440 research outputs found
Numerical simulations of the magnetorotational instability in protoneutron stars: I. Influence of buoyancy
The magneto-rotational instability (MRI) is considered to be a promising
mechanism to amplify the magnetic field in fast rotating protoneutron stars. In
contrast to accretion disks, radial buoyancy driven by entropy and lepton
fraction gradients is expected to have a dynamical role as important as
rotation and shear. We investigate the poorly known impact of buoyancy on the
non-linear phase of the MRI, by means of three dimensional numerical
simulations of a local model in the equatorial plane of a protoneutron star.
The use of the Boussinesq approximation allows us to utilise a shearing box
model with clean shearing periodic boundary conditions, while taking into
account the buoyancy driven by radial entropy and composition gradients. We
find significantly stronger turbulence and magnetic fields in buoyantly
unstable flows. On the other hand, buoyancy has only a limited impact on the
strength of turbulence and magnetic field amplification for buoyantly stable
flows in the presence of a realistic thermal diffusion. The properties of the
turbulence are, however, significantly affected in the latter case. In
particular, the toroidal components of the magnetic field and of the velocity
become even more dominant with respect to the poloidal ones. Furthermore, we
observed in the regime of stable buoyancy the formation of long lived coherent
structures such as channel flows and zonal flows. Overall, our results support
the ability of the MRI to amplify the magnetic field significantly even in
stably stratified regions of protoneutron stars.Comment: 22 pages, 15 figures, accepted for publication in MNRA
Global evolution of the magnetic field in a thin disc and its consequences for protoplanetary systems
The strength and structure of the large-scale magnetic field in
protoplanetary discs are still unknown, although they could have important
consequences for the dynamics and evolution of the disc. Using a mean-field
approach in which we model the effects of turbulence through enhanced diffusion
coefficients, we study the time-evolution of the large-scale poloidal magnetic
field in a global model of a thin accretion disc, with particular attention to
protoplanetary discs. With the transport coefficients usually assumed, the
magnetic field strength does not significantly increase radially inwards,
leading to a relatively weak magnetic field in the inner part of the disc. We
show that with more realistic transport coefficients that take into account the
vertical structure of the disc and the back-reaction of the magnetic field on
the flow as obtained by Guilet & Ogilvie (2012), the magnetic field can
significantly increase radially inwards. The magnetic-field profile adjusts to
reach an equilibrium value of the plasma parameter (the ratio of
midplane thermal pressure to magnetic pressure) in the inner part of the disc.
This value of depends strongly on the aspect ratio of the disc and on
the turbulent magnetic Prandtl number, and lies in the range for
protoplanetary discs. Such a magnetic field is expected to affect significantly
the dynamics of protoplanetary discs by increasing the strength of MHD
turbulence and launching an outflow. We discuss the implications of our results
for the evolution of protoplanetary discs and for the formation of powerful
jets as observed in T-Tauri star systems.Comment: 19 pages, 12 figures, accepted for publication in MNRA
A Shallow Water Analogue of the Standing Accretion Shock Instability: Experimental Demonstration and Two-Dimensional Model
Despite the sphericity of the collapsing stellar core, the birth conditions
of neutron stars can be highly non spherical due to a hydrodynamical
instability of the shocked accretion flow. Here we report the first laboratory
experiment of a shallow water analogue, based on the physics of hydraulic
jumps. Both the experiment and its shallow water modeling demonstrate a robust
linear instability and nonlinear properties of symmetry breaking, in a system
which is one million times smaller and about hundred times slower than its
astrophysical analogue.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Letters.
Supplementary Material (6 movies) available at
http://irfu.cea.fr/Projets/SN2NS/outreach.htm
Physical and chemical characterisation of crude meat and bone meal combustion residue: “waste or raw material?”
As a result of the recent bovine spongiform encephalopathy (BSE) crisis in the European beef industry, the use of animal by-product is now severely controlled. Meat and bone meal (MBM) production can no longer be used to feed cattle and must be safely disposed of or transformed. Main disposal option is incineration, producing huge amounts of ashes the valorisation of which becomes a major concern. The aim of this work is to characterise MBM combustion residue in order to evaluate their physical and chemical properties to propose new valorisation avenues. The thermal behaviour of crude meat and bone meal was followed by thermogravimetric analysis (TGA) and (24 wt.%) inorganic residue was collected. The resulting ashes were characterised by powder X-ray diffraction (XRD), particle size distribution, specific surface area (BET), scanning electron microscopy (SEM) couple with energy disperse X-ray analysis (EDX). Elemental analysis revealed the presence of chloride, sodium, potassium, magnesium with high level of phosphate (56 wt.%) and calcium (31 wt.%), two major constituents of bone, mainly as a mixture of Ca10(PO4)6(OH)2 and Ca3(PO4)2 phases. The impact of combustion temperature (from 550 to 1000 °C) on the constitution of ashes was followed by TGA, XRD and specific surface measurements. We observed a strong decrease of surface area for the ashes with crystallisation of calcium phosphates phases without major changes of chemical compositio
Toward a magnetohydrodynamic theory of the stationary accretion shock: toy model of the advective-acoustic cycle in a magnetized flow
The effect of a magnetic field on the linear phase of the advective-acoustic
instability is investigated, as a first step toward a magnetohydrodynamic (MHD)
theory of the stationary accretion shock instability taking place during
stellar core collapse. We study a toy model where the flow behind a planar
stationary accretion shock is adiabatically decelerated by an external
potential. Two magnetic field geometries are considered: parallel or
perpendicular to the shock. The entropy-vorticity wave, which is simply
advected in the unmagnetized limit, separates into five different waves: the
entropy perturbations are advected, while the vorticity can propagate along the
field lines through two Alfven waves and two slow magnetosonic waves. The two
cycles existing in the unmagnetized limit, advective-acoustic and purely
acoustic, are replaced by up to six distinct MHD cycles. The phase differences
among the cycles play an important role in determining the total cycle
efficiency and hence the growth rate. Oscillations in the growth rate as a
function of the magnetic field strength are due to this varying phase shift. A
vertical magnetic field hardly affects the cycle efficiency in the regime of
super-Alfvenic accretion that is considered. In contrast, we find that a
horizontal magnetic field strongly increases the efficiencies of the vorticity
cycles that bend the field lines, resulting in a significant increase of the
growth rate if the different cycles are in phase. These magnetic effects are
significant for large-scale modes if the Alfven velocity is a sizable fraction
of the flow velocity.Comment: 13 pages, 9 figures, accepted for publication in ApJ. Cosmetic
changes after proof reading corrections
Dynamics of an Alfven surface in core collapse supernovae
We investigate the dynamics of an Alfven surface (where the Alfven speed
equals the advection velocity) in the context of core collapse supernovae
during the phase of accretion on the proto-neutron star. Such a surface should
exist even for weak magnetic fields because the advection velocity decreases to
zero at the center of the collapsing core. In this decelerated flow, Alfven
waves created by the standing accretion shock instability (SASI) or convection
accumulate and amplify while approaching the Alfven surface. We study this
amplification using one dimensional MHD simulations with explicit physical
dissipation. In the linear regime, the amplification continues until the Alfven
wavelength becomes as small as the dissipative scale. A pressure feedback that
increases the pressure in the upstream flow is created via a non linear
coupling. We derive analytic formulae for the maximum amplification and the non
linear coupling and check them with numerical simulations to a very good
accuracy. We also characterize the non linear saturation of this amplification
when compression effects become important, leading to either a change of the
velocity gradient, or a steepening of the Alfven wave. Applying these results
to core collapse supernovae shows that the amplification can be fast enough to
affect the dynamics, if the magnetic field is strong enough for the Alfven
surface to lie in the region of strong velocity gradient just above the
neutrinosphere. This requires the presence of a strong magnetic field in the
progenitor star, which would correspond to the formation of a magnetar under
the assumption of magnetic flux conservation. An extrapolation of our analytic
formula (taking into account the nonlinear saturation) suggests that the Alfven
wave could reach an amplitude of B ~ 10^15 G, and that the pressure feedback
could significantly contribute to the pressure below the shock.Comment: 18 pages, 14 figures, accepted for publication in ApJ. Added a
discussion of the energy budget in subsection 7.
On the linear growth mechanism driving the stationary accretion shock instability
During stellar core collapse, which eventually leads to a supernovae
explosion, the stalled shock is unstable due to the standing accretion shock
instability (SASI). This instability induces large-scale non spherical
oscillations of the shock, which have crucial consequences on the dynamics and
the geometry of the explosion. While the existence of this instability has been
firmly established, its physical origin remains somewhat uncertain. Two
mechanisms have indeed been proposed to explain its linear growth. The first is
an advective-acoustic cycle, where the instability results from the interplay
between advected perturbations (entropy and vorticity) and an acoustic wave.
The second mechanism is purely acoustic and assumes that the shock is able to
amplify trapped acoustic waves. Several arguments favouring the
advective-acoustic cycle have already been proposed, however none was entirely
conclusive for realistic flow parameters. In this article we give two new
arguments which unambiguously show that the instability is not purely acoustic,
and should be attributed to the advective-acoustic cycle. First, we extract a
radial propagation timescale by comparing the frequencies of several unstable
harmonics that differ only by their radial structure. The extracted time
matches the advective-acoustic time but strongly disagrees with a purely
acoustic interpretation. Second, we present a method to compute purely acoustic
modes, by artificially removing advected perturbations below the shock. All
these purely acoustic modes are found to be stable, showing that the advected
wave is essential to the instability mechanism.Comment: 17 pages, 10 figures, accepted for publication in MNRA
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