is generated during the second bounce as the core exceeds nuclear matter density. The gravitational wave signal of these models does not fit into the standard classification. Therefore, in the first paper of this series we introduced a new type of gravitational wave signal, which we call type IV or âmagnetic typeâ. This signal type is more frequent for the approximate relativistic potential than for the Newtonian one. Most of our weak-field models are marginally detectable with the current LIGO interferometer for a source located at a distance of 10 kpc. Strongly magnetized models emit a substantial fraction of their GW power at very low frequencies. A flat spectrum between 10 Hz and <âź100 kHz denotes the generation of a jet-like hydromagnetic outflow.Aloy Toras, Miguel Angel, [email protected]
Magnetorotational supernovae: a nucleosynthetic analysis of sophisticated 3D models
Magnetorotational supernovae are a rare type of core-collapse supernovae where the magnetic field and rotation play a central role in the dynamics of the explosion. We present the post-processed nucleosynthesis of state-of-the-art neutrino-MHD supernova models that follow the post explosion evolution for few seconds. We find three different dynamical mechanisms to produce heavy r-process elements: (i) a prompt ejection of matter right after core bounce, (ii) neutron-rich matter that is ejected at late times due to a reconfiguration of the protoneutronstar shape, (iii) small amount of mass ejected with high entropies in the centre of the jet. We investigate total ejecta yields, including the ones of unstable nuclei such as 26Al, 44Ti, 56Ni, and 60Fe. The obtained 56Ni masses vary between 0.01â1Mââ . The latter maximum is compatible with hypernova observations. Furthermore, all of our models synthesize Zn masses in agreement with observations of old metal-poor stars. We calculate simplified light curves to investigate whether our models can be candidates for superluminous supernovae. The peak luminosities obtained from taking into account only nuclear heating reach up to a few âź1043ergsâ1â . Under certain conditions, we find a significant impact of the 66Ni decay chain that can raise the peak luminosity up to âź38 percent compared to models including only the 56Ni decay chain. This work reinforces the theoretical evidence on the critical role of magnetorotational supernovae to understand the occurrence of hypernovae, superluminous supernovae, and the synthesis of heavy elements
Semi-global simulations of the magneto-rotational instability in core collapse supernovae
Possible effects of magnetic fields in core collapse supernovae rely on an
efficient amplification of the weak pre-collapse fields. It has been suggested
that the magneto-rotational instability (MRI) leads to rapid field growth.
Although MRI studies exist for accretion discs, the application of their
results to core collapse supernovae is inhibited as the physics of supernova
cores is substantially different from that of accretion discs. We address the
problem of growth and saturation of the MRI by means of semi-global
simulations, which combine elements of global and local simulations by taking
the presence of global background gradients into account and using a local
computational grid. We analyze the dispersion relation of the MRI to identify
different regimes of the instability. This analysis is complemented by
simulations, where we consider a local computational box rotating at
sub-Keplerian velocity, and where we allow for a radial entropy gradient. We
identify six regimes of the MRI depending on the ratio of the entropy and
angular velocity gradient. Our numerical models confirm the instability
criteria and growth rates for all relevant regimes. The MRI grows exponentially
within milliseconds the flow and magnetic field geometries being dominated by
channel flows. The MRI growth ceases once the channels are disrupted by
resistive instabilities (due to finite numerical conductivity), and MHD
turbulence sets in. From an analysis of the growth rates of the resistive
instabilities, we deduce scaling laws for the termination amplitude of the MRI
which agree well with our numerical models. We determine the dependence of the
development of coherent flow structures in the saturated state on the aspect
ratio of the simulation boxes. [abridged]Comment: 32 pages, 33 figures. Accepted for publication in Astronomy &
Astrophysic
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