2,968 research outputs found

### Relativistic particle transport in extragalactic jets: I. Coupling MHD and kinetic theory

Multidimensional magneto-hydrodynamical (MHD) simulations coupled with
stochastic differential equations (SDEs) adapted to test particle acceleration
and transport in complex astrophysical flows are presented. The numerical
scheme allows the investigation of shock acceleration, adiabatic and radiative
losses as well as diffusive spatial transport in various diffusion regimes. The
applicability of SDEs to astrophysics is first discussed in regards to the
different regimes and the MHD code spatial resolution. The procedure is then
applied to 2.5D MHD-SDE simulations of kilo-parsec scale extragalactic jets.
The ability of SDE to reproduce analytical solutions of the
diffusion-convection equation for electrons is tested through the incorporation
of an increasing number of effects: shock acceleration, spatially dependent
diffusion coefficients and synchrotron losses. The SDEs prove to be efficient
in various shock configuration occurring in the inner jet during the
development of the Kelvin-Helmholtz instability. The particle acceleration in
snapshots of strong single and multiple shock acceleration including realistic
spatial transport is treated. In chaotic magnetic diffusion regime, turbulence
levels $\eta_T=/(B^2+)$ around $0.2-0.3$ are found to
be the most efficient to enable particles to reach the highest energies. The
spectrum, extending from 100 MeV to few TeV (or even 100 TeV for fast flows),
does not exhibit a power-law shape due to transverse momentum dependent
escapes. Out of this range, the confinement is not so efficient and the
spectrum cut-off above few hundreds of GeV, questioning the Chandra
observations of X-ray knots as being synchrotron radiation. The extension to
full time dependent simulations to X-ray extragalactic jets is discussed.Comment: Astronomy & Astrophysics (in press), 18 page

### Non-resonant magnetohydrodynamics streaming instability near magnetized relativistic shocks

We present in this paper both a linear study and numerical relativistic MHD
simulations of the non-resonant streaming instability occurring in the
precursor of relativistic shocks. In the shock front restframe, we perform a
linear analysis of this instability in a likely configuration for
ultra-relativistic shock precursors. This considers magneto-acoustic waves
having a wave vector perpendicular to the shock front and the large scale
magnetic field. Our linear analysis is achieved without any assumption on the
shock velocity and is thus valid for all velocity regimes. In order to check
our calculation, we also perform relativistic MHD simulations describing the
propagation of the aforementioned magneto-acoustic waves through the shock
precursor. The numerical calculations confirm our linear analysis, which
predicts that the growth rate of the instability is maximal for
ultra-relativistic shocks and exhibits a wavenumber dependence $\propto
k_x^{1/2}$. Our numerical simulations also depict the saturation regime of the
instability where we show that the magnetic amplification is moderate but
nevertheless significant ($\delta B/B\leq 10$). This latter fact may explain
the presence of strong turbulence in the vicinity of relativistic magnetized
shocks. Our numerical approach also introduces a convenient means to handle
isothermal (ultra-)relativistic MHD conditions.Comment: 14 pages, 6 figures, MNRAS (in press

### Vertical angular momentum transfer from accretion discs and the formation of large-scale collimated jets

Invited conference 35th European Physical society on Plasma PhysicsInternational audienceIn this paper I present an overview of the favoured scenario explaining the presence of twin cylindrical astrophysical jets in the vicinity of accretion discs. These jets are made of plasma and host large-scale magnetic fields. The twin jets flow away from the accreting system in opposite directions, perpendicular to the plane of the accretion disc. In the scenario presented in this paper, the accretion disc interacts with the magnetic field in such a way that the disc angular momentum is removed from the disc and transported away along the magnetic field lines. Such a transport is the source of the jet phenomenon as the angular momentum is given back to a tiny amount of material extracted from the disc. This outflow is then powered by the disc rotation as the disc is able to enter an accretion motion where matter releases its gravitational energy. The angular momentum carried by the jet is actually present through the existence of an electric current. In the jet cylindrical geometry, the presence of this current is able to provide a collimating mechanism where the magnetic field pinches the plasma column. This mechanism is very close to the one acting in tokamak reactors. Apart from explaining how the plasma outflow is able to be self-confined by the magnetic field present in the flow, this scenario is also able to explain how jet mass can be accelerated thanks to the magnetohydrodynamics Poynting flux escaping from the disc. In this presentation I finally present the constraints arising from the scenario, in particular upon the turbulent transport coefficient required to get a steady structure

### Formation and long-term evolution of 3D vortices in protoplanetary discs

In the context of planet formation, anticyclonic vortices have recently
received lots of attention for the role they can play in planetesimals
formation. Radial migration of intermediate size solids toward the central star
may prevent their growth to larger solid grains. On the other hand, vortices
can trap the dust and accelerate this growth, counteracting fast radial
transport. Multiple effects have been shown to affect this scenario, such as
vortex migration or decay. The aim of this paper is to study the formation of
vortices by the Rossby wave instability and their long term evolution in a full
three dimensional protoplanetary disc. We use a robust numerical scheme
combined with adaptive mesh refinement in cylindrical coordinates, allowing to
affordably compute long term 3D evolutions. We consider a full disc stratified
both radially and vertically that is prone to formation of vortices by the
Rossby wave instability. We show that the 3D Rossby vortices grow and survive
over hundreds of years without migration. The localized overdensity which
initiated the instability and vortex formation survives the growth of the
Rossby wave instability for very long times. When the vortices are no longer
sustained by the Rossby wave instability, their shape changes toward more
elliptical vortices. This allows them to survive shear-driven destruction, but
they may be prone to elliptical instability and slow decay. When the conditions
for growing Rossby wave-related instabilities are maintained in the disc,
large-scale vortices can survive over very long timescales and may be able to
concentrate solids.Comment: Accepted for publication in A&

### AMRVAC and Relativistic Hydrodynamic simulations for GRB afterglow phases

We apply a novel adaptive mesh refinement code, AMRVAC, to numerically
investigate the various evolutionary phases in the interaction of a
relativistic shell with its surrounding cold Interstellar Medium (ISM). We do
this for both 1D isotropic as well as full 2D jetlike fireball models. This is
relevant for Gamma Ray Bursts, and we demonstrate that, thanks to the AMR
strategy, we resolve the internal structure of the shocked shell-ISM matter,
which will leave its imprint on the GRB afterglow. We determine the
deceleration from an initial Lorentz factor $\gamma=100$ up to the almost
Newtonian $\gamma\sim{\cal O}(2)$ phase of the flow. We present axisymmetric 2D
shell evolutions, with the 2D extent characterized by their initial opening
angle. In such jetlike GRB models, we discuss the differences with the 1D
isotropic GRB equivalents. These are mainly due to thermally induced sideways
expansions of both the shocked shell and shocked ISM regions. We found that the
propagating 2D ultrarelativistic shell does not accrete all the surrounding
medium located within its initial opening angle. Part of this ISM matter gets
pushed away laterally and forms a wide bow-shock configuration with swirling
flow patterns trailing the thin shell. The resulting shell deceleration is
quite different from that found in isotropic GRB models. As long as the lateral
shell expansion is merely due to ballistic spreading of the shell, isotropic
and 2D models agree perfectly. As thermally induced expansions eventually lead
to significantly higher lateral speeds, the 2D shell interacts with comparably
more ISM matter and decelerates earlier than its isotropic counterpart.Comment: 12 pages, accepted in MNRAS, 12/01/200

### Time-dependent magnetohydrodynamic self-similar extragalactic jets

Extragalactic jets are visualized as dynamic erruptive events modelled by
time-dependent magnetohydrodynamic (MHD) equations. The jet structure comes
through the temporally self-similar solutions in two-dimensional axisymmetric
spherical geometry. The two-dimensional magnetic field is solved in the finite
plasma pressure regime, or finite $\beta$ regime, and it is described by an
equation where plasma pressure plays the role of an eigenvalue. This allows a
structure of magnetic lobes in space, among which the polar axis lobe is
strongly peaked in intensity and collimated in angular spread comparing to the
others. For this reason, the polar lobe overwhelmes the other lobes, and a jet
structure arises in the polar direction naturally. Furthermore, within each
magnetic lobe in space, there are small secondary regions with closed
two-dimensional field lines embedded along this primary lobe. In these embedded
magnetic toroids, plasma pressure and mass density are much higher accordingly.
These are termed as secondary plasmoids. The magnetic field lines in these
secondary plasmoids circle in alternating sequence such that adjacent plasmoids
have opposite field lines. In particular, along the polar primary lobe, such
periodic plasmoid structure happens to be compatible with radio observations
where islands of high radio intensities are mapped

### Two-flow magnetohydrodynamical jets around young stellar objects

We present the first-ever simulations of non-ideal magnetohydrodynamical
(MHD) stellar winds coupled with disc-driven jets where the resistive and
viscous accretion disc is self-consistently described. The transmagnetosonic,
collimated MHD outflows are investigated numerically using the VAC code. Our
simulations show that the inner outflow is accelerated from the central object
hot corona thanks to both the thermal pressure and the Lorentz force. In our
framework, the thermal acceleration is sustained by the heating produced by the
dissipated magnetic energy due to the turbulence. Conversely, the outflow
launched from the resistive accretion disc is mainly accelerated by the
magneto-centrifugal force. We also show that when a dense inner stellar wind
occurs, the resulting disc-driven jet have a different structure, namely a
magnetic structure where poloidal magnetic field lines are more inclined
because of the pressure caused by the stellar wind. This modification leads to
both an enhanced mass ejection rate in the disc-driven jet and a larger radial
extension which is in better agreement with the observations besides being more
consistent.Comment: Accepted for publication in Astrophysics & Space Science. Referred
proceeding of the fifth Mont Stromlo Symposium Dec. 1-8 2006, Canberra,
Australia. 5 pages, 3 figures. For high resolution version of the paper,
please click here http://www.apc.univ-paris7.fr/~fcasse/publications.htm

### Magnetized Accretion-Ejection Structures: 2.5D MHD simulations of continuous Ideal Jet launching from resistive accretion disks

We present numerical magnetohydrodynamic (MHD) simulations of a magnetized
accretion disk launching trans-Alfvenic jets. These simulations, performed in a
2.5 dimensional time-dependent polytropic resistive MHD framework, model a
resistive accretion disk threaded by an initial vertical magnetic field. The
resistivity is only important inside the disk, and is prescribed as eta =
alpha_m V_AH exp(-2Z^2/H^2), where V_A stands for Alfven speed, H is the disk
scale height and the coefficient alpha_m is smaller than unity. By performing
the simulations over several tens of dynamical disk timescales, we show that
the launching of a collimated outflow occurs self-consistently and the ejection
of matter is continuous and quasi-stationary. These are the first ever
simulations of resistive accretion disks launching non-transient ideal MHD
jets. Roughly 15% of accreted mass is persistently ejected. This outflow is
safely characterized as a jet since the flow becomes super-fastmagnetosonic,
well-collimated and reaches a quasi-stationary state. We present a complete
illustration and explanation of the `accretion-ejection' mechanism that leads
to jet formation from a magnetized accretion disk. In particular, the magnetic
torque inside the disk brakes the matter azimuthally and allows for accretion,
while it is responsible for an effective magneto-centrifugal acceleration in
the jet. As such, the magnetic field channels the disk angular momentum and
powers the jet acceleration and collimation. The jet originates from the inner
disk region where equipartition between thermal and magnetic forces is
achieved. A hollow, super-fastmagnetosonic shell of dense material is the
natural outcome of the inwards advection of a primordial field.Comment: ApJ (in press), 32 pages, Higher quality version available at
http://www-laog.obs.ujf-grenoble.fr/~fcass

### Magnetic Fields in Stellar Jets

Although several lines of evidence suggest that jets from young stars are
driven magnetically from accretion disks, existing observations of field
strengths in the bow shocks of these flows imply that magnetic fields play only
a minor role in the dynamics at these locations. To investigate this apparent
discrepancy we performed numerical simulations of expanding magnetized jets
with stochastically variable input velocities with the AstroBEAR MHD code.
Because the magnetic field B is proportional to the density n within
compression and rarefaction regions, the magnetic signal speed drops in
rarefactions and increases in the compressed areas of velocity-variable flows.
In contrast, B ~ n^0.5 for a steady-state conical flow with a toroidal field,
so the Alfven speed in that case is constant along the entire jet. The
simulations show that the combined effects of shocks, rarefactions, and
divergent flow cause magnetic fields to scale with density as an intermediate
power 1 > p > 0.5. Because p > 0.5, the Alfven speed in rarefactions decreases
on average as the jet propagates away from the star. This behavior is extremely
important to the flow dynamics because it means that a typical Alfven velocity
in the jet close to the star is significantly larger than it is in the
rarefactions ahead of bow shocks at larger distances, the one place where the
field is a measurable quantity. We find that the observed values of weak fields
at large distances are consistent with strong fields required to drive the
observed mass loss close to the star. For a typical stellar jet the crossover
point inside which velocity perturbations of 30 - 40 km/s no longer produce
shocks is ~ 300 AU from the source

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