608 research outputs found
Stellar winds, dead zones, and coronal mass ejections
Axisymmetric stellar wind solutions are presented, obtained by numerically
solving the ideal magnetohydrodynamic (MHD) equations. Stationary solutions are
critically analysed using the knowledge of the flux functions. These flux
functions enter in the general variational principle governing all axisymmetric
stationary ideal MHD equilibria. The magnetized wind solutions for
(differentially) rotating stars contain both a `wind' and a `dead' zone. We
illustrate the influence of the magnetic field topology on the wind
acceleration pattern, by varying the coronal field strength and the extent of
the dead zone. This is evident from the resulting variations in the location
and appearance of the critical curves where the wind speed equals the slow,
Alfven, and fast speed. Larger dead zones cause effective, fairly isotropic
acceleration to super-Alfvenic velocities as the polar, open field lines are
forced to fan out rapidly with radial distance. A higher field strength moves
the Alfven transition outwards. In the ecliptic, the wind outflow is clearly
modulated by the extent of the dead zone. The combined effect of a fast stellar
rotation and an equatorial `dead' zone in a bipolar field configuration can
lead to efficient thermo-centrifugal equatorial winds. Such winds show both a
strong poleward collimation and some equatorward streamline bending due to
significant toroidal field pressure at mid-latitudes. We discuss how coronal
mass ejections are then simulated on top of the transonic outflows.Comment: scheduled for Astrophys. J. 530 #2, Febr.20 2000 issue. 9 figures (as
6 jpeg and 8 eps files
Non-linear dynamics of Kelvin-Helmholtz unstable magnetized jets: three-dimensional effects
A numerical study of the Kelvin-Helmholtz instability in compressible
magnetohydrodynamics is presented. The three-dimensional simulations consider
shear flow in a cylindrical jet configuration, embedded in a uniform magnetic
field directed along the jet axis. The growth of linear perturbations at
specified poloidal and axial mode numbers demonstrate intricate non-linear
coupling effects. The physical mechanims leading to induced secondary
Kelvin-Helmholtz instabilities at higher mode numbers are identified. The
initially weak magnetic field becomes locally dominant in the non-linear
dynamics before and during saturation. Thereby, it controls the jet deformation
and eventual breakup. The results are obtained using the Versatile Advection
Code [G. Toth, Astrophys. Lett. Comm. 34, 245 (1996)], a software package
designed to solve general systems of conservation laws. An independent
calculation of the same Kelvin-Helmholtz unstable jet configuration using a
three-dimensional pseudo-spectral code gives important insights into the
coupling and excitation events of the various linear mode numbers.Comment: 10 (+7) pages, 6 figures, accepted for Phys. Plasmas 6, to appear
199
Numerical simulations of stellar winds: polytropic models
We discuss steady-state transonic outflows obtained by direct numerical
solution of the hydrodynamic and magnetohydrodynamic equations. We make use of
the Versatile Advection Code, a software package for solving systems of
(hyperbolic) partial differential equations. We proceed stepwise from a
spherically symmetric, isothermal, unmagnetized, non-rotating Parker wind to
arrive at axisymmetric, polytropic, magnetized, rotating models. These
represent 2D generalisations of the analytical 1D Weber-Davis wind solution,
which we obtain in the process. Axisymmetric wind solutions containing both a
`wind' and a `dead' zone are presented.
Since we are solving for steady-state solutions, we efficiently exploit fully
implicit time stepping. The method allows us to model thermally and/or
magneto-centrifugally driven stellar outflows. We particularly emphasize the
boundary conditions imposed at the stellar surface. For these axisymmetric,
steady-state solutions, we can use the knowledge of the flux functions to
verify the physical correctness of the numerical solutions.Comment: 11 pages, 6 figures, accepted for Astron. Astrophys. 342, to appear
199
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 . Our numerical simulations also depict the saturation regime of the
instability where we show that the magnetic amplification is moderate but
nevertheless significant (). 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
Modelling ripples in Orion with coupled dust dynamics and radiative transfer
In light of the recent detection of direct evidence for the formation of
Kelvin-Helmholtz instabilities in the Orion nebula, we expand upon previous
modelling efforts by numerically simulating the shear-flow driven gas and dust
dynamics in locations where the H region and the molecular cloud
interact. We aim to directly confront the simulation results with the infrared
observations. Methods: To numerically model the onset and full nonlinear
development of the Kelvin-Helmholtz instability we take the setup proposed to
interpret the observations, and adjust it to a full 3D hydrodynamical
simulation that includes the dynamics of gas as well as dust. A dust grain
distribution with sizes between 5-250 nm is used, exploiting the gas+dust
module of the MPI-AMRVAC code, in which the dust species are represented by
several pressureless dust fluids. The evolution of the model is followed well
into the nonlinear phase. The output of these simulations is then used as input
for the SKIRT dust radiative transfer code to obtain infrared images at several
stages of the evolution, which can be compared to the observations. Results: We
confirm that a 3D Kelvin-Helmholtz instability is able to develop in the
proposed setup, and that the formation of the instability is not inhibited by
the addition of dust. Kelvin-Helmholtz billows form at the end of the linear
phase, and synthetic observations of the billows show striking similarities to
the infrared observations. It is pointed out that the high density dust regions
preferentially collect on the flanks of the billows. To get agreement with the
observed Kelvin-Helmholtz ripples, the assumed geometry between the background
radiation, the billows and the observer is seen to be of critical importance.Comment: 8 pages, 10 figure
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 up to the almost
Newtonian 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
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&
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