68 research outputs found
Approximate Riemann Solvers and Robust High-Order Finite Volume Schemes for Multi-Dimensional Ideal MHD Equations
We design stable and high-order accurate finite volume schemes for the ideal MHD equations in multi-dimensions. We obtain excellent numerical stability due to some new elements in the algorithm. The schemes are based on three- and five-wave approximate Riemann solvers of the HLL-type, with the novelty that we allow a varying normal magnetic field. This is achieved by considering the semi-conservative Godunov-Powell form of the MHD equations. We show that it is important to discretize the Godunov-Powell source term in the right way, and that the HLL-type solvers naturally provide a stable upwind discretization. Second-order versions of the ENO- and WENO-type reconstructions are proposed, together with precise modifications necessary to preserve positive pressure and density. Extending the discrete source term to second order while maintaining stability requires non-standard techniques, which we present. The first- and second-order schemes are tested on a suite of numerical experiments demonstrating impressive numerical resolution as well as stability, even on very fine meshe
Statistical analysis of the mass-to-flux ratio in turbulent cores: effects of magnetic field reversals and dynamo amplification
We study the mass-to-flux ratio (M/\Phi) of clumps and cores in simulations
of supersonic, magnetohydrodynamical turbulence for different initial magnetic
field strengths. We investigate whether the (M/\Phi)-ratio of core and
envelope, R = (M/\Phi)_{core}/(M/\Phi)_{envelope} can be used to distinguish
between theories of ambipolar diffusion and turbulence-regulated star
formation. We analyse R for different Lines-of-Sight (LoS) in various sub-cubes
of our simulation box. We find that, 1) the average and median values of |R|
for different times and initial magnetic field strengths are typically greater,
but close to unity, 2) the average and median values of |R| saturate at average
values of |R| ~ 1 for smaller magnetic fields, 3) values of |R| < 1 for small
magnetic fields in the envelope are caused by field reversals when turbulence
twists the field lines such that field components in different directions
average out. Finally, we propose two mechanisms for generating values |R| ~< 1
for the weak and strong magnetic field limit in the context of a turbulent
model. First, in the weak field limit, the small-scale turbulent dynamo leads
to a significantly increased flux in the core and we find |R| ~< 1. Second, in
the strong field limit, field reversals in the envelope also lead to values |R|
~< 1. These reversals are less likely to occur in the core region where the
velocity field is more coherent and the internal velocity dispersion is
typically subsonic.Comment: 12 pages, 8 figures, accepted for publication in MNRA
Magnetic fields during the early stages of massive star formation - I. Accretion and disk evolution
We present simulations of collapsing 100 M_\sun mass cores in the context of
massive star formation. The effect of variable initial rotational and magnetic
energies on the formation of massive stars is studied in detail. We focus on
accretion rates and on the question under which conditions massive Keplerian
disks can form in the very early evolutionary stage of massive protostars. For
this purpose, we perform 12 simulations with different initial conditions
extending over a wide range in parameter space. The equations of
magnetohydrodynamics (MHD) are solved under the assumption of ideal MHD. We
find that the formation of Keplerian disks in the very early stages is
suppressed for a mass-to-flux ratio normalised to the critical value \mu below
10, in agreement with a series of low-mass star formation simulations. This is
caused by very efficient magnetic braking resulting in a nearly instantaneous
removal of angular momentum from the disk. For weak magnetic fields,
corresponding to \mu > 10, large-scale, centrifugally supported disks build up
with radii exceeding 100 AU. A stability analysis reveals that the disks are
supported against gravitationally induced perturbations by the magnetic field
and tend to form single stars rather than multiple objects. We find
protostellar accretion rates of the order of a few 10^-4 M_\sun yr^-1 which,
considering the large range covered by the initial conditions, vary only by a
factor of ~ 3 between the different simulations. We attribute this fact to two
competing effects of magnetic fields. On the one hand, magnetic braking
enhances accretion by removing angular momentum from the disk thus lowering the
centrifugal support against gravity. On the other hand, the combined effect of
magnetic pressure and magnetic tension counteracts gravity by exerting an
outward directed force on the gas in the disk thus reducing the accretion onto
the protostars.Comment: 22 pages, 17 figures, accepted for publication in MNRAS, updated to
final versio
A method for reconstructing the variance of a 3D physical field from 2D observations: Application to turbulence in the ISM
We introduce and test an expression for calculating the variance of a
physical field in three dimensions using only information contained in the
two-dimensional projection of the field. The method is general but assumes
statistical isotropy. To test the method we apply it to numerical simulations
of hydrodynamic and magnetohydrodynamic turbulence in molecular clouds, and
demonstrate that it can recover the 3D normalised density variance with ~10%
accuracy if the assumption of isotropy is valid. We show that the assumption of
isotropy breaks down at low sonic Mach number if the turbulence is
sub-Alfvenic. Theoretical predictions suggest that the 3D density variance
should increase proportionally to the square of the Mach number of the
turbulence. Application of our method will allow this prediction to be tested
observationally and therefore constrain a large body of analytic models of star
formation that rely on it.Comment: 8 pages, 9 figures, accepted for publication in MNRA
Magnetohydrodynamics on an unstructured moving grid
Magnetic fields play an important role in astrophysics on a wide variety of
scales, ranging from the Sun and compact objects to galaxies and galaxy
clusters. Here we discuss a novel implementation of ideal magnetohydrodynamics
(MHD) in the moving mesh code AREPO which combines many of the advantages of
Eulerian and Lagrangian methods in a single computational technique. The
employed grid is defined as the Voronoi tessellation of a set of
mesh-generating points which can move along with the flow, yielding an
automatic adaptivity of the mesh and a substantial reduction of advection
errors. Our scheme solves the MHD Riemann problem in the rest frame of the
Voronoi interfaces using the HLLD Riemann solver. To satisfy the divergence
constraint of the magnetic field in multiple dimensions, the Dedner divergence
cleaning method is applied. In a set of standard test problems we show that the
new code produces accurate results, and that the divergence of the magnetic
field is kept sufficiently small to closely preserve the correct physical
solution. We also apply the code to two first application problems, namely
supersonic MHD turbulence and the spherical collapse of a magnetized cloud. We
verify that the code is able to handle both problems well, demonstrating the
applicability of this MHD version of AREPO to a wide range of problems in
astrophysics.Comment: 11 pages, 9 figures, accepted by MNRA
A new Jeans resolution criterion for (M)HD simulations of self-gravitating gas: Application to magnetic field amplification by gravity-driven turbulence
Cosmic structure formation is characterized by the complex interplay between
gravity, turbulence, and magnetic fields. The processes by which gravitational
energy is converted into turbulent and magnetic energies, however, remain
poorly understood. Here, we show with high-resolution, adaptive-mesh
simulations that MHD turbulence is efficiently driven by extracting energy from
the gravitational potential during the collapse of a dense gas cloud.
Compressible motions generated during the contraction are converted into
solenoidal, turbulent motions, leading to a natural energy ratio of E_sol/E_tot
of approximately 2/3. We find that the energy injection scale of gravity-driven
turbulence is close to the local Jeans scale. If small seeds of the magnetic
field are present, they are amplified exponentially fast via the small-scale
dynamo process. The magnetic field grows most efficiently on the smallest
scales, for which the stretching, twisting, and folding of field lines, and the
turbulent vortices are sufficiently resolved. We find that this scale
corresponds to about 30 grid cells in the simulations. We thus suggest a new
minimum resolution criterion of 30 cells per Jeans length in
(magneto)hydrodynamical simulations of self-gravitating gas, in order to
resolve turbulence on the Jeans scale, and to capture minimum dynamo
amplification of the magnetic field. Due to numerical diffusion, however, any
existing simulation today can at best provide lower limits on the physical
growth rates. We conclude that a small, initial magnetic field can grow to
dynamically important strength on time scales significantly shorter than the
free-fall time of the cloud.Comment: 17 pages, 13 figures, ApJ accepted, more info at
http://www.ita.uni-heidelberg.de/~chfeder/pubs/dynamo/dynamo.shtml?lang=e
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