710 research outputs found
Magnetohydrodynamic jets from different magnetic field configurations
Using axisymmetric MHD simulations we investigate how the overall jet
formation is affected by a variation in the disk magnetic flux profile and/or
the existence of a central stellar magnetosphere. Our simulations evolve from
an initial, hydrostatic equilibrium state in a force-free magnetic field
configuration. We find a unique relation between the collimation degree and the
disk wind magnetization power law exponent. The collimation degree decreases
for steeper disk magnetic field profiles. Highly collimated outflows resulting
from a flat profile tend to be unsteady. We further consider a magnetic field
superposed of a stellar dipole and a disk field in parallel or anti-parallel
alignment. Both stellar and disk wind may evolve in a pair of outflows,
however, a reasonably strong disk wind component is essential for jet
collimation. Strong flares may lead to a sudden change in mass flux by a factor
two. We hypothesize that such flares may eventually trigger jet knots.Comment: 5 pages, 4 figures; proceedings from conference: Protostellar Jets in
Context, held in Rhodes, July 7-12, 200
Optimising Boltzmann codes for the Planck era
High precision measurements of the Cosmic Microwave Background (CMB)
anisotropies, as can be expected from the Planck satellite, will require
high-accuracy theoretical predictions as well. One possible source of
theoretical uncertainty is the numerical error in the output of the Boltzmann
codes used to calculate angular power spectra. In this work, we carry out an
extensive study of the numerical accuracy of the public Boltzmann code CAMB,
and identify a set of parameters which determine the error of its output. We
show that at the current default settings, the cosmological parameters
extracted from data of future experiments like Planck can be biased by several
tenths of a standard deviation for the six parameters of the standard
Lambda-CDM model, and potentially more seriously for extended models. We
perform an optimisation procedure that leads the code to achieve sufficient
precision while at the same time keeping the computation time within reasonable
limits. Our conclusion is that the contribution of numerical errors to the
theoretical uncertainty of model predictions is well under control -- the main
challenges for more accurate calculations of CMB spectra will be of an
astrophysical nature instead.Comment: 13 pages, 4 figure
The Axisymmetric Pulsar Magnetosphere
We present, for the first time, the structure of the axisymmetric force-free
magnetosphere of an aligned rotating magnetic dipole, in the case in which
there exists a sufficiently large charge density (whose origin we do not
question) to satisfy the ideal MHD condition, , everywhere.
The unique distribution of electric current along the open magnetic field lines
which is required for the solution to be continuous and smooth is obtained
numerically. With the geometry of the field lines thus determined we compute
the dynamics of the associated MHD wind. The main result is that the
relativistic outflow contained in the magnetosphere is not accelerated to the
extremely relativistic energies required for the flow to generate gamma rays.
We expect that our solution will be useful as the starting point for detailed
studies of pulsar magnetospheres under more general conditions, namely when
either the force-free and/or the ideal MHD condition are not
valid in the entire magnetosphere. Based on our solution, we consider that the
most likely positions of such an occurrence are the polar cap, the crossings of
the zero space charge surface by open field lines, and the return current
boundary, but not the light cylinder.Comment: 15 pages AAS Latex, 5 postscript figure
Magnetic interaction of jets and molecular clouds in NGC 4258
NGC 4258 is a well known spiral galaxy with a peculiar large scale jet flow detected in the radio and in H-alpha. Due to the special geometry of the galaxy, the jets emerge from the nuclear region through the galactic disk. Also the distribution of molecular gas looks different from that in other spiral galaxies: [12]CO(1-0)emission has only been detected in the center and along the jets and only up to distances of about 50 arcsec (1.8 kpc) from the nucleus. The reason for the CO concentration along the inner jets in NGC 4258 was not understood and is the motivation for the observations presented here. Using the IRAM interferometer, we mapped the [12]CO(1-0) emission of the central part of NGC 4258 along the nuclear jet direction in the inner 3 kpc. We detected two parallel CO ridges along a position angle of -25 degr with a total length of about 80 arcsec (2.8 kpc), separated by a CO-depleted funnel with a width of about 5 arcsec (175 pc). The H-alpha emission is more extended and broader than the CO emission with its maximum just in between the two CO ridges. In CO we see a peculiar velocity distribution in the iso-velocity map and p-v diagrams. We discuss different scenarios for an interpretation and present a model which can explain the observational results consistently. We propose here that the concentration of CO along the ridges is due to interaction of the rotating gas clouds with the jet's magnetic field by ambipolar diffusion. This magnetic interaction is thought to increase the time the molecular clouds reside near the jet thus leading to the quasi-static CO ridge
Magnetospheric Accretion and Ejection of Matter in Resistive Magnetohydrodynamic Simulations
The ejection of matter in the close vicinity of a young stellar object is
investigated, treating the accretion disk as a gravitationally bound reservoir
of matter. By solving the resistive MHD equations in 2D axisymmetry using our
version of the Zeus-3D code with newly implemented resistivity, we study the
effect of magnetic diffusivity in the magnetospheric accretion-ejection
mechanism. Physical resistivity was included in the whole computational domain
so that reconnection is enabled by the physical as well as the numerical
resistivity. We show, for the first time, that quasi-stationary fast ejecta of
matter, which we call {\em micro-ejections}, of small mass and angular momentum
fluxes, can be launched from a purely resistive magnetosphere. They are
produced by a combination of pressure gradient and magnetic forces, in presence
of ongoing magnetic reconnection along the boundary layer between the star and
the disk, where a current sheet is formed. Mass flux of micro-ejection
increases with increasing magnetic field strength and stellar rotation rate,
and is not dependent on the disk to corona density ratio and amount of
resistivity.Comment: 18 pages, many revisions from previous version, accepted in Ap
Particle acceleration close to the supermassive black hole horizon: the case of M87
The radio galaxy M87 has recently been found to be a rapidly variable TeV
emitting source. We analyze the implications of the observed TeV
characteristics and show that it proves challenging to account for them within
conventional acceleration and emission models. We discuss a new pulsar-type
scenario for the origin of variable, very high energy (VHE) emission close to
the central supermassive black hole and show that magneto-centrifugally
accelerated electrons could efficiently Compton upscatter sub-mm ADAF disk
photons to the TeV regime, leading to VHE characteristics close to the observed
ones. This suggests, conversely, that VHE observations of highly under-luminous
AGNs could provide an important diagnostic tool for probing the conditions
prevalent in the inner accretion disk of these sources.Comment: 5 pages, one figure (typos corrected); based on presentation at "High
Energy Phenomena in Relativistic Outflows", Dublin, Sept. 2007; accepted for
publication in International Journal of Modern Physics
Collimation of astrophysical jets - the role of the accretion disk magnetic field distribution
We have applied axisymmetric MHD simulations to investigate the impact of the
accretion disk magnetic flux profile on the jet collimation. Using the ZEUS-3D
code modified for magnetic diffusivity, our simulations evolve from an initial
hydrostatic equilibrium state in a force-free magnetic field. Considering a
power law for the disk poloidal magnetic field profile Bp ~ r^{-mu} and for the
disk wind density profile rho ~ r^{-mu_rho} we performed a systematic study
over a wide parameter range mu and mu_rho. We find a degree of collimation
(ratio of mass flow rates in axial and lateral direction) decreasing for
steeper disk magnetic field profiles (increasing mu). Varying the total
magnetic flux doesn't change the degree of jet collimation substantially, it
only affects the time scale of outflow evolution and the terminal jet speed. As
our major result we find a general relation between the collimation degree with
the disk wind magnetization power law exponent. Outflows with high collimation
degree resulting from a flat disk magnetic field profile tend to be unsteady,
producing axially propagating knots as discussed earlier. Depending slightly on
the inflow density profile this unsteady behavior sets in for mu < 0.4. We also
performed simulations of jet formation with artificially enhanced decay of the
toroidal magnetic field in order to investigate the idea of a purely "poloidal
collimation" discussed in the literature. These outflows remain weakly
collimated and propagate with lower velocity. Thanks to our large numerical
grid size (7x14 AU for protostars), we may apply our results to recently
observed hints of jet rotation (DG Tau) indicating a relatively flat disk
magnetic field profile, mu ~ 0.5. In general, our results are applicable to
both stellar and extragalactic sources of MHD jets.Comment: accepted by ApJ, high resolution version under
www.mpia-hd.mpg.de/homes/fendt
Formation of protostellar jets - effects of magnetic diffusion
We investigate the evolution of a disk wind into a collimated jet under the
influence of magnetic diffusivity, assuming that the turbulent pattern in the
disk will also enter the disk corona and the jet. Using the ZEUS-3D code in the
axisymmetry option we solve the time-dependent resistive MHD equations for a
model setup of a central star surrounded by an accretion disk. We find that the
diffusive jets propagate slower into the ambient medium. Close to the star we
find that a quasi stationary state evolves after several hundred (weak
diffusion) or thousand (strong diffusion) disk rotations. Magnetic diffusivity
affects the protostellar jet structure as follows. The jet poloidal magnetic
field becomes de-collimated. The jet velocity increases with increasing
diffusivity, while the degree of collimation for the hydrodynamic flow remains
more or less the same. We suggest that the mass flux is a proper tracer for the
degree of jet collimation and find indications of a critical value for the
magnetic diffusivity above which the jet collimation is only weak.Comment: 16 pages, 12 figs, accepted by Astron. and Astrop
Ultra-Relativistic Magneto-Hydro-Dynamic Jets in the context of Gamma Ray Bursts
We present a detailed numerical study of the dynamics and evolution of
ultrarelativistic magnetohydrodynamic jets in the black hole-disk system under
extreme magnetization conditions. We find that Lorentz factors of up to 3000
are achieved and derived a modifiedMichel scaling (Gamma ~ sigma) which allows
for a wide variation in the flow Lorentz factor. Pending contamination induced
by mass-entrainment, the linear Michel scaling links modulations in the
ultrarelativistic wind to variations in mass accretion in the disk for a given
magnetization. The jet is asymptotically dominated by the toroidal magnetic
field allowing for efficient collimation. We discuss our solutions (jets) in
the context of Gamma ray bursts and describe the relevant features such as the
high variability in the Lorentz factor and how high collimation angles (~ 0-5
degrees), or cylindrical jets, can be achieved. We isolate a jet instability
mechanism we refer to as the "bottle-neck" instability which essentially relies
on a high magnetization and a recollimation of the magnetic flux surfaces. The
instability occurs at large radii where any dissipation of the magnetic energy
into radiation would in principle result in an optically thin emission.Comment: 31 pages, 6 figures. Submitted to ApJ. Higher Quality figures at
http://www.capca.ucalgary.ca/paper
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